Design Pattern Used by Struts 2 for Implementing Interceptors

17. Web MVC framework

17.1 Introduction to Spring Web MVC framework

The Spring Web model-view-controller (MVC) framework is designed around a DispatcherServlet that dispatches requests to handlers, with configurable handler mappings, view resolution, locale and theme resolution as well as support for uploading files. The default handler is based on the @Controller and @RequestMapping annotations, offering a wide range of flexible handling methods. With the introduction of Spring 3.0, the @Controller mechanism also allows you to create RESTful Web sites and applications, through the @PathVariable annotation and other features.

In Spring Web MVC you can use any object as a command or form-backing object; you do not need to implement a framework-specific interface or base class. Spring's data binding is highly flexible: for example, it treats type mismatches as validation errors that can be evaluated by the application, not as system errors. Thus you need not duplicate your business objects' properties as simple, untyped strings in your form objects simply to handle invalid submissions, or to convert the Strings properly. Instead, it is often preferable to bind directly to your business objects.

Spring's view resolution is extremely flexible. A Controller is typically responsible for preparing a model Map with data and selecting a view name but it can also write directly to the response stream and complete the request. View name resolution is highly configurable through file extension or Accept header content type negotiation, through bean names, a properties file, or even a custom ViewResolver implementation. The model (the M in MVC) is a Map interface, which allows for the complete abstraction of the view technology. You can integrate directly with template based rendering technologies such as JSP, Velocity and Freemarker, or directly generate XML, JSON, Atom, and many other types of content. The model Map is simply transformed into an appropriate format, such as JSP request attributes, a Velocity template model.

17.1.1 Features of Spring Web MVC

Spring's web module includes many unique web support features:

Clear separation of roles . Each role — controller, validator, command object, form object, model object, DispatcherServlet, handler mapping, view resolver, and so on — can be fulfilled by a specialized object.
Powerful and straightforward configuration of both framework and application classes as JavaBeans . This configuration capability includes easy referencing across contexts, such as from web controllers to business objects and validators.
Adaptability, non-intrusiveness, and flexibility. Define any controller method signature you need, possibly using one of the parameter annotations (such as @RequestParam, @RequestHeader, @PathVariable, and more) for a given scenario.
Reusable business code , no need for duplication . Use existing business objects as command or form objects instead of mirroring them to extend a particular framework base class.
Customizable binding and validation . Type mismatches as application-level validation errors that keep the offending value, localized date and number binding, and so on instead of String-only form objects with manual parsing and conversion to business objects.
Customizable handler mapping and view resolution . Handler mapping and view resolution strategies range from simple URL-based configuration, to sophisticated, purpose-built resolution strategies. Spring is more flexible than web MVC frameworks that mandate a particular technique.
Flexible model transfer . Model transfer with a name/value Map supports easy integration with any view technology.
Customizable locale and theme resolution, support for JSPs with or without Spring tag library, support for JSTL, support for Velocity without the need for extra bridges, and so on.
A simple yet powerful JSP tag library known as the Spring tag library that provides support for features such as data binding and themes . The custom tags allow for maximum flexibility in terms of markup code. For information on the tag library descriptor, see the appendix entitled Appendix G, spring.tld
A JSP form tag library, introduced in Spring 2.0, that makes writing forms in JSP pages much easier. For information on the tag library descriptor, see the appendix entitled Appendix H, spring-form.tld
Beans whose lifecycle is scoped to the current HTTP request or HTTP Session. This is not a specific feature of Spring MVC itself, but rather of the WebApplicationContext container(s) that Spring MVC uses. These bean scopes are described in Section 5.5.4, "Request, session, and global session scopes"

17.1.2 Pluggability of other MVC implementations

Non-Spring MVC implementations are preferable for some projects. Many teams expect to leverage their existing investment in skills and tools. A large body of knowledge and experience exist for the Struts framework. If you can abide Struts' architectural flaws, it can be a viable choice for the web layer; the same applies to WebWork and other web MVC frameworks.

If you do not want to use Spring's web MVC, but intend to leverage other solutions that Spring offers, you can integrate the web MVC framework of your choice with Spring easily. Simply start up a Spring root application context through its ContextLoaderListener, and access it through its ServletContext attribute (or Spring's respective helper method) from within a Struts or WebWork action. No "plug-ins" are involved, so no dedicated integration is necessary. From the web layer's point of view, you simply use Spring as a library, with the root application context instance as the entry point.

Your registered beans and Spring's services can be at your fingertips even without Spring's Web MVC. Spring does not compete with Struts or WebWork in this scenario. It simply addresses the many areas that the pure web MVC frameworks do not, from bean configuration to data access and transaction handling. So you can enrich your application with a Spring middle tier and/or data access tier, even if you just want to use, for example, the transaction abstraction with JDBC or Hibernate.

17.2 The `DispatcherServlet`

Spring's web MVC framework is, like many other web MVC frameworks, request-driven, designed around a central Servlet that dispatches requests to controllers and offers other functionality that facilitates the development of web applications. Spring's DispatcherServlet however, does more than just that. It is completely integrated with the Spring IoC container and as such allows you to use every other feature that Spring has.

The request processing workflow of the Spring Web MVC DispatcherServlet is illustrated in the following diagram. The pattern-savvy reader will recognize that the DispatcherServlet is an expression of the "Front Controller" design pattern (this is a pattern that Spring Web MVC shares with many other leading web frameworks).

The DispatcherServlet is an actual Servlet (it inherits from the HttpServlet base class), and as such is declared in the web.xml of your web application. You need to map requests that you want the DispatcherServlet to handle, by using a URL mapping in the same web.xml file. This is standard Java EE Servlet configuration; the following example shows such a DispatcherServlet declaration and mapping:

            <web-app>            <servlet>            <servlet-name>example</servlet-name>            <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>            <load-on-startup>1</load-on-startup>            </servlet>            <servlet-mapping>            <servlet-name>example</servlet-name>            <url-pattern>/example/*</url-pattern>            </servlet-mapping>            </web-app>

In the preceding example, all requests starting with /example will be handled by the DispatcherServlet instance named example. In a Servlet 3.0+ environment, you also have the option of configuring the Servlet container programmatically. Below is the code based equivalent of the above web.xml example:

            public            class            MyWebApplicationInitializer            implements            WebApplicationInitializer {                          @Override                        public            void            onStartup(ServletContext container) {         ServletRegistration.Dynamic registration = container.addServlet("dispatcher",            new            DispatcherServlet());         registration.setLoadOnStartup(1);         registration.addMapping("/example/*");     }  }

WebApplicationInitializer is an interface provided by Spring MVC that ensures your code-based configuration is detected and automatically used to initialize any Servlet 3 container. An abstract base class implementation of this interace named AbstractDispatcherServletInitializer makes it even easier to register the DispatcherServlet by simply specifying its servlet mapping. See Code-based Servlet container initialization for more details.

The above is only the first step in setting up Spring Web MVC. You now need to configure the various beans used by the Spring Web MVC framework (over and above the DispatcherServlet itself).

As detailed in Section 5.14, "Additional Capabilities of the ApplicationContext", ApplicationContext instances in Spring can be scoped. In the Web MVC framework, each DispatcherServlet has its own WebApplicationContext, which inherits all the beans already defined in the root WebApplicationContext. These inherited beans can be overridden in the servlet-specific scope, and you can define new scope-specific beans local to a given Servlet instance.

Upon initialization of a DispatcherServlet, Spring MVC looks for a file named [servlet-name]-servlet.xml in the WEB-INF directory of your web application and creates the beans defined there, overriding the definitions of any beans defined with the same name in the global scope.

Consider the following DispatcherServlet Servlet configuration (in the web.xml file):

            <web-app>            <servlet>            <servlet-name>                          golfing                        </servlet-name>            <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>            <load-on-startup>1</load-on-startup>            </servlet>            <servlet-mapping>            <servlet-name>                          golfing                        </servlet-name>            <url-pattern>/golfing/*</url-pattern>            </servlet-mapping>            </web-app>

With the above Servlet configuration in place, you will need to have a file called /WEB-INF/ golfing -servlet.xml in your application; this file will contain all of your Spring Web MVC-specific components (beans). You can change the exact location of this configuration file through a Servlet initialization parameter (see below for details).

The WebApplicationContext is an extension of the plain ApplicationContext that has some extra features necessary for web applications. It differs from a normal ApplicationContext in that it is capable of resolving themes (see Section 17.9, "Using themes"), and that it knows which Servlet it is associated with (by having a link to the ServletContext). The WebApplicationContext is bound in the ServletContext, and by using static methods on the RequestContextUtils class you can always look up the WebApplicationContext if you need access to it.

17.2.1 Special Bean Types In the `WebApplicationContext`

The Spring DispatcherServlet uses special beans to process requests and render the appropriate views. These beans are part of Spring MVC. You can choose which special beans to use by simply configuring one or more of them in the WebApplicationContext. However, you don't need to do that initially since Spring MVC maintains a list of default beans to use if you don't configure any. More on that in the next section. First see the table below listing the special bean types the DispatcherServlet relies on.

Table 17.1. Special bean types in the WebApplicationContext

Bean type	Explanation
HandlerMapping	Maps incoming requests to handlers and a list of pre- and post-processors (handler interceptors) based on some criteria the details of which vary by `HandlerMapping` implementation. The most popular implementation supports annotated controllers but other implementations exists as well.
HandlerAdapter	Helps the `DispatcherServlet` to invoke a handler mapped to a request regardless of the handler is actually invoked. For example, invoking an annotated controller requires resolving various annotations. Thus the main purpose of a `HandlerAdapter` is to shield the `DispatcherServlet` from such details.
HandlerExceptionResolver	Maps exceptions to views also allowing for more complex exception handling code.
ViewResolver	Resolves logical String-based view names to actual `View` types.
LocaleResolver	Resolves the locale a client is using, in order to be able to offer internationalized views
ThemeResolver	Resolves themes your web application can use, for example, to offer personalized layouts
MultipartResolver	Parses multi-part requests for example to support processing file uploads from HTML forms.
FlashMapManager	Stores and retrieves the "input" and the "output" `FlashMap` that can be used to pass attributes from one request to another, usually across a redirect.

17.2.2 Default DispatcherServlet Configuration

As mentioned in the previous section for each special bean the DispatcherServlet maintains a list of implementations to use by default. This information is kept in the file DispatcherServlet.properties in the package org.springframework.web.servlet.

All special beans have some reasonable defaults of their own. Sooner or later though you'll need to customize one or more of the properties these beans provide. For example it's quite common to configure an InternalResourceViewResolver settings its prefix property to the parent location of view files.

Regardless of the details, the important concept to understand here is that once you configure a special bean such as an InternalResourceViewResolver in your WebApplicationContext, you effectively override the list of default implementations that would have been used otherwise for that special bean type. For example if you configure an InternalResourceViewResolver, the default list of ViewResolver implementations is ignored.

In Section 17.15, "Configuring Spring MVC" you'll learn about other options for configuring Spring MVC including MVC Java config and the MVC XML namespace both of which provide a simple starting point and assume little knowledge of how Spring MVC works. Regardless of how you choose to configure your application, the concepts explained in this section are fundamental should be of help to you.

17.2.3 DispatcherServlet Processing Sequence

After you set up a DispatcherServlet, and a request comes in for that specific DispatcherServlet, the DispatcherServlet starts processing the request as follows:

The WebApplicationContext is searched for and bound in the request as an attribute that the controller and other elements in the process can use. It is bound by default under the key DispatcherServlet.WEB_APPLICATION_CONTEXT_ATTRIBUTE.
The locale resolver is bound to the request to enable elements in the process to resolve the locale to use when processing the request (rendering the view, preparing data, and so on). If you do not need locale resolving, you do not need it.
The theme resolver is bound to the request to let elements such as views determine which theme to use. If you do not use themes, you can ignore it.
If you specify a multipart file resolver, the request is inspected for multiparts; if multiparts are found, the request is wrapped in a MultipartHttpServletRequest for further processing by other elements in the process. See Section 17.10, "Spring's multipart (file upload) support" for further information about multipart handling.
An appropriate handler is searched for. If a handler is found, the execution chain associated with the handler (preprocessors, postprocessors, and controllers) is executed in order to prepare a model or rendering.
If a model is returned, the view is rendered. If no model is returned, (may be due to a preprocessor or postprocessor intercepting the request, perhaps for security reasons), no view is rendered, because the request could already have been fulfilled.

Handler exception resolvers that are declared in the WebApplicationContext pick up exceptions that are thrown during processing of the request. Using these exception resolvers allows you to define custom behaviors to address exceptions.

The Spring DispatcherServlet also supports the return of the last-modification-date , as specified by the Servlet API. The process of determining the last modification date for a specific request is straightforward: the DispatcherServlet looks up an appropriate handler mapping and tests whether the handler that is found implements the LastModified interface. If so, the value of the long getLastModified(request) method of the LastModified interface is returned to the client.

You can customize individual DispatcherServlet instances by adding Servlet initialization parameters (init-param elements) to the Servlet declaration in the web.xml file. See the following table for the list of supported parameters.

Table 17.2.DispatcherServlet initialization parameters

Parameter	Explanation
`contextClass`	Class that implements `WebApplicationContext`, which instantiates the context used by this Servlet. By default, the `XmlWebApplicationContext` is used.
`contextConfigLocation`	String that is passed to the context instance (specified by `contextClass`) to indicate where context(s) can be found. The string consists potentially of multiple strings (using a comma as a delimiter) to support multiple contexts. In case of multiple context locations with beans that are defined twice, the latest location takes precedence.
`namespace`	Namespace of the `WebApplicationContext`. Defaults to `[servlet-name]-servlet`.

17.3 Implementing Controllers

Controllers provide access to the application behavior that you typically define through a service interface. Controllers interpret user input and transform it into a model that is represented to the user by the view. Spring implements a controller in a very abstract way, which enables you to create a wide variety of controllers.

Spring 2.5 introduced an annotation-based programming model for MVC controllers that uses annotations such as @RequestMapping, @RequestParam, @ModelAttribute, and so on. This annotation support is available for both Servlet MVC and Portlet MVC. Controllers implemented in this style do not have to extend specific base classes or implement specific interfaces. Furthermore, they do not usually have direct dependencies on Servlet or Portlet APIs, although you can easily configure access to Servlet or Portlet facilities.

	Tip
	Available in the samples repository, a number of web applications leverage the annotation support described in this section including MvcShowcase , MvcAjax , MvcBasic , PetClinic , PetCare , and others.

                          @Controller                        public            class            HelloWorldController {                          @RequestMapping("/helloWorld")                        public            String helloWorld(Model model) {         model.addAttribute("message",            "Hello World!");            return            "helloWorld";     } }

As you can see, the @Controller and @RequestMapping annotations allow flexible method names and signatures. In this particular example the method accepts a Model and returns a view name as a String, but various other method parameters and return values can be used as explained later in this section. @Controller and @RequestMapping and a number of other annotations form the basis for the Spring MVC implementation. This section documents these annotations and how they are most commonly used in a Servlet environment.

17.3.1 Defining a controller with `@Controller`

The @Controller annotation indicates that a particular class serves the role of a controller . Spring does not require you to extend any controller base class or reference the Servlet API. However, you can still reference Servlet-specific features if you need to.

The @Controller annotation acts as a stereotype for the annotated class, indicating its role. The dispatcher scans such annotated classes for mapped methods and detects @RequestMapping annotations (see the next section).

You can define annotated controller beans explicitly, using a standard Spring bean definition in the dispatcher's context. However, the @Controller stereotype also allows for autodetection, aligned with Spring general support for detecting component classes in the classpath and auto-registering bean definitions for them.

To enable autodetection of such annotated controllers, you add component scanning to your configuration. Use the spring-context schema as shown in the following XML snippet:

              <?xml version="1.0" encoding="UTF-8"?>              <beans              xmlns="http://www.springframework.org/schema/beans"              xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"              xmlns:p="http://www.springframework.org/schema/p"              xmlns:context="http://www.springframework.org/schema/context"              xsi:schemaLocation="         http://www.springframework.org/schema/beans         http://www.springframework.org/schema/beans/spring-beans.xsd         http://www.springframework.org/schema/context         http://www.springframework.org/schema/context/spring-context.xsd"              >              <context:component-scan              base-package="org.springframework.samples.petclinic.web"              />                              <!-- ... -->                            </beans>

17.3.2 Mapping Requests With `@RequestMapping`

You use the @RequestMapping annotation to map URLs such as /appointments onto an entire class or a particular handler method. Typically the class-level annotation maps a specific request path (or path pattern) onto a form controller, with additional method-level annotations narrowing the primary mapping for a specific HTTP method request method ("GET", "POST", etc.) or an HTTP request parameter condition.

The following example from the Petcare sample shows a controller in a Spring MVC application that uses this annotation:

                              @Controller                                            @RequestMapping("/appointments")                            public              class              AppointmentsController {              private              final              AppointmentBook appointmentBook;                              @Autowired                            public              AppointmentsController(AppointmentBook appointmentBook) {              this.appointmentBook = appointmentBook;     }                              @RequestMapping(method = RequestMethod.GET)                            public              Map<String, Appointment> get() {              return              appointmentBook.getAppointmentsForToday();     }                              @RequestMapping(value="/{day}", method = RequestMethod.GET)                            public              Map<String, Appointment> getForDay(                @PathVariable                                            @DateTimeFormat(iso=ISO.DATE)                            Date day, Model model) {              return              appointmentBook.getAppointmentsForDay(day);     }                              @RequestMapping(value="/new", method = RequestMethod.GET)                            public              AppointmentForm getNewForm() {              return              new              AppointmentForm();     }                              @RequestMapping(method = RequestMethod.POST)                            public              String add(                @Valid                            AppointmentForm appointment, BindingResult result) {              if              (result.hasErrors()) {              return              "appointments/new";         }         appointmentBook.addAppointment(appointment);              return              "redirect:/appointments";     } }

In the example, the @RequestMapping is used in a number of places. The first usage is on the type (class) level, which indicates that all handling methods on this controller are relative to the /appointments path. The get() method has a further @RequestMapping refinement: it only accepts GET requests, meaning that an HTTP GET for /appointments invokes this method. The post() has a similar refinement, and the getNewForm() combines the definition of HTTP method and path into one, so that GET requests for appointments/new are handled by that method.

The getForDay() method shows another usage of @RequestMapping: URI templates. (See the next section ).

A @RequestMapping on the class level is not required. Without it, all paths are simply absolute, and not relative. The following example from the PetClinic sample application shows a multi-action controller using @RequestMapping:

                              @Controller                            public              class              ClinicController {              private              final              Clinic clinic;                              @Autowired                            public              ClinicController(Clinic clinic) {              this.clinic = clinic;     }                              @RequestMapping("/")                            public              void              welcomeHandler() {     }                              @RequestMapping("/vets")                            public              ModelMap vetsHandler() {              return              new              ModelMap(this.clinic.getVets());     }  }

Using @RequestMapping On Interface Methods

	Using `@RequestMapping` On Interface Methods
A common pitfall when working with annotated controller classes happens when applying functionality that requires creating a proxy for the controller object (e.g. `@Transactional` methods). Usually you will introduce an interface for the controller in order to use JDK dynamic proxies. To make this work you must move the `@RequestMapping` annotations, as well as any other type and method-level annotations (e.g. `@ModelAttribute`, `@InitBinder`) to the interface as well as the mapping mechanism can only "see" the interface exposed by the proxy. Alternatively, you could activate `proxy-target-class="true"` in the configuration for the functionality applied to the controller (in our transaction scenario in `<tx:annotation-driven />`). Doing so indicates that CGLIB-based subclass proxies should be used instead of interface-based JDK proxies. For more information on various proxying mechanisms see Section 9.6, "Proxying mechanisms". Note however that method argument annotations, e.g. `@RequestParam`, must be present in the method signatures of the controller class.

A common pitfall when working with annotated controller classes happens when applying functionality that requires creating a proxy for the controller object (e.g. @Transactional methods). Usually you will introduce an interface for the controller in order to use JDK dynamic proxies. To make this work you must move the @RequestMapping annotations, as well as any other type and method-level annotations (e.g. @ModelAttribute, @InitBinder) to the interface as well as the mapping mechanism can only "see" the interface exposed by the proxy. Alternatively, you could activate proxy-target-class="true" in the configuration for the functionality applied to the controller (in our transaction scenario in <tx:annotation-driven />). Doing so indicates that CGLIB-based subclass proxies should be used instead of interface-based JDK proxies. For more information on various proxying mechanisms see Section 9.6, "Proxying mechanisms".

Note however that method argument annotations, e.g. @RequestParam, must be present in the method signatures of the controller class.

New Support Classes for `@RequestMapping` methods in Spring MVC 3.1

Spring 3.1 introduced a new set of support classes for @RequestMapping methods called RequestMappingHandlerMapping and RequestMappingHandlerAdapter respectively. They are recommended for use and even required to take advantage of new features in Spring MVC 3.1 and going forward. The new support classes are enabled by default by the MVC namespace and the MVC Java config but must be configured explicitly if using neither. This section describes a few important differences between the old and the new support classes.

Prior to Spring 3.1, type and method-level request mappings were examined in two separate stages -- a controller was selected first by the DefaultAnnotationHandlerMapping and the actual method to invoke was narrowed down second by the AnnotationMethodHandlerAdapter.

With the new support classes in Spring 3.1, the RequestMappingHandlerMapping is the only place where a decision is made about which method should process the request. Think of controller methods as a collection of unique endpoints with mappings for each method derived from type and method-level @RequestMapping information.

This enables some new possibilities. For once a HandlerInterceptor or a HandlerExceptionResolver can now expect the Object-based handler to be a HandlerMethod, which allows them to examine the exact method, its parameters and associated annotations. The processing for a URL no longer needs to be split across different controllers.

There are also several things no longer possible:

Select a controller first with a SimpleUrlHandlerMapping or BeanNameUrlHandlerMapping and then narrow the method based on @RequestMapping annotations.
Rely on method names as a fall-back mechanism to disambiguate between two @RequestMapping methods that don't have an explicit path mapping URL path but otherwise match equally, e.g. by HTTP method. In the new support classes @RequestMapping methods have to be mapped uniquely.
Have a single default method (without an explicit path mapping) with which requests are processed if no other controller method matches more concretely. In the new support classes if a matching method is not found a 404 error is raised.

The above features are still supported with the existing support classes. However to take advantage of new Spring MVC 3.1 features you'll need to use the new support classes.

URI Template Patterns

URI templates can be used for convenient access to selected parts of a URL in a @RequestMapping method.

A URI Template is a URI-like string, containing one or more variable names. When you substitute values for these variables, the template becomes a URI. The proposed RFC for URI Templates defines how a URI is parameterized. For example, the URI Template http://www.example.com/users/{userId} contains the variable userId . Assigning the value fred to the variable yields http://www.example.com/users/fred.

In Spring MVC you can use the @PathVariable annotation on a method argument to bind it to the value of a URI template variable:

                                  @RequestMapping(value="/owners/{ownerId}", method=RequestMethod.GET)                                public                String findOwner(                  @PathVariable                                String ownerId, Model model) {   Owner owner = ownerService.findOwner(ownerId);   model.addAttribute("owner", owner);                return                "displayOwner"; }

The URI Template "/owners/{ownerId}" specifies the variable name ownerId. When the controller handles this request, the value of ownerId is set to the value found in the appropriate part of the URI. For example, when a request comes in for /owners/fred, the value of ownerId is fred.

Tip

To process the @PathVariable annotation, Spring MVC needs to find the matching URI template variable by name. You can specify it in the annotation:

                                                      @RequestMapping(value="/owners/{ownerId}", method=RequestMethod.GET)                                                    public                          String findOwner(                            @PathVariable                          ("ownerId") String theOwner, Model model) {    }

Or if the URI template variable name matches the method argument name you can omit that detail. As long as your code is not compiled without debugging information, Spring MVC will match the method argument name to the URI template variable name:

                                                      @RequestMapping(value="/owners/{ownerId}", method=RequestMethod.GET)                                                    public                          String findOwner(                            @PathVariable                                                    String ownerId, Model model) {    }

A method can have any number of @PathVariable annotations:

                                  @RequestMapping(value="/owners/{ownerId}/pets/{petId}", method=RequestMethod.GET)                                public                String findPet(                  @PathVariable                                String ownerId,                                  @PathVariable                                String petId, Model model) {   Owner owner = ownerService.findOwner(ownerId);   Pet pet = owner.getPet(petId);   model.addAttribute("pet", pet);                return                "displayPet"; }

When a @PathVariable annotation is used on a Map<String, String> argument, the map is populated with all URI template variables.

A URI template can be assembled from type and path level @RequestMapping annotations. As a result the findPet() method can be invoked with a URL such as /owners/42/pets/21.

                                  @Controller                                @RequestMapping(                  "/owners/{ownerId}"                )                public                class                RelativePathUriTemplateController {    @RequestMapping(                  "/pets/{petId}"                )                public                void                findPet(                  @PathVariable                                String ownerId,                                  @PathVariable                                String petId, Model model) {        } }

A @PathVariable argument can be of any simple type such as int, long, Date, etc. Spring automatically converts to the appropriate type or throws a TypeMismatchException if it fails to do so. You can also register support for parsing additional data types. See the section called "Method Parameters And Type Conversion" and the section called "Customizing WebDataBinder initialization".

URI Template Patterns with Regular Expressions

Sometimes you need more precision in defining URI template variables. Consider the URL "/spring-web/spring-web-3.0.5.jar". How do you break it down into multiple parts?

The @RequestMapping annotation supports the use of regular expressions in URI template variables. The syntax is {varName:regex} where the first part defines the variable name and the second - the regular expression.For example:

                                  @RequestMapping("/spring-web/{symbolicName:[a-z-]+}-{version:\\d\\.\\d\\.\\d}{extension:\\.[a-z]+}")                                public                void                handle(                  @PathVariable                                String version,                                  @PathVariable                                String extension) {        } }

Path Patterns

In addition to URI templates, the @RequestMapping annotation also supports Ant-style path patterns (for example, /myPath/*.do). A combination of URI templates and Ant-style globs is also supported (for example, /owners/*/pets/{petId}).

Patterns with Placeholders

Patterns in @RequestMapping annotations support ${...} placeholders against local properties and/or system properties and environment variables. This may be useful in cases where the path a controller is mapped to may need to be customized through configuration. For more information on placeholders see the Javadoc for PropertyPlaceholderConfigurer.

Matrix Variables

The URI specification RFC 3986 defines the possibility of including name-value pairs within path segments. There is no specific term used in the spec. The general "URI path parameters" could be applied although the more unique "Matrix URIs", originating from an old post by Tim Berners-Lee, is also frequently used and fairly well known. Within Spring MVC these are referred to as matrix variables.

Matrix variables can appear in any path segment, each matrix variable separated with a ";" (semicolon). For example: "/cars;color=red;year=2012". Multiple values may be either "," (comma) separated "color=red,green,blue" or the variable name may be repeated "color=red;color=green;color=blue".

If a URL is expected to contain matrix variables, the request mapping pattern must represent them with a URI template. This ensures the request can be matched correctly regardless of whether matrix variables are present or not and in what order they are provided.

Below is an example of extracting the matrix variable "q":

                                    @RequestMapping(value = "/pets/{petId}", method = RequestMethod.GET)                                public                void                findPet(                  @PathVariable                                String petId,                                  @MatrixVariable                                int                q) {         }

Since all path segments may contain matrix variables, in some cases you need to be more specific to identify where the variable is expected to be:

                                    @RequestMapping(value = "/owners/{ownerId}/pets/{petId}", method = RequestMethod.GET)                                public                void                findPet(                                  @MatrixVariable(value="q", pathVar="ownerId")                                int                q1,                                  @MatrixVariable(value="q", pathVar="petId")                                int                q2) {         }

A matrix variable may be defined as optional and a default value specified:

                                    @RequestMapping(value = "/pets/{petId}", method = RequestMethod.GET)                                public                void                findPet(                  @MatrixVariable(required=true, defaultValue="1")                                int                q) {          }

All matrix variables may be obtained in a Map:

                                    @RequestMapping(value = "/owners/{ownerId}/pets/{petId}", method = RequestMethod.GET)                                public                void                findPet(                                  @MatrixVariable                                Map<String, String> matrixVars,                                  @MatrixVariable(pathVar="petId"")                                Map<String, String> petMatrixVars) {               }

Note that to enable the use of matrix variables, you must set the removeSemicolonContent property of RequestMappingHandlerMapping to false. By default it is set to false.

In the MVC namespace, the `mvc:annotation-driven` element has an `enableMatrixVariables` attribute that should be set to `true`. By default it is set to `false`.

Consumable Media Types

You can narrow the primary mapping by specifying a list of consumable media types. The request will be matched only if the Content-Type request header matches the specified media type. For example:

                                  @Controller                                @RequestMapping(value =                "/pets", method = RequestMethod.POST,                                  consumes="application/json"                )                public                void                addPet(                  @RequestBody                                Pet pet, Model model) {      }

Consumable media type expressions can also be negated as in !text/plain to match to all requests other than those with Content-Type of text/plain .

	Tip
	The consumes condition is supported on the type and on the method level. Unlike most other conditions, when used at the type level, method-level consumable types override rather than extend type-level consumable types.

Producible Media Types

You can narrow the primary mapping by specifying a list of producible media types. The request will be matched only if the Accept request header matches one of these values. Furthermore, use of the produces condition ensures the actual content type used to generate the response respects the media types specified in the produces condition. For example:

                                  @Controller                                @RequestMapping(value =                "/pets/{petId}", method = RequestMethod.GET,                                  produces="application/json"                )                                  @ResponseBody                                public                Pet getPet(                  @PathVariable                                String petId, Model model) {      }

Just like with consumes , producible media type expressions can be negated as in !text/plain to match to all requests other than those with an Accept header value of text/plain .

	Tip
	The produces condition is supported on the type and on the method level. Unlike most other conditions, when used at the type level, method-level producible types override rather than extend type-level producible types.

Request Parameters and Header Values

You can narrow request matching through request parameter conditions such as "myParam", "!myParam", or "myParam=myValue". The first two test for request parameter presence/absence and the third for a specific parameter value. Here is an example with a request parameter value condition:

                                  @Controller                                                  @RequestMapping("/owners/{ownerId}")                                public                class                RelativePathUriTemplateController {    @RequestMapping(value =                "/pets/{petId}", method = RequestMethod.GET,                                  params="myParam=myValue"                )                public                void                findPet(                  @PathVariable                                String ownerId,                                  @PathVariable                                String petId, Model model) {        } }

The same can be done to test for request header presence/absence or to match based on a specific request header value:

                                  @Controller                                                  @RequestMapping("/owners/{ownerId}")                                public                class                RelativePathUriTemplateController {  @RequestMapping(value =                "/pets", method = RequestMethod.GET,                                  headers="myHeader=myValue"                )                public                void                findPet(                  @PathVariable                                String ownerId,                                  @PathVariable                                String petId, Model model) {        } }

	Tip
	Although you can match to Content-Type and Accept header values using media type wild cards (for example "content-type=text/"* will match to "text/plain" and "text/html" ), it is recommended to use the consumes and produces conditions respectively instead. They are intended specifically for that purpose.

17.3.3 Defining `@RequestMapping` handler methods

An @RequestMapping handler method can have a very flexible signatures. The supported method arguments and return values are described in the following section. Most arguments can be used in arbitrary order with the only exception of BindingResult arguments. This is described in the next section.

Note

Spring 3.1 introduced a new set of support classes for @RequestMapping methods called RequestMappingHandlerMapping and RequestMappingHandlerAdapter respectively. They are recommended for use and even required to take advantage of new features in Spring MVC 3.1 and going forward. The new support classes are enabled by default from the MVC namespace and with use of the MVC Java config but must be configured explicitly if using neither.

Supported method argument types

The following are the supported method arguments:

Request or response objects (Servlet API). Choose any specific request or response type, for example ServletRequest or HttpServletRequest.

Session object (Servlet API): of type HttpSession. An argument of this type enforces the presence of a corresponding session. As a consequence, such an argument is never null.

	Note
	Session access may not be thread-safe, in particular in a Servlet environment. Consider setting the `RequestMappingHandlerAdapter`'s "synchronizeOnSession" flag to "true" if multiple requests are allowed to access a session concurrently.

org.springframework.web.context.request.WebRequest or org.springframework.web.context.request.NativeWebRequest. Allows for generic request parameter access as well as request/session attribute access, without ties to the native Servlet/Portlet API.
java.util.Locale for the current request locale, determined by the most specific locale resolver available, in effect, the configured LocaleResolver in a Servlet environment.
java.io.InputStream / java.io.Reader for access to the request's content. This value is the raw InputStream/Reader as exposed by the Servlet API.
java.io.OutputStream / java.io.Writer for generating the response's content. This value is the raw OutputStream/Writer as exposed by the Servlet API.
java.security.Principal containing the currently authenticated user.
@PathVariable annotated parameters for access to URI template variables. See the section called "URI Template Patterns".
@MatrixVariable annotated parameters for access to name-value pairs located in URI path segments. See the section called "Matrix Variables".
@RequestParam annotated parameters for access to specific Servlet request parameters. Parameter values are converted to the declared method argument type. See the section called "Binding request parameters to method parameters with @RequestParam".
@RequestHeader annotated parameters for access to specific Servlet request HTTP headers. Parameter values are converted to the declared method argument type.
@RequestBody annotated parameters for access to the HTTP request body. Parameter values are converted to the declared method argument type using HttpMessageConverters. See the section called "Mapping the request body with the @RequestBody annotation".
@RequestPart annotated parameters for access to the content of a "multipart/form-data" request part. See Section 17.10.5, "Handling a file upload request from programmatic clients" and Section 17.10, "Spring's multipart (file upload) support".
HttpEntity<?> parameters for access to the Servlet request HTTP headers and contents. The request stream will be converted to the entity body using HttpMessageConverters. See the section called "Using HttpEntity<?>".
java.util.Map / org.springframework.ui.Model / org.springframework.ui.ModelMap for enriching the implicit model that is exposed to the web view.
org.springframework.web.servlet.mvc.support.RedirectAttributes to specify the exact set of attributes to use in case of a redirect and also to add flash attributes (attributes stored temporarily on the server-side to make them available to the request after the redirect). RedirectAttributes is used instead of the implicit model if the method returns a "redirect:" prefixed view name or RedirectView.
Command or form objects to bind request parameters to bean properties (via setters) or directly to fields, with customizable type conversion, depending on @InitBinder methods and/or the HandlerAdapter configuration. See the webBindingInitializer property on RequestMappingHandlerAdapter. Such command objects along with their validation results will be exposed as model attributes by default, using the command class class name - e.g. model attribute "orderAddress" for a command object of type "some.package.OrderAddress". The ModelAttribute annotation can be used on a method argument to customize the model attribute name used.
org.springframework.validation.Errors / org.springframework.validation.BindingResult validation results for a preceding command or form object (the immediately preceding method argument).
org.springframework.web.bind.support.SessionStatus status handle for marking form processing as complete, which triggers the cleanup of session attributes that have been indicated by the @SessionAttributes annotation at the handler type level.
org.springframework.web.util.UriComponentsBuilder a builder for preparing a URL relative to the current request's host, port, scheme, context path, and the literal part of the servlet mapping.

The Errors or BindingResult parameters have to follow the model object that is being bound immediately as the method signature might have more that one model object and Spring will create a separate BindingResult instance for each of them so the following sample won't work:

Example 17.1. Invalid ordering of BindingResult and @ModelAttribute

                                          @RequestMapping(method = RequestMethod.POST)                                        public                    String processSubmit(                      @ModelAttribute("pet") Pet pet                    ,     Model model,                                          BindingResult result                    ) { … }

Note, that there is a Model parameter in between Pet and BindingResult. To get this working you have to reorder the parameters as follows:

                                          @RequestMapping(method = RequestMethod.POST)                                        public                    String processSubmit(                      @ModelAttribute("pet") Pet pet                    ,                                          BindingResult result                    , Model model) { … }

Supported method return types

The following are the supported return types:

A ModelAndView object, with the model implicitly enriched with command objects and the results of @ModelAttribute annotated reference data accessor methods.
A Model object, with the view name implicitly determined through a RequestToViewNameTranslator and the model implicitly enriched with command objects and the results of @ModelAttribute annotated reference data accessor methods.
A Map object for exposing a model, with the view name implicitly determined through a RequestToViewNameTranslator and the model implicitly enriched with command objects and the results of @ModelAttribute annotated reference data accessor methods.
A View object, with the model implicitly determined through command objects and @ModelAttribute annotated reference data accessor methods. The handler method may also programmatically enrich the model by declaring a Model argument (see above).
A String value that is interpreted as the logical view name, with the model implicitly determined through command objects and @ModelAttribute annotated reference data accessor methods. The handler method may also programmatically enrich the model by declaring a Model argument (see above).
void if the method handles the response itself (by writing the response content directly, declaring an argument of type ServletResponse / HttpServletResponse for that purpose) or if the view name is supposed to be implicitly determined through a RequestToViewNameTranslator (not declaring a response argument in the handler method signature).
If the method is annotated with @ResponseBody, the return type is written to the response HTTP body. The return value will be converted to the declared method argument type using HttpMessageConverters. See the section called "Mapping the response body with the @ResponseBody annotation".
A HttpEntity<?> or ResponseEntity<?> object to provide access to the Servlet response HTTP headers and contents. The entity body will be converted to the response stream using HttpMessageConverters. See the section called "Using HttpEntity<?>".
A Callable<?> can be returned when the application wants to produce the return value asynchronously in a thread managed by Spring MVC.
A DeferredResult<?> can be returned when the application wants to produce the return value from a thread of its own choosing.
Any other return type is considered to be a single model attribute to be exposed to the view, using the attribute name specified through @ModelAttribute at the method level (or the default attribute name based on the return type class name). The model is implicitly enriched with command objects and the results of @ModelAttribute annotated reference data accessor methods.

Binding request parameters to method parameters with `@RequestParam`

Use the @RequestParam annotation to bind request parameters to a method parameter in your controller.

The following code snippet shows the usage:

                                  @Controller                                                  @RequestMapping("/pets")                                                  @SessionAttributes("pet")                                public                class                EditPetForm {                                  // ...                                                  @RequestMapping(method = RequestMethod.GET)                                public                String setupForm(                  @RequestParam("petId") int petId                , ModelMap model) {         Pet pet =                this.clinic.loadPet(petId);         model.addAttribute("pet", pet);                return                "petForm";     }                                  // ...

Parameters using this annotation are required by default, but you can specify that a parameter is optional by setting @RequestParam's required attribute to false (e.g., @RequestParam(value="id", required=false)).

Type conversion is applied automatically if the target method parameter type is not String. See the section called "Method Parameters And Type Conversion".

Mapping the request body with the @RequestBody annotation

The @RequestBody method parameter annotation indicates that a method parameter should be bound to the value of the HTTP request body. For example:

                                  @RequestMapping(value = "/something", method = RequestMethod.PUT)                                public                void                handle(                  @RequestBody                                String body, Writer writer)                throws                IOException {   writer.write(body); }

You convert the request body to the method argument by using an HttpMessageConverter. HttpMessageConverter is responsible for converting from the HTTP request message to an object and converting from an object to the HTTP response body. The RequestMappingHandlerAdapter supports the @RequestBody annotation with the following default HttpMessageConverters:

ByteArrayHttpMessageConverter converts byte arrays.
StringHttpMessageConverter converts strings.
FormHttpMessageConverter converts form data to/from a MultiValueMap<String, String>.
SourceHttpMessageConverter converts to/from a javax.xml.transform.Source.

For more information on these converters, see Message Converters. Also note that if using the MVC namespace or the MVC Java config, a wider range of message converters are registered by default. See Enabling the MVC Java Config or the MVC XML Namespace for more information.

If you intend to read and write XML, you will need to configure the MarshallingHttpMessageConverter with a specific Marshaller and an Unmarshaller implementation from the org.springframework.oxm package. The example below shows how to do that directly in your configuration but if your application is configured through the MVC namespace or the MVC Java config see Enabling the MVC Java Config or the MVC XML Namespace instead.

                <bean                class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter"                >                <property                name="messageConverters"                >                <util:list                id="beanList"                >                <ref                bean="stringHttpMessageConverter"                />                <ref                bean="marshallingHttpMessageConverter"                />                </util:list>                </property </bean>                <bean                id="stringHttpMessageConverter"                class="org.springframework.http.converter.StringHttpMessageConverter"                />                <bean                id="marshallingHttpMessageConverter"                class="org.springframework.http.converter.xml.MarshallingHttpMessageConverter"                >                <property                name="marshaller"                ref="castorMarshaller"                                  />                <property                name="unmarshaller"                ref="castorMarshaller"                                  />                </bean>                <bean                id="castorMarshaller"                class="org.springframework.oxm.castor.CastorMarshaller"                />

An @RequestBody method parameter can be annotated with @Valid, in which case it will be validated using the configured Validator instance. When using the MVC namespace or the MVC Java config, a JSR-303 validator is configured automatically assuming a JSR-303 implementation is available on the classpath.

Just like with @ModelAttribute parameters, an Errors argument can be used to examine the errors. If such an argument is not declared, a MethodArgumentNotValidException will be raised. The exception is handled in the DefaultHandlerExceptionResolver, which sends a 400 error back to the client.

Mapping the response body with the `@ResponseBody` annotation

The @ResponseBody annotation is similar to @RequestBody. This annotation can be put on a method and indicates that the return type should be written straight to the HTTP response body (and not placed in a Model, or interpreted as a view name). For example:

                                  @RequestMapping(value = "/something", method = RequestMethod.PUT)                                                  @ResponseBody                                public                String helloWorld()  {                return                "Hello World"; }

The above example will result in the text Hello World being written to the HTTP response stream.

As with @RequestBody, Spring converts the returned object to a response body by using an HttpMessageConverter. For more information on these converters, see the previous section and Message Converters.

Using `HttpEntity<?>`

The HttpEntity is similar to @RequestBody and @ResponseBody. Besides getting access to the request and response body, HttpEntity (and the response-specific subclass ResponseEntity) also allows access to the request and response headers, like so:

                                  @RequestMapping("/something")                                public                ResponseEntity<String> handle(HttpEntity<byte[]> requestEntity)                throws                UnsupportedEncodingException {   String requestHeader = requestEntity.getHeaders().getFirst("MyRequestHeader"));                byte[] requestBody = requestEntity.getBody();       HttpHeaders responseHeaders =                new                HttpHeaders();   responseHeaders.set("MyResponseHeader",                "MyValue");                return                new                ResponseEntity<String>("Hello World", responseHeaders, HttpStatus.CREATED); }

The above example gets the value of the MyRequestHeader request header, and reads the body as a byte array. It adds the MyResponseHeader to the response, writes Hello World to the response stream, and sets the response status code to 201 (Created).

As with @RequestBody and @ResponseBody, Spring uses HttpMessageConverter to convert from and to the request and response streams. For more information on these converters, see the previous section and Message Converters.

Using `@ModelAttribute` on a method

The @ModelAttribute annotation can be used on methods or on method arguments. This section explains its usage on methods while the next section explains its usage on method arguments.

An @ModelAttribute on a method indicates the purpose of that method is to add one or more model attributes. Such methods support the same argument types as @RequestMapping methods but cannot be mapped directly to requests. Instead @ModelAttribute methods in a controller are invoked before @RequestMapping methods, within the same controller. A couple of examples:

                                                     @ModelAttribute                                public                Account addAccount(                  @RequestParam                                String number) {                return                accountManager.findAccount(number); }                                    @ModelAttribute                                public                void                populateModel(                  @RequestParam                                String number, Model model) {     model.addAttribute(accountManager.findAccount(number));      }

@ModelAttribute methods are used to populate the model with commonly needed attributes for example to fill a drop-down with states or with pet types, or to retrieve a command object like Account in order to use it to represent the data on an HTML form. The latter case is further discussed in the next section.

Note the two styles of @ModelAttribute methods. In the first, the method adds an attribute implicitly by returning it. In the second, the method accepts a Model and adds any number of model attributes to it. You can choose between the two styles depending on your needs.

A controller can have any number of @ModelAttribute methods. All such methods are invoked before @RequestMapping methods of the same controller.

@ModelAttribute methods can also be defined in an @ControllerAdvice-annotated class and such methods apply to all controllers. The @ControllerAdvice annotation is a component annotation allowing implementation classes to be autodetected through classpath scanning.

Tip

What happens when a model attribute name is not explicitly specified? In such cases a default name is assigned to the model attribute based on its type. For example if the method returns an object of type Account, the default name used is "account". You can change that through the value of the @ModelAttribute annotation. If adding attributes directly to the Model, use the appropriate overloaded addAttribute(..) method - i.e., with or without an attribute name.

The @ModelAttribute annotation can be used on @RequestMapping methods as well. In that case the return value of the @RequestMapping method is interpreted as a model attribute rather than as a view name. The view name is derived from view name conventions instead much like for methods returning void — see Section 17.12.3, "The View - RequestToViewNameTranslator".

Using `@ModelAttribute` on a method argument

As explained in the previous section @ModelAttribute can be used on methods or on method arguments. This section explains its usage on method arguments.

An @ModelAttribute on a method argument indicates the argument should be retrieved from the model. If not present in the model, the argument should be instantiated first and then added to the model. Once present in the model, the argument's fields should be populated from all request parameters that have matching names. This is known as data binding in Spring MVC, a very useful mechanism that saves you from having to parse each form field individually.

                                  @RequestMapping(value="/owners/{ownerId}/pets/{petId}/edit", method = RequestMethod.POST)                                public                String processSubmit(                  @ModelAttribute Pet pet                ) {  }

Given the above example where can the Pet instance come from? There are several options:

It may already be in the model due to use of @SessionAttributes — see the section called "Using @SessionAttributes to store model attributes in the HTTP session between requests".
It may already be in the model due to an @ModelAttribute method in the same controller — as explained in the previous section.
It may be retrieved based on a URI template variable and type converter (explained in more detail below).
It may be instantiated using its default constructor.

An @ModelAttribute method is a common way to to retrieve an attribute from the database, which may optionally be stored between requests through the use of @SessionAttributes. In some cases it may be convenient to retrieve the attribute by using an URI template variable and a type converter. Here is an example:

                                  @RequestMapping(value="/accounts/{account}", method = RequestMethod.PUT)                                public                String save(                  @ModelAttribute("account")                                Account account) {  }

In this example the name of the model attribute (i.e. "account") matches the name of a URI template variable. If you register Converter<String, Account> that can turn the String account value into an Account instance, then the above example will work without the need for an @ModelAttribute method.

The next step is data binding. The WebDataBinder class matches request parameter names — including query string parameters and form fields — to model attribute fields by name. Matching fields are populated after type conversion (from String to the target field type) has been applied where necessary. Data binding and validation are covered in Chapter 7, Validation, Data Binding, and Type Conversion. Customizing the data binding process for a controller level is covered in the section called "Customizing WebDataBinder initialization".

As a result of data binding there may be errors such as missing required fields or type conversion errors. To check for such errors add a BindingResult argument immediately following the @ModelAttribute argument:

                                  @RequestMapping(value="/owners/{ownerId}/pets/{petId}/edit", method = RequestMethod.POST)                                public                String processSubmit(                  @ModelAttribute("pet") Pet pet                , BindingResult result) {                if                (result.hasErrors()) {                return                "petForm";     }        }

With a BindingResult you can check if errors were found in which case it's common to render the same form where the errors can be shown with the help of Spring's <errors> form tag.

In addition to data binding you can also invoke validation using your own custom validator passing the same BindingResult that was used to record data binding errors. That allows for data binding and validation errors to be accumulated in one place and subsequently reported back to the user:

                                  @RequestMapping(value="/owners/{ownerId}/pets/{petId}/edit", method = RequestMethod.POST)                                public                String processSubmit(                  @ModelAttribute("pet") Pet pet                , BindingResult result) {                new                PetValidator().validate(pet, result);                if                (result.hasErrors()) {                return                "petForm";     }       }

Or you can have validation invoked automatically by adding the JSR-303 @Valid annotation:

                                  @RequestMapping(value="/owners/{ownerId}/pets/{petId}/edit", method = RequestMethod.POST)                                public                String processSubmit(                  @Valid @ModelAttribute("pet") Pet pet                , BindingResult result) {                if                (result.hasErrors()) {                return                "petForm";     }       }

See Section 7.8, "Spring 3 Validation" and Chapter 7, Validation, Data Binding, and Type Conversion for details on how to configure and use validation.

Using `@SessionAttributes` to store model attributes in the HTTP session between requests

The type-level @SessionAttributes annotation declares session attributes used by a specific handler. This will typically list the names of model attributes or types of model attributes which should be transparently stored in the session or some conversational storage, serving as form-backing beans between subsequent requests.

The following code snippet shows the usage of this annotation, specifying the model attribute name:

                                  @Controller                                                  @RequestMapping("/editPet.do")                                                  @SessionAttributes("pet")                                public                class                EditPetForm {                                  // ...                                }

	Note
	When using controller interfaces (e.g., for AOP proxying), make sure to consistently put all your mapping annotations - such as `@RequestMapping` and `@SessionAttributes` - on the controller interface rather than on the implementation class.

Specifying redirect and flash attributes

By default all model attributes are considered to be exposed as URI template variables in the redirect URL. Of the remaining attributes those that are primitive types or collections/arrays of primitive types are automatically appended as query parameters.

In annotated controllers however the model may contain additional attributes originally added for rendering purposes (e.g. drop-down field values). To gain precise control over the attributes used in a redirect scenario, an @RequestMapping method can declare an argument of type RedirectAttributes and use it to add attributes for use in RedirectView. If the controller method does redirect, the content of RedirectAttributes is used. Otherwise the content of the default Model is used.

The RequestMappingHandlerAdapter provides a flag called "ignoreDefaultModelOnRedirect" that can be used to indicate the content of the default Model should never be used if a controller method redirects. Instead the controller method should declare an attribute of type RedirectAttributes or if it doesn't do so no attributes should be passed on to RedirectView. Both the MVC namespace and the MVC Java config keep this flag set to false in order to maintain backwards compatibility. However, for new applications we recommend setting it to true

The RedirectAttributes interface can also be used to add flash attributes. Unlike other redirect attributes, which end up in the target redirect URL, flash attributes are saved in the HTTP session (and hence do not appear in the URL). The model of the controller serving the target redirect URL automatically receives these flash attributes after which they are removed from the session. See Section 17.6, "Using flash attributes" for an overview of the general support for flash attributes in Spring MVC.

Working with `"application/x-www-form-urlencoded"` data

The previous sections covered use of @ModelAttribute to support form submission requests from browser clients. The same annotation is recommended for use with requests from non-browser clients as well. However there is one notable difference when it comes to working with HTTP PUT requests. Browsers can submit form data via HTTP GET or HTTP POST. Non-browser clients can also submit forms via HTTP PUT. This presents a challenge because the Servlet specification requires the ServletRequest.getParameter*() family of methods to support form field access only for HTTP POST, not for HTTP PUT.

To support HTTP PUT and PATCH requests, the spring-web module provides the filter HttpPutFormContentFilter, which can be configured in web.xml:

                <filter>                <filter-name>httpPutFormFilter</filter-name>                <filter-class>org.springframework.web.filter.HttpPutFormContentFilter</filter-class>                </filter>                <filter-mapping>                <filter-name>httpPutFormFilter</filter-name>                <servlet-name>dispatcherServlet</servlet-name>                </filter-mapping>                <servlet>                <servlet-name>dispatcherServlet</servlet-name>                <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>                </servlet>

The above filter intercepts HTTP PUT and PATCH requests with content type application/x-www-form-urlencoded, reads the form data from the body of the request, and wraps the ServletRequest in order to make the form data available through the ServletRequest.getParameter*() family of methods.

	Note
	As `HttpPutFormContentFilter` consumes the body of the request, it should not be configured for PUT or PATCH URLs that rely on other converters for `application/x-www-form-urlencoded`. This includes `@RequestBody MultiValueMap<String, String>` and `HttpEntity<MultiValueMap<String, String>>`.

Mapping cookie values with the @CookieValue annotation

The @CookieValue annotation allows a method parameter to be bound to the value of an HTTP cookie.

Let us consider that the following cookie has been received with an http request:

JSESSIONID=415A4AC178C59DACE0B2C9CA727CDD84

The following code sample demonstrates how to get the value of the JSESSIONID cookie:

                                  @RequestMapping("/displayHeaderInfo.do")                                public                void                displayHeaderInfo(                  @CookieValue("JSESSIONID")                                String cookie)  {      }

Type conversion is applied automatically if the target method parameter type is not String. See the section called "Method Parameters And Type Conversion".

This annotation is supported for annotated handler methods in Servlet and Portlet environments.

Mapping request header attributes with the @RequestHeader annotation

The @RequestHeader annotation allows a method parameter to be bound to a request header.

Here is a sample request header:

Host                    localhost:8080 Accept                  text/html,application/xhtml+xml,application/xml;q=0.9 Accept-Language         fr,en-gb;q=0.7,en;q=0.3 Accept-Encoding         gzip,deflate Accept-Charset          ISO-8859-1,utf-8;q=0.7,*;q=0.7 Keep-Alive              300

The following code sample demonstrates how to get the value of the Accept-Encoding and Keep-Alive headers:

                                  @RequestMapping("/displayHeaderInfo.do")                                public                void                displayHeaderInfo(                  @RequestHeader("Accept-Encoding")                                String encoding,                                  @RequestHeader("Keep-Alive")                                long                keepAlive)  {      }

Type conversion is applied automatically if the method parameter is not String. See the section called "Method Parameters And Type Conversion".

	Tip
	Built-in support is available for converting a comma-separated string into an array/collection of strings or other types known to the type conversion system. For example a method parameter annotated with `@RequestHeader("Accept")` may be of type `String` but also `String[]` or `List<String>`.

This annotation is supported for annotated handler methods in Servlet and Portlet environments.

Method Parameters And Type Conversion

String-based values extracted from the request including request parameters, path variables, request headers, and cookie values may need to be converted to the target type of the method parameter or field (e.g., binding a request parameter to a field in an @ModelAttribute parameter) they're bound to. If the target type is not String, Spring automatically converts to the appropriate type. All simple types such as int, long, Date, etc. are supported. You can further customize the conversion process through a WebDataBinder (see the section called "Customizing WebDataBinder initialization") or by registering Formatters with the FormattingConversionService (see Section 7.6, "Spring 3 Field Formatting").

Customizing `WebDataBinder` initialization

To customize request parameter binding with PropertyEditors through Spring's WebDataBinder, you can use @InitBinder-annotated methods within your controller, @InitBinder methods within an @ControllerAdvice class, or provide a custom WebBindingInitializer.

Customizing data binding with `@InitBinder`

Annotating controller methods with @InitBinder allows you to configure web data binding directly within your controller class. @InitBinder identifies methods that initialize the WebDataBinder that will be used to populate command and form object arguments of annotated handler methods.

Such init-binder methods support all arguments that @RequestMapping supports, except for command/form objects and corresponding validation result objects. Init-binder methods must not have a return value. Thus, they are usually declared as void. Typical arguments include WebDataBinder in combination with WebRequest or java.util.Locale, allowing code to register context-specific editors.

The following example demonstrates the use of @InitBinder to configure a CustomDateEditor for all java.util.Date form properties.

                                      @Controller                                    public                  class                  MyFormController {                                      @InitBinder                                    public                  void                  initBinder(WebDataBinder binder) {         SimpleDateFormat dateFormat =                  new                  SimpleDateFormat("yyyy-MM-dd");         dateFormat.setLenient(false);         binder.registerCustomEditor(Date.class,                  new                  CustomDateEditor(dateFormat, false));     }                                      // ...                                    }

Configuring a custom `WebBindingInitializer`

To externalize data binding initialization, you can provide a custom implementation of the WebBindingInitializer interface, which you then enable by supplying a custom bean configuration for an AnnotationMethodHandlerAdapter, thus overriding the default configuration.

The following example from the PetClinic application shows a configuration using a custom implementation of the WebBindingInitializer interface, org.springframework.samples.petclinic.web.ClinicBindingInitializer, which configures PropertyEditors required by several of the PetClinic controllers.

                  <bean                  class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter"                  >                  <property                  name="cacheSeconds"                  value="0"                                      />                  <property                  name="webBindingInitializer"                  >                  <bean                  class="org.springframework.samples.petclinic.web.ClinicBindingInitializer"                                      />                  </property>                  </bean>

Customizing data binding with externalized `@InitBinder` methods

@InitBinder methods can also be defined in an @ControllerAdvice-annotated class in which case they apply to all controllers. This provides an alternative to using a WebBindingInitializer.

The @ControllerAdvice annotation is a component annotation allowing implementation classes to be autodetected through classpath scanning.

Support for the 'Last-Modified' Response Header To Facilitate Content Caching

An @RequestMapping method may wish to support 'Last-Modified' HTTP requests, as defined in the contract for the Servlet API's getLastModified method, to facilitate content caching. This involves calculating a lastModified long value for a given request, comparing it against the 'If-Modified-Since' request header value, and potentially returning a response with status code 304 (Not Modified). An annotated controller method can achieve that as follows:

                                  @RequestMapping                                public                String myHandleMethod(WebRequest webRequest, Model model) {                long                lastModified =                 if                (request.checkNotModified(lastModified)) {                         return                null;      }           model.addAttribute(...);                return                "myViewName"; }

There are two key elements to note: calling request.checkNotModified(lastModified) and returning null. The former sets the response status to 304 before it returns true. The latter, in combination with the former, causes Spring MVC to do no further processing of the request.

17.3.4 Asynchronous Request Processing

Spring MVC 3.2 introduced Servlet 3 based asynchronous request processing. Instead of returning a value, as usual, a controller method can now return a java.util.concurrent.Callable and produce the return value from a separate thread. Meanwhile the main Servlet container thread is released and allowed to process other requests. Spring MVC invokes the Callable in a separate thread with the help of a TaskExecutor and when the Callable returns, the request is dispatched back to the Servlet container to resume processing with the value returned by the Callable. Here is an example controller method:

                              @RequestMapping(method=RequestMethod.POST)                            public              Callable<String> processUpload(final              MultipartFile file) {              return              new              Callable<String>() {              public              Object call()              throws              Exception {                     return              "someView";     }   }; }

A second option is for the controller to return an instance of DeferredResult. In this case the return value will also be produced from a separate thread. However, that thread is not known to Spring MVC. For example the result may be produced in response to some external event such as a JMS message, a scheduled task, etc. Here is an example controller method:

                              @RequestMapping("/quotes")                                            @ResponseBody                            public              DeferredResult<String> quotes() {   DeferredResult<String> deferredResult =              new              DeferredResult<String>();                 return              deferredResult; }   deferredResult.setResult(data);

This may be difficult to understand without any knowledge of the Servlet 3 async processing feature. It would certainly help to read up on it. At a very minimum consider the following basic facts:

A ServletRequest can be put in asynchronous mode by calling request.startAsync(). The main effect of doing so is that the Servlet, as well as any Filters, can exit but the response will remain open allowing some other thread to complete processing.
The call to request.startAsync() returns an AsyncContext, which can be used for further control over async processing. For example it provides the method dispatch, which can be called from an application thread in order to "dispatch" the request back to the Servlet container. An async dispatch is similar to a forward except it is made from one (application) thread to another (Servlet container) thread whereas a forward occurs synchronously in the same (Servlet container) thread.
ServletRequest provides access to the current DispatcherType, which can be used to distinguish if a Servlet or a Filter is processing on the initial request processing thread and when it is processing in an async dispatch.

With the above in mind, the following is the sequence of events for async request processing with a Callable: (1) Controller returns a Callable, (2) Spring MVC starts async processing and submits the Callable to a TaskExecutor for processing in a separate thread, (3) the DispatcherServlet and all Filter's exit the request processing thread but the response remains open, (4) the Callable produces a result and Spring MVC dispatches the request back to the Servlet container, (5) the DispatcherServlet is invoked again and processing resumes with the asynchronously produced result from the Callable. The exact sequencing of (2), (3), and (4) may vary depending on the speed of execution of the concurrent threads.

The sequence of events for async request processing with a DeferredResult is the same in principal except it's up to the application to produce the asynchronous result from some thread: (1) Controller returns a DeferredResult and saves it in some in-memory queue or list where it can be accessed, (2) Spring MVC starts async processing, (3) the DispatcherServlet and all configured Filter's exit the request processing thread but the response remains open, (4) the application sets the DeferredResult from some thread and Spring MVC dispatches the request back to the Servlet container, (5) the DispatcherServlet is invoked again and processing resumes with the asynchronously produced result.

Explaining the motivation for async request processing and when or why to use it are beyond the scope of this document. For further information you may wish to read this blog post series.

Exception Handling for Async Requests

What happens if a Callable returned from a controller method raises an Exception while being executed? The effect is similar to what happens when any controller method raises an exception. It is handled by a matching @ExceptionHandler method in the same controller or by one of the configured HandlerExceptionResolver instances.

	Note
	Under the covers, when a `Callable` raises an Exception, Spring MVC still dispatches to the Servlet container to resume processing. The only difference is that the result of executing the `Callable` is an `Exception` that must be processed with the configured `HandlerExceptionResolver` instances.

When using a DeferredResult, you have a choice of calling its setErrorResult(Object) method and provide an Exception or any other Object you'd like to use as the result. If the result is an Exception, it will be processed with a matching @ExceptionHandler method in the same controller or with any configured HandlerExceptionResolver instance.

Intercepting Async Requests

An existing HandlerInterceptor can implement AsyncHandlerInterceptor, which provides one additional method afterConcurrentHandlingStarted. It is invoked after async processing starts and when the initial request processing thread is being exited. See the Javadoc of AsyncHandlerInterceptor for more details on that.

Further options for async request lifecycle callbacks are provided directly on DeferredResult, which has the methods onTimeout(Runnable) and onCompletion(Runnable). Those are called when the async request is about to time out or has completed respectively. The timeout event can be handled by setting the DeferredResult to some value. The completion callback however is final and the result can no longer be set.

Similar callbacks are also available with a Callable. However, you will need to wrap the Callable in an instance of WebAsyncTask and then use that to register the timeout and completion callbacks. Just like with DeferredResult, the timeout event can be handled and a value can be returned while the completion event is final.

You can also register a CallableProcessingInterceptor or a DeferredResultProcessingInterceptor globally through the MVC Java config or the MVC namespace. Those interceptors provide a full set of callbacks and apply every time a Callable or a DeferredResult is used.

Configuration for Async Request Processing

Servlet 3 Async Config

To use Servlet 3 async request processing, you need to update web.xml to version 3.0:

                  <web-app                  xmlns="http://java.sun.com/xml/ns/javaee"                  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"                  xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"                  version="3.0"                  >                  ...                  </web-app>

The DispatcherServlet and any Filter configuration need to have the <async-supported>true</async-supported> sub-element. Additionally, any Filter that also needs to get involved in async dispatches should also be configured to support the ASYNC dispatcher type. Note that it is safe to enable the ASYNC dispatcher type for all filters provided with the Spring Framework since they will not get involved in async dispatches unless needed.

If using Servlet 3, Java based configuration, e.g. via WebApplicationInitializer, you'll also need to set the "asyncSupported" flag as well as the ASYNC dispatcher type just like with web.xml. To simplify all this configuration, consider extending AbstractDispatcherServletInitializer or AbstractAnnotationConfigDispatcherServletInitializer, which automatically set those options and make it very easy to register Filter instances.

Spring MVC Async Config

The MVC Java config and the MVC namespace both provide options for configuring async request processing. WebMvcConfigurer has the method configureAsyncSupport while <mvc:annotation-driven> has an <async-support> sub-element.

Those allow you to configure the default timeout value to use for async requests, which if not set depends on the underlying Servlet container (e.g. 10 seconds on Tomcat). You can also configure an AsyncTaskExecutor to use for executing Callable instances returned from controller methods. It is highly recommended to configure this property since by default Spring MVC uses SimpleAsyncTaskExecutor. The MVC Java config and the MVC namespace also allow you to register CallableProcessingInterceptor and DeferredResultProcessingInterceptor instances.

If you need to override the default timeout value for a specific DeferredResult, you can do so by using the appropriate class constructor. Similarly, for a Callable, you can wrap it in a WebAsyncTask and use the appropriate class constructor to customize the timeout value. The class constructor of WebAsyncTask also allows providing an AsyncTaskExecutor.

17.4 Handler mappings

In previous versions of Spring, users were required to define one or more HandlerMapping beans in the web application context to map incoming web requests to appropriate handlers. With the introduction of annotated controllers, you generally don't need to do that because the RequestMappingHandlerMapping automatically looks for @RequestMapping annotations on all @Controller beans. However, do keep in mind that all HandlerMapping classes extending from AbstractHandlerMapping have the following properties that you can use to customize their behavior:

interceptors: List of interceptors to use. HandlerInterceptors are discussed in Section 17.4.1, "Intercepting requests with a HandlerInterceptor".
defaultHandler: Default handler to use, when this handler mapping does not result in a matching handler.
order: Based on the value of the order property (see the org.springframework.core.Ordered interface), Spring sorts all handler mappings available in the context and applies the first matching handler.
alwaysUseFullPath: If true , Spring uses the full path within the current Servlet context to find an appropriate handler. If false (the default), the path within the current Servlet mapping is used. For example, if a Servlet is mapped using /testing/* and the alwaysUseFullPath property is set to true, /testing/viewPage.html is used, whereas if the property is set to false, /viewPage.html is used.
urlDecode: Defaults to true, as of Spring 2.5. If you prefer to compare encoded paths, set this flag to false. However, the HttpServletRequest always exposes the Servlet path in decoded form. Be aware that the Servlet path will not match when compared with encoded paths.

The following example shows how to configure an interceptor:

            <beans>            <bean            class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping"            >            <property            name="interceptors"            >            <bean            class="example.MyInterceptor"            />            </property>            </bean>            <beans>

17.4.1 Intercepting requests with a `HandlerInterceptor`

Spring's handler mapping mechanism includes handler interceptors, which are useful when you want to apply specific functionality to certain requests, for example, checking for a principal.

Interceptors located in the handler mapping must implement HandlerInterceptor from the org.springframework.web.servlet package. This interface defines three methods: preHandle(..) is called before the actual handler is executed; postHandle(..) is called after the handler is executed; and afterCompletion(..) is called after the complete request has finished . These three methods should provide enough flexibility to do all kinds of preprocessing and postprocessing.

The preHandle(..) method returns a boolean value. You can use this method to break or continue the processing of the execution chain. When this method returns true, the handler execution chain will continue; when it returns false, the DispatcherServlet assumes the interceptor itself has taken care of requests (and, for example, rendered an appropriate view) and does not continue executing the other interceptors and the actual handler in the execution chain.

Interceptors can be configured using the interceptors property, which is present on all HandlerMapping classes extending from AbstractHandlerMapping. This is shown in the example below:

              <beans>              <bean              id="handlerMapping"              class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping"              >              <property              name="interceptors"              >              <list>              <ref              bean="officeHoursInterceptor"              />              </list>              </property>              </bean>              <bean              id="officeHoursInterceptor"              class="samples.TimeBasedAccessInterceptor"              >              <property              name="openingTime"              value="9"              />              <property              name="closingTime"              value="18"              />              </bean>              <beans>

              package              samples;              public              class              TimeBasedAccessInterceptor              extends              HandlerInterceptorAdapter {              private              int              openingTime;              private              int              closingTime;              public              void              setOpeningTime(int              openingTime) {              this.openingTime = openingTime;     }              public              void              setClosingTime(int              closingTime) {              this.closingTime = closingTime;     }              public              boolean              preHandle(             HttpServletRequest request,             HttpServletResponse response,             Object handler)              throws              Exception {          Calendar cal = Calendar.getInstance();              int              hour = cal.get(HOUR_OF_DAY);              if              (openingTime <= hour && hour < closingTime) {              return              true;         }              else              {             response.sendRedirect("http://host.com/outsideOfficeHours.html");              return              false;         }     } }

Any request handled by this mapping is intercepted by the TimeBasedAccessInterceptor. If the current time is outside office hours, the user is redirected to a static HTML file that says, for example, you can only access the website during office hours.

	Note
	When using the `RequestMappingHandlerMapping` the actual handler is an instance of `HandlerMethod` which identifies the specific controller method that will be invoked.

As you can see, the Spring adapter class HandlerInterceptorAdapter makes it easier to extend the HandlerInterceptor interface.

	Tip
	In the example above, the configured interceptor will apply to all requests handled with annotated controller methods. If you want to narrow down the URL paths to which an interceptor applies, you can use the MVC namespace or the MVC Java config, or declare bean instances of type `MappedInterceptor` to do that. See Enabling the MVC Java Config or the MVC XML Namespace.

17.5 Resolving views

All MVC frameworks for web applications provide a way to address views. Spring provides view resolvers, which enable you to render models in a browser without tying you to a specific view technology. Out of the box, Spring enables you to use JSPs, Velocity templates and XSLT views, for example. See Chapter 18, View technologies for a discussion of how to integrate and use a number of disparate view technologies.

The two interfaces that are important to the way Spring handles views are ViewResolver and View. The ViewResolver provides a mapping between view names and actual views. The View interface addresses the preparation of the request and hands the request over to one of the view technologies.

17.5.1 Resolving views with the `ViewResolver` interface

As discussed in Section 17.3, "Implementing Controllers", all handler methods in the Spring Web MVC controllers must resolve to a logical view name, either explicitly (e.g., by returning a String, View, or ModelAndView) or implicitly (i.e., based on conventions). Views in Spring are addressed by a logical view name and are resolved by a view resolver. Spring comes with quite a few view resolvers. This table lists most of them; a couple of examples follow.

Table 17.3. View resolvers

`ViewResolver`	Description
`AbstractCachingViewResolver`	Abstract view resolver that caches views. Often views need preparation before they can be used; extending this view resolver provides caching.
`XmlViewResolver`	Implementation of `ViewResolver` that accepts a configuration file written in XML with the same DTD as Spring's XML bean factories. The default configuration file is `/WEB-INF/views.xml`.
`ResourceBundleViewResolver`	Implementation of `ViewResolver` that uses bean definitions in a `ResourceBundle`, specified by the bundle base name. Typically you define the bundle in a properties file, located in the classpath. The default file name is `views.properties`.
`UrlBasedViewResolver`	Simple implementation of the `ViewResolver` interface that effects the direct resolution of logical view names to URLs, without an explicit mapping definition. This is appropriate if your logical names match the names of your view resources in a straightforward manner, without the need for arbitrary mappings.
`InternalResourceViewResolver`	Convenient subclass of `UrlBasedViewResolver` that supports `InternalResourceView` (in effect, Servlets and JSPs) and subclasses such as `JstlView` and `TilesView`. You can specify the view class for all views generated by this resolver by using `setViewClass(..)`. See the Javadocs for the `UrlBasedViewResolver` class for details.
`VelocityViewResolver` / `FreeMarkerViewResolver`	Convenient subclass of `UrlBasedViewResolver` that supports `VelocityView` (in effect, Velocity templates) or `FreeMarkerView` ,respectively, and custom subclasses of them.
`ContentNegotiatingViewResolver`	Implementation of the `ViewResolver` interface that resolves a view based on the request file name or `Accept` header. See Section 17.5.4, "`ContentNegotiatingViewResolver`".

As an example, with JSP as a view technology, you can use the UrlBasedViewResolver. This view resolver translates a view name to a URL and hands the request over to the RequestDispatcher to render the view.

              <bean              id="viewResolver"              class="org.springframework.web.servlet.view.UrlBasedViewResolver"              >              <property              name="viewClass"              value="org.springframework.web.servlet.view.JstlView"              />              <property              name="prefix"              value="/WEB-INF/jsp/"              />              <property              name="suffix"              value=".jsp"              />              </bean>

When returning test as a logical view name, this view resolver forwards the request to the RequestDispatcher that will send the request to /WEB-INF/jsp/test.jsp.

When you combine different view technologies in a web application, you can use the ResourceBundleViewResolver:

              <bean              id="viewResolver"              class="org.springframework.web.servlet.view.ResourceBundleViewResolver"              >              <property              name="basename"              value="views"              />              <property              name="defaultParentView"              value="parentView"              />              </bean>

The ResourceBundleViewResolver inspects the ResourceBundle identified by the basename, and for each view it is supposed to resolve, it uses the value of the property [viewname].(class) as the view class and the value of the property [viewname].url as the view url. Examples can be found in the next chapter which covers view technologies. As you can see, you can identify a parent view, from which all views in the properties file "extend". This way you can specify a default view class, for example.

Note

Subclasses of AbstractCachingViewResolver cache view instances that they resolve. Caching improves performance of certain view technologies. It's possible to turn off the cache by setting the cache property to false. Furthermore, if you must refresh a certain view at runtime (for example when a Velocity template is modified), you can use the removeFromCache(String viewName, Locale loc) method.

17.5.2 Chaining ViewResolvers

Spring supports multiple view resolvers. Thus you can chain resolvers and, for example, override specific views in certain circumstances. You chain view resolvers by adding more than one resolver to your application context and, if necessary, by setting the order property to specify ordering. Remember, the higher the order property, the later the view resolver is positioned in the chain.

In the following example, the chain of view resolvers consists of two resolvers, an InternalResourceViewResolver, which is always automatically positioned as the last resolver in the chain, and an XmlViewResolver for specifying Excel views. Excel views are not supported by the InternalResourceViewResolver.

              <bean              id="jspViewResolver"              class="org.springframework.web.servlet.view.InternalResourceViewResolver"              >              <property              name="viewClass"              value="org.springframework.web.servlet.view.JstlView"              />              <property              name="prefix"              value="/WEB-INF/jsp/"              />              <property              name="suffix"              value=".jsp"              />              </bean>              <bean              id="excelViewResolver"              class="org.springframework.web.servlet.view.XmlViewResolver"              >              <property              name="order"              value="1"              />              <property              name="location"              value="/WEB-INF/views.xml"              />              </bean>                              <!-- in views.xml -->                            <beans>              <bean              name="report"              class="org.springframework.example.ReportExcelView"              />              </beans>

If a specific view resolver does not result in a view, Spring examines the context for other view resolvers. If additional view resolvers exist, Spring continues to inspect them until a view is resolved. If no view resolver returns a view, Spring throws a ServletException.

The contract of a view resolver specifies that a view resolver can return null to indicate the view could not be found. Not all view resolvers do this, however, because in some cases, the resolver simply cannot detect whether or not the view exists. For example, the InternalResourceViewResolver uses the RequestDispatcher internally, and dispatching is the only way to figure out if a JSP exists, but this action can only execute once. The same holds for the VelocityViewResolver and some others. Check the Javadoc for the view resolver to see whether it reports non-existing views. Thus, putting an InternalResourceViewResolver in the chain in a place other than the last, results in the chain not being fully inspected, because the InternalResourceViewResolver will always return a view!

17.5.3 Redirecting to views

As mentioned previously, a controller typically returns a logical view name, which a view resolver resolves to a particular view technology. For view technologies such as JSPs that are processed through the Servlet or JSP engine, this resolution is usually handled through the combination of InternalResourceViewResolver and InternalResourceView, which issues an internal forward or include via the Servlet API's RequestDispatcher.forward(..) method or RequestDispatcher.include() method. For other view technologies, such as Velocity, XSLT, and so on, the view itself writes the content directly to the response stream.

It is sometimes desirable to issue an HTTP redirect back to the client, before the view is rendered. This is desirable, for example, when one controller has been called with POSTed data, and the response is actually a delegation to another controller (for example on a successful form submission). In this case, a normal internal forward will mean that the other controller will also see the same POST data, which is potentially problematic if it can confuse it with other expected data. Another reason to perform a redirect before displaying the result is to eliminate the possibility of the user submitting the form data multiple times. In this scenario, the browser will first send an initial POST; it will then receive a response to redirect to a different URL; and finally the browser will perform a subsequent GET for the URL named in the redirect response. Thus, from the perspective of the browser, the current page does not reflect the result of a POST but rather of a GET. The end effect is that there is no way the user can accidentally re-POST the same data by performing a refresh. The refresh forces a GET of the result page, not a resend of the initial POST data.

`RedirectView`

One way to force a redirect as the result of a controller response is for the controller to create and return an instance of Spring's RedirectView. In this case, DispatcherServlet does not use the normal view resolution mechanism. Rather because it has been given the (redirect) view already, the DispatcherServlet simply instructs the view to do its work.

The RedirectView issues an HttpServletResponse.sendRedirect() call that returns to the client browser as an HTTP redirect. By default all model attributes are considered to be exposed as URI template variables in the redirect URL. Of the remaining attributes those that are primitive types or collections/arrays of primitive types are automatically appended as query parameters.

Appending primitive type attributes as query parameters may be the desired result if a model instance was prepared specifically for the redirect. However, in annotated controllers the model may contain additional attributes added for rendering purposes (e.g. drop-down field values). To avoid the possibility of having such attributes appear in the URL an annotated controller can declare an argument of type RedirectAttributes and use it to specify the exact attributes to make available to RedirectView. If the controller method decides to redirect, the content of RedirectAttributes is used. Otherwise the content of the model is used.

Note that URI template variables from the present request are automatically made available when expanding a redirect URL and do not need to be added explicitly neither through Model nor RedirectAttributes. For example:

                                  @RequestMapping(value = "/files/{path}", method = RequestMethod.POST)                                public                String upload(...) {                     return                "redirect:files/{path}"; }

If you use RedirectView and the view is created by the controller itself, it is recommended that you configure the redirect URL to be injected into the controller so that it is not baked into the controller but configured in the context along with the view names. The next section discusses this process.

The `redirect:` prefix

While the use of RedirectView works fine, if the controller itself creates the RedirectView, there is no avoiding the fact that the controller is aware that a redirection is happening. This is really suboptimal and couples things too tightly. The controller should not really care about how the response gets handled. In general it should operate only in terms of view names that have been injected into it.

The special redirect: prefix allows you to accomplish this. If a view name is returned that has the prefix redirect:, the UrlBasedViewResolver (and all subclasses) will recognize this as a special indication that a redirect is needed. The rest of the view name will be treated as the redirect URL.

The net effect is the same as if the controller had returned a RedirectView, but now the controller itself can simply operate in terms of logical view names. A logical view name such as redirect:/myapp/some/resource will redirect relative to the current Servlet context, while a name such as redirect:http://myhost.com/some/arbitrary/path will redirect to an absolute URL.

The `forward:` prefix

It is also possible to use a special forward: prefix for view names that are ultimately resolved by UrlBasedViewResolver and subclasses. This creates an InternalResourceView (which ultimately does a RequestDispatcher.forward()) around the rest of the view name, which is considered a URL. Therefore, this prefix is not useful with InternalResourceViewResolver and InternalResourceView (for JSPs for example). But the prefix can be helpful when you are primarily using another view technology, but still want to force a forward of a resource to be handled by the Servlet/JSP engine. (Note that you may also chain multiple view resolvers, instead.)

As with the redirect: prefix, if the view name with the forward: prefix is injected into the controller, the controller does not detect that anything special is happening in terms of handling the response.

17.5.4`ContentNegotiatingViewResolver`

The ContentNegotiatingViewResolver does not resolve views itself but rather delegates to other view resolvers, selecting the view that resembles the representation requested by the client. Two strategies exist for a client to request a representation from the server:

Use a distinct URI for each resource, typically by using a different file extension in the URI. For example, the URI http://www.example.com/users/fred.pdf requests a PDF representation of the user fred, and http://www.example.com/users/fred.xml requests an XML representation.

Use the same URI for the client to locate the resource, but set the Accept HTTP request header to list the media types that it understands. For example, an HTTP request for http://www.example.com/users/fred with an Accept header set to application/pdf requests a PDF representation of the user fred, while http://www.example.com/users/fred with an Accept header set to text/xml requests an XML representation. This strategy is known as content negotiation.

Note

One issue with the Accept header is that it is impossible to set it in a web browser within HTML. For example, in Firefox, it is fixed to:

Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8

For this reason it is common to see the use of a distinct URI for each representation when developing browser based web applications.

To support multiple representations of a resource, Spring provides the ContentNegotiatingViewResolver to resolve a view based on the file extension or Accept header of the HTTP request. ContentNegotiatingViewResolver does not perform the view resolution itself but instead delegates to a list of view resolvers that you specify through the bean property ViewResolvers.

The ContentNegotiatingViewResolver selects an appropriate View to handle the request by comparing the request media type(s) with the media type (also known as Content-Type) supported by the View associated with each of its ViewResolvers. The first View in the list that has a compatible Content-Type returns the representation to the client. If a compatible view cannot be supplied by the ViewResolver chain, then the list of views specified through the DefaultViews property will be consulted. This latter option is appropriate for singleton Views that can render an appropriate representation of the current resource regardless of the logical view name. The Accept header may include wild cards, for example text/*, in which case a View whose Content-Type was text/xml is a compatible match.

To support the resolution of a view based on a file extension, use the ContentNegotiatingViewResolver bean property mediaTypes to specify a mapping of file extensions to media types. For more information on the algorithm used to determine the request media type, refer to the API documentation for ContentNegotiatingViewResolver.

Here is an example configuration of a ContentNegotiatingViewResolver:

              <bean              class="org.springframework.web.servlet.view.ContentNegotiatingViewResolver"              >              <property              name="mediaTypes"              >              <map>              <entry              key="atom"              value="application/atom+xml"              />              <entry              key="html"              value="text/html"              />              <entry              key="json"              value="application/json"              />              </map>              </property>              <property              name="viewResolvers"              >              <list>              <bean              class="org.springframework.web.servlet.view.BeanNameViewResolver"              />              <bean              class="org.springframework.web.servlet.view.InternalResourceViewResolver"              >              <property              name="prefix"              value="/WEB-INF/jsp/"              />              <property              name="suffix"              value=".jsp"              />              </bean>              </list>              </property>              <property              name="defaultViews"              >              <list>              <bean              class="org.springframework.web.servlet.view.json.MappingJackson2JsonView"                              />              </list>              </property>              </bean>              <bean              id="content"              class="com.springsource.samples.rest.SampleContentAtomView"              />

The InternalResourceViewResolver handles the translation of view names and JSP pages, while the BeanNameViewResolver returns a view based on the name of a bean. (See "Resolving views with the ViewResolver interface" for more details on how Spring looks up and instantiates a view.) In this example, the content bean is a class that inherits from AbstractAtomFeedView, which returns an Atom RSS feed. For more information on creating an Atom Feed representation, see the section Atom Views.

In the above configuration, if a request is made with an .html extension, the view resolver looks for a view that matches the text/html media type. The InternalResourceViewResolver provides the matching view for text/html. If the request is made with the file extension .atom, the view resolver looks for a view that matches the application/atom+xml media type. This view is provided by the BeanNameViewResolver that maps to the SampleContentAtomView if the view name returned is content. If the request is made with the file extension .json, the MappingJackson2JsonViewinstance from the DefaultViews list will be selected regardless of the view name. Alternatively, client requests can be made without a file extension but with the Accept header set to the preferred media-type, and the same resolution of request to views would occur.

	Note
	If `ContentNegotiatingViewResolver`'s list of ViewResolvers is not configured explicitly, it automatically uses any ViewResolvers defined in the application context.

The corresponding controller code that returns an Atom RSS feed for a URI of the form http://localhost/content.atom or http://localhost/content with an Accept header of application/atom+xml is shown below.

                              @Controller                            public              class              ContentController {              private              List<SampleContent> contentList =              new              ArrayList<SampleContent>();                              @RequestMapping(value="/content", method=RequestMethod.GET)                            public              ModelAndView getContent() {         ModelAndView mav =              new              ModelAndView();         mav.setViewName("content");         mav.addObject("sampleContentList", contentList);              return              mav;     }  }

17.6 Using flash attributes

Flash attributes provide a way for one request to store attributes intended for use in another. This is most commonly needed when redirecting — for example, the Post/Redirect/Get pattern. Flash attributes are saved temporarily before the redirect (typically in the session) to be made available to the request after the redirect and removed immediately.

Spring MVC has two main abstractions in support of flash attributes. FlashMap is used to hold flash attributes while FlashMapManager is used to store, retrieve, and manage FlashMap instances.

Flash attribute support is always "on" and does not need to enabled explicitly although if not used, it never causes HTTP session creation. On each request there is an "input" FlashMap with attributes passed from a previous request (if any) and an "output" FlashMap with attributes to save for a subsequent request. Both FlashMap instances are accessible from anywhere in Spring MVC through static methods in RequestContextUtils.

Annotated controllers typically do not need to work with FlashMap directly. Instead an @RequestMapping method can accept an argument of type RedirectAttributes and use it to add flash attributes for a redirect scenario. Flash attributes added via RedirectAttributes are automatically propagated to the "output" FlashMap. Similarly after the redirect attributes from the "input" FlashMap are automatically added to the Model of the controller serving the target URL.

17.7 Building `URI`s

Spring MVC provides a mechanism for building and encoding a URI using UriComponentsBuilder and UriComponents.

For example you can expand and encode a URI template string:

UriComponents uriComponents =         UriComponentsBuilder.fromUriString("http://example.com/hotels/{hotel}/bookings/{booking}").build();  URI uri = uriComponents.expand("42",            "21").encode().toUri();

Note that UriComponents is immutable and the expand() and encode() operations return new instances if necessary.

You can also expand and encode using individual URI components:

UriComponents uriComponents =         UriComponentsBuilder.newInstance()             .scheme("http").host("example.com").path("/hotels/{hotel}/bookings/{booking}").build()             .expand("42",            "21")             .encode();

In a Servlet environment the ServletUriComponentsBuilder sub-class provides static factory methods to copy available URL information from a Servlet requests:

HttpServletRequest request = ...     ServletUriComponentsBuilder ucb =         ServletUriComponentsBuilder.fromRequest(request).replaceQueryParam("accountId",            "{id}").build()             .expand("123")             .encode();

Alternatively, you may choose to copy a subset of the available information up to and including the context path:

                ServletUriComponentsBuilder ucb =         ServletUriComponentsBuilder.fromContextPath(request).path("/accounts").build()

Or in cases where the DispatcherServlet is mapped by name (e.g. /main/*), you can also have the literal part of the servlet mapping included:

                 ServletUriComponentsBuilder ucb =         ServletUriComponentsBuilder.fromServletMapping(request).path("/accounts").build()

17.8 Using locales

Most parts of Spring's architecture support internationalization, just as the Spring web MVC framework does. DispatcherServlet enables you to automatically resolve messages using the client's locale. This is done with LocaleResolver objects.

When a request comes in, the DispatcherServlet looks for a locale resolver, and if it finds one it tries to use it to set the locale. Using the RequestContext.getLocale() method, you can always retrieve the locale that was resolved by the locale resolver.

In addition to automatic locale resolution, you can also attach an interceptor to the handler mapping (see Section 17.4.1, "Intercepting requests with a HandlerInterceptor" for more information on handler mapping interceptors) to change the locale under specific circumstances, for example, based on a parameter in the request.

Locale resolvers and interceptors are defined in the org.springframework.web.servlet.i18n package and are configured in your application context in the normal way. Here is a selection of the locale resolvers included in Spring.

17.8.1`AcceptHeaderLocaleResolver`

This locale resolver inspects the accept-language header in the request that was sent by the client (e.g., a web browser). Usually this header field contains the locale of the client's operating system.

17.8.2`CookieLocaleResolver`

This locale resolver inspects a Cookie that might exist on the client to see if a locale is specified. If so, it uses the specified locale. Using the properties of this locale resolver, you can specify the name of the cookie as well as the maximum age. Find below an example of defining a CookieLocaleResolver.

              <bean              id="localeResolver"              class="org.springframework.web.servlet.i18n.CookieLocaleResolver"              >              <property              name="cookieName"              value="clientlanguage"              />                              <!-- in seconds. If set to -1, the cookie is not persisted (deleted when browser shuts down) -->                            <property              name="cookieMaxAge"              value="100000"              >              </bean>

Table 17.4.CookieLocaleResolver properties

Property	Default	Description
cookieName	classname + LOCALE	The name of the cookie
cookieMaxAge	Integer.MAX_INT	The maximum time a cookie will stay persistent on the client. If -1 is specified, the cookie will not be persisted; it will only be available until the client shuts down his or her browser.
cookiePath	/	Limits the visibility of the cookie to a certain part of your site. When cookiePath is specified, the cookie will only be visible to that path and the paths below it.

17.8.3`SessionLocaleResolver`

The SessionLocaleResolver allows you to retrieve locales from the session that might be associated with the user's request.

17.8.4`LocaleChangeInterceptor`

You can enable changing of locales by adding the LocaleChangeInterceptor to one of the handler mappings (see Section 17.4, "Handler mappings"). It will detect a parameter in the request and change the locale. It calls setLocale() on the LocaleResolver that also exists in the context. The following example shows that calls to all *.view resources containing a parameter named siteLanguage will now change the locale. So, for example, a request for the following URL, http://www.sf.net/home.view?siteLanguage=nl will change the site language to Dutch.

              <bean              id="localeChangeInterceptor"              class="org.springframework.web.servlet.i18n.LocaleChangeInterceptor"              >              <property              name="paramName"              value="siteLanguage"              />              </bean>              <bean              id="localeResolver"              class="org.springframework.web.servlet.i18n.CookieLocaleResolver"              />              <bean              id="urlMapping"              class="org.springframework.web.servlet.handler.SimpleUrlHandlerMapping"              >              <property              name="interceptors"              >              <list>              <ref              bean="localeChangeInterceptor"              />              </list>              </property>              <property              name="mappings"              >              <value>/**/*.view=someController</value>              </property>              </bean>

17.9 Using themes

17.9.1 Overview of themes

You can apply Spring Web MVC framework themes to set the overall look-and-feel of your application, thereby enhancing user experience. A theme is a collection of static resources, typically style sheets and images, that affect the visual style of the application.

17.9.2 Defining themes

To use themes in your web application, you must set up an implementation of the org.springframework.ui.context.ThemeSource interface. The WebApplicationContext interface extends ThemeSource but delegates its responsibilities to a dedicated implementation. By default the delegate will be an org.springframework.ui.context.support.ResourceBundleThemeSource implementation that loads properties files from the root of the classpath. To use a custom ThemeSource implementation or to configure the base name prefix of the ResourceBundleThemeSource, you can register a bean in the application context with the reserved name themeSource. The web application context automatically detects a bean with that name and uses it.

When using the ResourceBundleThemeSource, a theme is defined in a simple properties file. The properties file lists the resources that make up the theme. Here is an example:

styleSheet=/themes/cool/style.css background=/themes/cool/img/coolBg.jpg

The keys of the properties are the names that refer to the themed elements from view code. For a JSP, you typically do this using the spring:theme custom tag, which is very similar to the spring:message tag. The following JSP fragment uses the theme defined in the previous example to customize the look and feel:

              <%@              taglib              prefix="spring"              uri="http://www.springframework.org/tags"              %>              <html>              <head>              <link              rel="stylesheet"              href="<spring:theme code='styleSheet'/>"              type="text/css"              />              </head>              <body              style="background=<spring:theme code='background'/>"              >              ...              </body>              </html>

By default, the ResourceBundleThemeSource uses an empty base name prefix. As a result, the properties files are loaded from the root of the classpath. Thus you would put the cool.properties theme definition in a directory at the root of the classpath, for example, in /WEB-INF/classes. The ResourceBundleThemeSource uses the standard Java resource bundle loading mechanism, allowing for full internationalization of themes. For example, we could have a /WEB-INF/classes/cool_nl.properties that references a special background image with Dutch text on it.

17.9.3 Theme resolvers

After you define themes, as in the preceding section, you decide which theme to use. The DispatcherServlet will look for a bean named themeResolver to find out which ThemeResolver implementation to use. A theme resolver works in much the same way as a LocaleResolver. It detects the theme to use for a particular request and can also alter the request's theme. The following theme resolvers are provided by Spring:

Table 17.5.ThemeResolver implementations

Class	Description
`FixedThemeResolver`	Selects a fixed theme, set using the `defaultThemeName` property.
`SessionThemeResolver`	The theme is maintained in the user's HTTP session. It only needs to be set once for each session, but is not persisted between sessions.
`CookieThemeResolver`	The selected theme is stored in a cookie on the client.

Spring also provides a ThemeChangeInterceptor that allows theme changes on every request with a simple request parameter.

17.10 Spring's multipart (file upload) support

17.10.1 Introduction

Spring's built-in multipart support handles file uploads in web applications. You enable this multipart support with pluggable MultipartResolver objects, defined in the org.springframework.web.multipart package. Spring provides one MultipartResolver implementation for use with Commons FileUpload and another for use with Servlet 3.0 multipart request parsing.

By default, Spring does no multipart handling, because some developers want to handle multiparts themselves. You enable Spring multipart handling by adding a multipart resolver to the web application's context. Each request is inspected to see if it contains a multipart. If no multipart is found, the request continues as expected. If a multipart is found in the request, the MultipartResolver that has been declared in your context is used. After that, the multipart attribute in your request is treated like any other attribute.

17.10.2 Using a `MultipartResolver` with Commons FileUpload

The following example shows how to use the CommonsMultipartResolver:

              <bean              id="multipartResolver"              class="org.springframework.web.multipart.commons.CommonsMultipartResolver"              >                              <!-- one of the properties available; the maximum file size in bytes -->                            <property              name="maxUploadSize"              value="100000"              />              </bean>

Of course you also need to put the appropriate jars in your classpath for the multipart resolver to work. In the case of the CommonsMultipartResolver, you need to use commons-fileupload.jar.

When the Spring DispatcherServlet detects a multi-part request, it activates the resolver that has been declared in your context and hands over the request. The resolver then wraps the current HttpServletRequest into a MultipartHttpServletRequest that supports multipart file uploads. Using the MultipartHttpServletRequest, you can get information about the multiparts contained by this request and actually get access to the multipart files themselves in your controllers.

17.10.3 Using a `MultipartResolver` with Servlet 3.0

In order to use Servlet 3.0 based multipart parsing, you need to mark the DispatcherServlet with a "multipart-config" section in web.xml, or with a javax.servlet.MultipartConfigElement in programmatic Servlet registration, or in case of a custom Servlet class possibly with a javax.servlet.annotation.MultipartConfig annotation on your Servlet class. Configuration settings such as maximum sizes or storage locations need to be applied at that Servlet registration level as Servlet 3.0 does not allow for those settings to be done from the MultipartResolver.

Once Servlet 3.0 multipart parsing has been enabled in one of the above mentioned ways you can add the StandardServletMultipartResolver to your Spring configuration:

              <bean              id="multipartResolver"              class="org.springframework.web.multipart.support.StandardServletMultipartResolver"              >              </bean>

17.10.4 Handling a file upload in a form

After the MultipartResolver completes its job, the request is processed like any other. First, create a form with a file input that will allow the user to upload a form. The encoding attribute (enctype="multipart/form-data") lets the browser know how to encode the form as multipart request:

              <html>              <head>              <title>Upload a file please</title>              </head>              <body>              <h1>Please upload a file</h1>              <form              method="post"              action="/form"              enctype="multipart/form-data"              >              <input              type="text"              name="name"              />              <input              type="file"              name="file"              />              <input              type="submit"              />              </form>              </body>              </html>

The next step is to create a controller that handles the file upload. This controller is very similar to a normal annotated @Controller, except that we use MultipartHttpServletRequest or MultipartFile in the method parameters:

                              @Controller                            public              class              FileUploadController {                              @RequestMapping(value = "/form", method = RequestMethod.POST)                            public              String handleFormUpload(                @RequestParam("name")                            String name,                              @RequestParam("file")                            MultipartFile file) {              if              (!file.isEmpty()) {              byte[] bytes = file.getBytes();                              // store the bytes somewhere                            return              "redirect:uploadSuccess";        }              else              {              return              "redirect:uploadFailure";        }     }  }

Note how the @RequestParam method parameters map to the input elements declared in the form. In this example, nothing is done with the byte[], but in practice you can save it in a database, store it on the file system, and so on.

When using Servlet 3.0 multipart parsing you can also use javax.servlet.http.Part for the method parameter:

                              @Controller                            public              class              FileUploadController {                              @RequestMapping(value = "/form", method = RequestMethod.POST)                            public              String handleFormUpload(                @RequestParam("name")                            String name,                              @RequestParam("file")                            Part file) {          InputStream inputStream = file.getInputStream();                              // store bytes from uploaded file somewhere                            return              "redirect:uploadSuccess";     }  }

17.10.5 Handling a file upload request from programmatic clients

Multipart requests can also be submitted from non-browser clients in a RESTful service scenario. All of the above examples and configuration apply here as well. However, unlike browsers that typically submit files and simple form fields, a programmatic client can also send more complex data of a specific content type — for example a multipart request with a file and second part with JSON formatted data:

POST /someUrl Content-Type: multipart/mixed  --edt7Tfrdusa7r3lNQc79vXuhIIMlatb7PQg7Vp Content-Disposition: form-data; name="meta-data" Content-Type: application/json; charset=UTF-8 Content-Transfer-Encoding: 8bit  {   "name": "value" } --edt7Tfrdusa7r3lNQc79vXuhIIMlatb7PQg7Vp Content-Disposition: form-data; name="file-data"; filename="file.properties" Content-Type: text/xml Content-Transfer-Encoding: 8bit ... File Data ...

You could access the part named "meta-data" with a @RequestParam("meta-data") String metadata controller method argument. However, you would probably prefer to accept a strongly typed object initialized from the JSON formatted data in the body of the request part, very similar to the way @RequestBody converts the body of a non-multipart request to a target object with the help of an HttpMessageConverter.

You can use the @RequestPart annotation instead of the @RequestParam annotation for this purpose. It allows you to have the content of a specific multipart passed through an HttpMessageConverter taking into consideration the 'Content-Type' header of the multipart:

                              @RequestMapping(value="/someUrl", method = RequestMethod.POST)                            public              String onSubmit(                @RequestPart("meta-data") MetaData metadata,                        @RequestPart("file-data") MultipartFile file              ) {                              // ...                            }

Notice how MultipartFile method arguments can be accessed with @RequestParam or with @RequestPart interchangeably. However, the @RequestPart("meta-data") MetaData method argument in this case is read as JSON content based on its 'Content-Type' header and converted with the help of the MappingJackson2HttpMessageConverter.

17.11 Handling exceptions

17.11.1`HandlerExceptionResolver`

Spring HandlerExceptionResolver implementations deal with unexpected exceptions that occur during controller execution. A HandlerExceptionResolver somewhat resembles the exception mappings you can define in the web application descriptor web.xml. However, they provide a more flexible way to do so. For example they provide information about which handler was executing when the exception was thrown. Furthermore, a programmatic way of handling exceptions gives you more options for responding appropriately before the request is forwarded to another URL (the same end result as when you use the Servlet specific exception mappings).

Besides implementing the HandlerExceptionResolver interface, which is only a matter of implementing the resolveException(Exception, Handler) method and returning a ModelAndView, you may also use the provided SimpleMappingExceptionResolver or create @ExceptionHandler methods. The SimpleMappingExceptionResolver enables you to take the class name of any exception that might be thrown and map it to a view name. This is functionally equivalent to the exception mapping feature from the Servlet API, but it is also possible to implement more finely grained mappings of exceptions from different handlers. The @ExceptionHandler annotation on the other hand can be used on methods that should be invoked to handle an exception. Such methods may be defined locally within an @Controller or may apply globally to all @RequestMapping methods when defined within an @ControllerAdvice class. The following sections explain this in more detail.

17.11.2`@ExceptionHandler`

The HandlerExceptionResolver interface and the SimpleMappingExceptionResolver implementations allow you to map Exceptions to specific views declaratively along with some optional Java logic before forwarding to those views. However, in some cases, especially when relying on @ResponseBody methods rather than on view resolution, it may be more convenient to directly set the status of the response and optionally write error content to the body of the response.

You can do that with @ExceptionHandler methods. When declared within a controller such methods apply to exceptions raised by @RequestMapping methods of that contoroller (or any of its sub-classes). You can also declare an @ExceptionHandler method within an @ControllerAdvice class in which case it handles exceptions from @RequestMapping methods from any controller. The @ControllerAdvice annotation is a component annotation, which can be used with classpath scanning. It is automatically enabled when using the MVC namespace and the MVC Java config, or otherwise depending on whether the ExceptionHandlerExceptionResolver is configured or not. Below is an example of a controller-local @ExceptionHandler method:

                              @Controller                            public              class              SimpleController {                                   @ExceptionHandler(IOException.class)                            public              ResponseEntity<String> handleIOException(IOException ex) {                    return              responseEntity;   }  }

The @ExceptionHandler value can be set to an array of Exception types. If an exception is thrown that matches one of the types in the list, then the method annotated with the matching @ExceptionHandler will be invoked. If the annotation value is not set then the exception types listed as method arguments are used.

Much like standard controller methods annotated with a @RequestMapping annotation, the method arguments and return values of @ExceptionHandler methods can be flexible. For example, the HttpServletRequest can be accessed in Servlet environments and the PortletRequest in Portlet environments. The return type can be a String, which is interpreted as a view name, a ModelAndView object, a ResponseEntity, or you can also add the @ResponseBody to have the method return value converted with message converters and written to the response stream.

17.11.3 Handling Standard Spring MVC Exceptions

Spring MVC may raise a number of exceptions while processing a request. The SimpleMappingExceptionResolver can easily map any exception to a default error view as needed. However, when working with clients that interpret responses in an automated way you will want to set specific status code on the response. Depending on the exception raised the status code may indicate a client error (4xx) or a server error (5xx).

The DefaultHandlerExceptionResolver translates Spring MVC exceptions to specific error status codes. It is registered by default with the MVC namespace, the MVC Java config, and also by the the DispatcherServlet (i.e. when not using the MVC namespace or Java config). Listed below are some of the exceptions handled by this resolver and the corresponding status codes:

Exception	HTTP Status Code
`BindException`	400 (Bad Request)
`ConversionNotSupportedException`	500 (Internal Server Error)
`HttpMediaTypeNotAcceptableException`	406 (Not Acceptable)
`HttpMediaTypeNotSupportedException`	415 (Unsupported Media Type)
`HttpMessageNotReadableException`	400 (Bad Request)
`HttpMessageNotWritableException`	500 (Internal Server Error)
`HttpRequestMethodNotSupportedException`	405 (Method Not Allowed)
`MethodArgumentNotValidException`	400 (Bad Request)
`MissingServletRequestParameterException`	400 (Bad Request)
`MissingServletRequestPartException`	400 (Bad Request)
`NoSuchRequestHandlingMethodException`	404 (Not Found)
`TypeMismatchException`	400 (Bad Request)

The DefaultHandlerExceptionResolver works transparently by setting the status of the response. However, it stops short of writing any error content to the body of the response while your application may need to add developer-friendly content to every error response for example when providing a REST API. You can prepare a ModelAndView and render error content through view resolution -- i.e. by configuring a ContentNegotiatingViewResolver, MappingJacksonJsonView, and so on. However, you may prefer to use @ExceptionHandler methods instead.

If you prefer to write error content via @ExceptionHandler methods you can extend ResponseEntityExceptionHandler instead. This is a convenient base for @ControllerAdvice classes providing an @ExceptionHandler method to handle standard Spring MVC exceptions and return ResponseEntity. That allows you to customize the response and write error content with message converters. See the Javadoc of ResponseEntityExceptionHandler for more details.

17.11.4 Annotating Business Exceptions With `@ResponseStatus`

A business exception can be annotated with @ResponseStatus. When the exception is raised, the ResponseStatusExceptionResolver handles it by setting the status of the response accordingly. By default the DispatcherServlet registers the ResponseStatusExceptionResolver and it is available for use.

17.11.5 Customizing the Default Servlet Container Error Page

When the status of the response is set to an error status code and the body of the response is empty, Servlet containers commonly render an HTML formatted error page. To customize the default error page of the container, you can declare an <error-page> element in web.xml. Up until Servlet 3, that element had to be mapped to a specific status code or exception type. Starting with Servlet 3 an error page does not need to be mapped, which effectively means the specified location customizes the default Servlet container error page.

              <error-page>              <location>/error</location>              </error-page>

Note that the actual location for the error page can be a JSP page or some other URL within the container including one handled through an @Controller method:

When writing error information, the status code and the error message set on the HttpServletResponse can be accessed through request attributes in a controller:

                              @Controller                            public              class              ErrorController {                              @RequestMapping(value="/error", produces="application/json")                                            @ResponseBody                            public              Map<String, Object> handle(HttpServletRequest request) {          Map<String, Object> map =              new              HashMap<String, Object>();         map.put("status", request.getAttribute("javax.servlet.error.status_code"));         map.put("reason", request.getAttribute("javax.servlet.error.message"));              return              map;     }  }

or in a JSP:

              <%@              page              contentType="application/json"              pageEncoding="UTF-8"              %>              {              status:<%=request.getAttribute("javax.servlet.error.status_code")              %>,              reason:<%=request.getAttribute("javax.servlet.error.message")              %>              }

17.12 Convention over configuration support

For a lot of projects, sticking to established conventions and having reasonable defaults is just what they (the projects) need, and Spring Web MVC now has explicit support for convention over configuration . What this means is that if you establish a set of naming conventions and suchlike, you can substantially cut down on the amount of configuration that is required to set up handler mappings, view resolvers, ModelAndView instances, etc. This is a great boon with regards to rapid prototyping, and can also lend a degree of (always good-to-have) consistency across a codebase should you choose to move forward with it into production.

Convention-over-configuration support addresses the three core areas of MVC: models, views, and controllers.

17.12.1 The Controller `ControllerClassNameHandlerMapping`

The ControllerClassNameHandlerMapping class is a HandlerMapping implementation that uses a convention to determine the mapping between request URLs and the Controller instances that are to handle those requests.

Consider the following simple Controller implementation. Take special notice of the name of the class.

              public              class                              ViewShoppingCartController                            implements              Controller {              public              ModelAndView handleRequest(HttpServletRequest request, HttpServletResponse response) {                              // the implementation is not hugely important for this example...                            } }

Here is a snippet from the corresponding Spring Web MVC configuration file:

              <bean              class="org.springframework.web.servlet.mvc.support.ControllerClassNameHandlerMapping"              />              <bean              id="                              viewShoppingCart              " class="x.y.z.ViewShoppingCartController">                              <!-- inject dependencies as required... -->                            </bean>

The ControllerClassNameHandlerMapping finds all of the various handler (or Controller) beans defined in its application context and strips Controller off the name to define its handler mappings. Thus, ViewShoppingCartController maps to the /viewshoppingcart* request URL.

Let's look at some more examples so that the central idea becomes immediately familiar. (Notice all lowercase in the URLs, in contrast to camel-cased Controller class names.)

WelcomeController maps to the /welcome* request URL
HomeController maps to the /home* request URL
IndexController maps to the /index* request URL
RegisterController maps to the /register* request URL

In the case of MultiActionController handler classes, the mappings generated are slightly more complex. The Controller names in the following examples are assumed to be MultiActionController implementations:

AdminController maps to the /admin/* request URL
CatalogController maps to the /catalog/* request URL

If you follow the convention of naming your Controller implementations as xxx Controller , the ControllerClassNameHandlerMapping saves you the tedium of defining and maintaining a potentially looooong SimpleUrlHandlerMapping (or suchlike).

The ControllerClassNameHandlerMapping class extends the AbstractHandlerMapping base class so you can define HandlerInterceptor instances and everything else just as you would with many other HandlerMapping implementations.

17.12.2 The Model `ModelMap` (`ModelAndView`)

The ModelMap class is essentially a glorified Map that can make adding objects that are to be displayed in (or on) a View adhere to a common naming convention. Consider the following Controller implementation; notice that objects are added to the ModelAndView without any associated name specified.

              public              class              DisplayShoppingCartController              implements              Controller {              public              ModelAndView handleRequest(HttpServletRequest request, HttpServletResponse response) {          List cartItems =                              // get a List of CartItem objects                            User user =                              // get the User doing the shopping                            ModelAndView mav =              new              ModelAndView("displayShoppingCart");                              <-- the logical view name                            mav.addObject(cartItems);                              <-- look ma, no name, just the object                            mav.addObject(user);                              <-- and again ma!                            return              mav;     } }

The ModelAndView class uses a ModelMap class that is a custom Map implementation that automatically generates a key for an object when an object is added to it. The strategy for determining the name for an added object is, in the case of a scalar object such as User, to use the short class name of the object's class. The following examples are names that are generated for scalar objects put into a ModelMap instance.

An x.y.User instance added will have the name user generated.
An x.y.Registration instance added will have the name registration generated.
An x.y.Foo instance added will have the name foo generated.
A java.util.HashMap instance added will have the name hashMap generated. You probably want to be explicit about the name in this case because hashMap is less than intuitive.
Adding null will result in an IllegalArgumentException being thrown. If the object (or objects) that you are adding could be null, then you will also want to be explicit about the name.

The strategy for generating a name after adding a Set or a List is to peek into the collection, take the short class name of the first object in the collection, and use that with List appended to the name. The same applies to arrays although with arrays it is not necessary to peek into the array contents. A few examples will make the semantics of name generation for collections clearer:

An x.y.User[] array with zero or more x.y.User elements added will have the name userList generated.
An x.y.Foo[] array with zero or more x.y.User elements added will have the name fooList generated.
A java.util.ArrayList with one or more x.y.User elements added will have the name userList generated.
A java.util.HashSet with one or more x.y.Foo elements added will have the name fooList generated.
An empty java.util.ArrayList will not be added at all (in effect, the addObject(..) call will essentially be a no-op).

17.12.3 The View - `RequestToViewNameTranslator`

The RequestToViewNameTranslator interface determines a logical View name when no such logical view name is explicitly supplied. It has just one implementation, the DefaultRequestToViewNameTranslator class.

The DefaultRequestToViewNameTranslator maps request URLs to logical view names, as with this example:

              public              class              RegistrationController              implements              Controller {              public              ModelAndView handleRequest(HttpServletRequest request, HttpServletResponse response) {                              // process the request...                            ModelAndView mav =              new              ModelAndView();                              // add data as necessary to the model...                            return              mav;                              // notice that no View or logical view name has been set                            } }

              <?xml version="1.0" encoding="UTF-8"?>              <beans              xmlns="http://www.springframework.org/schema/beans"              xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"              xsi:schemaLocation="         http://www.springframework.org/schema/beans         http://www.springframework.org/schema/beans/spring-beans.xsd"              >                              <!-- this bean with the well known name generates view names for us -->                            <bean              id="viewNameTranslator"              class="org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator"              />              <bean              class="x.y.RegistrationController"              >                              <!-- inject dependencies as necessary -->                            </bean>                              <!-- maps request URLs to Controller names -->                            <bean              class="org.springframework.web.servlet.mvc.support.ControllerClassNameHandlerMapping"              />              <bean              id="viewResolver"              class="org.springframework.web.servlet.view.InternalResourceViewResolver"              >              <property              name="prefix"              value="/WEB-INF/jsp/"              />              <property              name="suffix"              value=".jsp"              />              </bean>              </beans>

Notice how in the implementation of the handleRequest(..) method no View or logical view name is ever set on the ModelAndView that is returned. The DefaultRequestToViewNameTranslator is tasked with generating a logical view name from the URL of the request. In the case of the above RegistrationController, which is used in conjunction with the ControllerClassNameHandlerMapping, a request URL of http://localhost/registration.html results in a logical view name of registration being generated by the DefaultRequestToViewNameTranslator. This logical view name is then resolved into the /WEB-INF/jsp/registration.jsp view by the InternalResourceViewResolver bean.

	Tip
	You do not need to define a `DefaultRequestToViewNameTranslator` bean explicitly. If you like the default settings of the `DefaultRequestToViewNameTranslator`, you can rely on the Spring Web MVC `DispatcherServlet` to instantiate an instance of this class if one is not explicitly configured.

Of course, if you need to change the default settings, then you do need to configure your own DefaultRequestToViewNameTranslator bean explicitly. Consult the comprehensive Javadoc for the DefaultRequestToViewNameTranslator class for details of the various properties that can be configured.

17.13 ETag support

An ETag (entity tag) is an HTTP response header returned by an HTTP/1.1 compliant web server used to determine change in content at a given URL. It can be considered to be the more sophisticated successor to the Last-Modified header. When a server returns a representation with an ETag header, the client can use this header in subsequent GETs, in an If-None-Match header. If the content has not changed, the server returns 304: Not Modified.

Support for ETags is provided by the Servlet filter ShallowEtagHeaderFilter. It is a plain Servlet Filter, and thus can be used in combination with any web framework. The ShallowEtagHeaderFilter filter creates so-called shallow ETags (as opposed to deep ETags, more about that later).The filter caches the content of the rendered JSP (or other content), generates an MD5 hash over that, and returns that as an ETag header in the response. The next time a client sends a request for the same resource, it uses that hash as the If-None-Match value. The filter detects this, renders the view again, and compares the two hashes. If they are equal, a 304 is returned. This filter will not save processing power, as the view is still rendered. The only thing it saves is bandwidth, as the rendered response is not sent back over the wire.

You configure the ShallowEtagHeaderFilter in web.xml:

            <filter>            <filter-name>etagFilter</filter-name>            <filter-class>org.springframework.web.filter.ShallowEtagHeaderFilter</filter-class>            </filter>            <filter-mapping>            <filter-name>etagFilter</filter-name>            <servlet-name>petclinic</servlet-name>            </filter-mapping>

17.14 Code-based Servlet container initialization

In a Servlet 3.0+ environment, you have the option of configuring the Servlet container programmatically as an alternative or in combination with a web.xml file. Below is an example of registering a DispatcherServlet:

            import            org.springframework.web.WebApplicationInitializer;            public            class            MyWebApplicationInitializer            implements            WebApplicationInitializer {                          @Override                        public            void            onStartup(ServletContext container) {         XmlWebApplicationContext appContext =            new            XmlWebApplicationContext();         appContext.setConfigLocation("/WEB-INF/spring/dispatcher-config.xml");          ServletRegistration.Dynamic registration = container.addServlet("dispatcher",            new            DispatcherServlet(appContext));         registration.setLoadOnStartup(1);         registration.addMapping("/");     }  }

WebApplicationInitializer is an interface provided by Spring MVC that ensures your implementation is detected and automatically used to initialize any Servlet 3 container. An abstract base class implementation of WebApplicationInitializer named AbstractDispatcherServletInitializer makes it even easier to register the DispatcherServlet by simply overriding methods to specify the servlet mapping and the location of the DispatcherServlet configuration:

            public            class            MyWebAppInitializer            extends            AbstractAnnotationConfigDispatcherServletInitializer {                          @Override                        protected            Class<?>[] getRootConfigClasses() {            return            null;     }                          @Override                        protected            Class<?>[] getServletConfigClasses() {            return            new            Class[] { MyWebConfig.class            };     }                          @Override                        protected            String[] getServletMappings() {            return            new            String[] {            "/"            };     }  }

The above example is for an application that uses Java-based Spring configuration. If using XML-based Spring configuration, extend directly from AbstractDispatcherServletInitializer:

            public            class            MyWebAppInitializer            extends            AbstractDispatcherServletInitializer {                          @Override                        protected            WebApplicationContext createRootApplicationContext() {            return            null;     }                          @Override                        protected            WebApplicationContext createServletApplicationContext() {         XmlWebApplicationContext cxt =            new            XmlWebApplicationContext();         cxt.setConfigLocation("/WEB-INF/spring/dispatcher-config.xml");            return            cxt;     }                          @Override                        protected            String[] getServletMappings() {            return            new            String[] {            "/"            };     }  }

AbstractDispatcherServletInitializer also provides a convenient way to add Filter instances and have them automatically mapped to the DispatcherServlet:

            public            class            MyWebAppInitializer            extends            AbstractDispatcherServletInitializer {                                @Override                        protected            Filter[] getServletFilters() {            return            new            Filter[] {            new            HiddenHttpMethodFilter(),            new            CharacterEncodingFilter() };     }  }

Each filter is added with a default name based on its concrete type and automatically mapped to the DispatcherServlet.

The isAsyncSupported protected method of AbstractDispatcherServletInitializer provides a single place to enable async support on the DispatcherServlet and all filters mapped to it. By default this flag is set to true.

17.15 Configuring Spring MVC

Section 17.2.1, "Special Bean Types In the WebApplicationContext" and Section 17.2.2, "Default DispatcherServlet Configuration" explained about Spring MVC's special beans and the default implementations used by the DispatcherServlet. In this section you'll learn about two additional ways of configuring Spring MVC. Namely the MVC Java config and the MVC XML namespace.

The MVC Java config and the MVC namespace provide similar default configuration that overrides the DispatcherServlet defaults. The goal is to spare most applications from having to having to create the same configuration and also to provide higher-level constructs for configuring Spring MVC that serve as a simple starting point and require little or no prior knowledge of the underlying configuration.

You can choose either the MVC Java config or the MVC namespace depending on your preference. Also as you will see further below, with the MVC Java config it is easier to see the underlying configuration as well as to make fine-grained customizations directly to the created Spring MVC beans. But let's start from the beginning.

17.15.1 Enabling the MVC Java Config or the MVC XML Namespace

To enable MVC Java config add the annotation @EnableWebMvc to one of your @Configuration classes:

                              @Configuration                                            @EnableWebMvc                            public              class              WebConfig {  }

To achieve the same in XML use the mvc:annotation-driven element:

              <?xml version="1.0" encoding="UTF-8"?>              <beans              xmlns="http://www.springframework.org/schema/beans"              xmlns:mvc="http://www.springframework.org/schema/mvc"              xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"              xsi:schemaLocation="         http://www.springframework.org/schema/beans         http://www.springframework.org/schema/beans/spring-beans.xsd         http://www.springframework.org/schema/mvc         http://www.springframework.org/schema/mvc/spring-mvc.xsd"              >              <mvc:annotation-driven />              </beans>

The above registers a RequestMappingHandlerMapping, a RequestMappingHandlerAdapter, and an ExceptionHandlerExceptionResolver (among others) in support of processing requests with annotated controller methods using annotations such as @RequestMapping , @ExceptionHandler, and others.

It also enables the following:

Spring 3 style type conversion through a ConversionService instance in addition to the JavaBeans PropertyEditors used for Data Binding.
Support for formatting Number fields using the @NumberFormat annotation through the ConversionService.
Support for formatting Date, Calendar, Long, and Joda Time fields using the @DateTimeFormat annotation.
Support for validating @Controller inputs with @Valid, if a JSR-303 Provider is present on the classpath.
HttpMessageConverter support for @RequestBody method parameters and @ResponseBody method return values from @RequestMapping or @ExceptionHandler methods.

This is the complete list of HttpMessageConverters set up by mvc:annotation-driven:
- ByteArrayHttpMessageConverter converts byte arrays.
- StringHttpMessageConverter converts strings.
- ResourceHttpMessageConverter converts to/from org.springframework.core.io.Resource for all media types.
- SourceHttpMessageConverter converts to/from a javax.xml.transform.Source.
- FormHttpMessageConverter converts form data to/from a MultiValueMap<String, String>.
- Jaxb2RootElementHttpMessageConverter converts Java objects to/from XML — added if JAXB2 is present on the classpath.
- MappingJackson2HttpMessageConverter (or MappingJacksonHttpMessageConverter) converts to/from JSON — added if Jackson 2 (or Jackson) is present on the classpath.
- AtomFeedHttpMessageConverter converts Atom feeds — added if Rome is present on the classpath.
- RssChannelHttpMessageConverter converts RSS feeds — added if Rome is present on the classpath.

17.15.2 Customizing the Provided Configuration

To customize the default configuration in Java you simply implement the WebMvcConfigurer interface or more likely extend the class WebMvcConfigurerAdapter and override the methods you need. Below is an example of some of the available methods to override. See WebMvcConifgurer for a list of all methods and the Javadoc for further details:

                              @Configuration                                            @EnableWebMvc                            public              class              WebConfig              extends              WebMvcConfigurerAdapter {                              @Override                            protected              void              addFormatters(FormatterRegistry registry) {        }                              @Override                            public              void              configureMessageConverters(List<HttpMessageConverter<?>> converters) {        }  }

To customize the default configuration of <mvc:annotation-driven /> check what attributes and sub-elements it supports. You can view the Spring MVC XML schema or use the code completion feature of your IDE to discover what attributes and sub-elements are available. The sample below shows a subset of what is available:

              <mvc:annotation-driven              conversion-service="conversionService"              >              <mvc:message-converters>              <bean              class="org.example.MyHttpMessageConverter"              />              <bean              class="org.example.MyOtherHttpMessageConverter"              />              </mvc:message-converters>              </mvc:annotation-driven>              <bean              id="conversionService"              class="org.springframework.format.support.FormattingConversionServiceFactoryBean"              >              <property              name="formatters"              >              <list>              <bean              class="org.example.MyFormatter"              />              <bean              class="org.example.MyOtherFormatter"              />              </list>              </property>              </bean>

17.15.3 Configuring Interceptors

You can configure HandlerInterceptors or WebRequestInterceptors to be applied to all incoming requests or restricted to specific URL path patterns.

An example of registering interceptors in Java:

                              @Configuration                                            @EnableWebMvc                            public              class              WebConfig              extends              WebMvcConfigurerAdapter {                              @Override                            public              void              addInterceptors(InterceptorRegistry registry) {     registry.addInterceptor(new              LocaleInterceptor());     registry.addInterceptor(new              ThemeInterceptor()).addPathPatterns("/**").excludePathPatterns("/admin/**");     registry.addInterceptor(new              SecurityInterceptor()).addPathPatterns("/secure/*");  }  }

And in XML use the <mvc:interceptors> element:

              <mvc:interceptors>              <bean              class="org.springframework.web.servlet.i18n.LocaleChangeInterceptor"                              />              <mvc:interceptor>              <mapping              path="/**"              />              <exclude-mapping              path="/admin/**"              />              <bean              class="org.springframework.web.servlet.theme.ThemeChangeInterceptor"                              />              </mvc:interceptor>              <mvc:interceptor>              <mapping              path="/secure/*"              />              <bean              class="org.example.SecurityInterceptor"                              />              </mvc:interceptor>              </mvc:interceptors>

17.15.4 Configuring Content Negotiation

Staring with Spring Framework 3.2, you can configure how Spring MVC determines the requested media types from the client for request mapping as well as for content negotiation purposes. The available options are to check the file extension in the request URI, the "Accept" header, a request parameter, as well as to fall back on a default content type. By default, file extension in the request URI is checked first and the "Accept" header is checked next.

For file extensions in the request URI, the MVC Java config and the MVC namespace, automatically register extensions such as .json, .xml, .rss, and .atom if the corresponding dependencies such as Jackson, JAXB2, or Rome are present on the classpath. Additional extensions may be not need to be registered explicitly if they can be discovered via ServletContext.getMimeType(String) or the Java Activation Framework (see javax.activation.MimetypesFileTypeMap).

Below is an example of customizing content negotiation options through the MVC Java config:

                              @Configuration                                            @EnableWebMvc                            public              class              WebConfig              extends              WebMvcConfigurerAdapter {                              @Override                            public              void              configureContentNegotiation(ContentNegotiationConfigurer configurer) {     configurer.favorPathExtension(false).favorParameter(true);   } }

In the MVC namespace, the <mvc:annotation-driven> element has a content-negotiation-manager attribute, which expects a ContentNegotiationManager that in turn can be created with a ContentNegotiationManagerFactoryBean:

              <mvc:annotation-driven              content-negotiation-manager="contentNegotiationManager"                              />              <bean              id="contentNegotiationManager"              class="org.springframework.web.accept.ContentNegotiationManagerFactoryBean"              >              <property              name="favorPathExtension"              value="false"                              />              <property              name="favorParameter"              value="true"                              />              <property              name="mediaTypes"                              >              <value>              json=application/json             xml=application/xml              </value>              </property>              </bean>

If not using the MVC Java config or the MVC namespace, you'll need to create an instance of ContentNegotiationManager and use it to configure RequestMappingHandlerMapping for request mapping purposes, and RequestMappingHandlerAdapter and ExceptionHandlerExceptionResolver for content negotiation purposes.

Note that ContentNegotiatingViewResolver now can also be configured with a ContentNegotiationManager, so you can use one instance throughout Spring MVC.

In more advanced cases, it may be useful to configure multiple ContentNegotiationManager instances that in turn may contain custom ContentNegotiationStrategy implementations. For example you could configure ExceptionHandlerExceptionResolver with a ContentNegotiationManager that always resolves the requested media type to "application/json". Or you may want to plug a custom strategy that has some logic to select a default content type (e.g. either XML or JSON) if no content types were requested.

17.15.5 Configuring View Controllers

This is a shortcut for defining a ParameterizableViewController that immediately forwards to a view when invoked. Use it in static cases when there is no Java controller logic to execute before the view generates the response.

An example of forwarding a request for "/" to a view called "home" in Java:

                              @Configuration                                            @EnableWebMvc                            public              class              WebConfig              extends              WebMvcConfigurerAdapter {                              @Override                            public              void              addViewControllers(ViewControllerRegistry registry) {     registry.addViewController("/").setViewName("home");   }  }

And the same in XML use the <mvc:view-controller> element:

              <mvc:view-controller              path="/"              view-name="home"              />

17.15.6 Configuring Serving of Resources

This option allows static resource requests following a particular URL pattern to be served by a ResourceHttpRequestHandler from any of a list of Resource locations. This provides a convenient way to serve static resources from locations other than the web application root, including locations on the classpath. The cache-period property may be used to set far future expiration headers (1 year is the recommendation of optimization tools such as Page Speed and YSlow) so that they will be more efficiently utilized by the client. The handler also properly evaluates the Last-Modified header (if present) so that a 304 status code will be returned as appropriate, avoiding unnecessary overhead for resources that are already cached by the client. For example, to serve resource requests with a URL pattern of /resources/** from a public-resources directory within the web application root you would use:

                              @Configuration                                            @EnableWebMvc                            public              class              WebConfig              extends              WebMvcConfigurerAdapter {                              @Override                            public              void              addResourceHandlers(ResourceHandlerRegistry registry) {     registry.addResourceHandler("/resources/**").addResourceLocations("/public-resources/");   }  }

And the same in XML:

              <mvc:resources              mapping="/resources/**"              location="/public-resources/"              />

To serve these resources with a 1-year future expiration to ensure maximum use of the browser cache and a reduction in HTTP requests made by the browser:

                              @Configuration                                            @EnableWebMvc                            public              class              WebConfig              extends              WebMvcConfigurerAdapter {                              @Override                            public              void              addResourceHandlers(ResourceHandlerRegistry registry) {     registry.addResourceHandler("/resources/**").addResourceLocations("/public-resources/").setCachePeriod(31556926);   }  }

And in XML:

              <mvc:resources              mapping="/resources/**"              location="/public-resources/"              cache-period="31556926"              />

The mapping attribute must be an Ant pattern that can be used by SimpleUrlHandlerMapping, and the location attribute must specify one or more valid resource directory locations. Multiple resource locations may be specified using a comma-separated list of values. The locations specified will be checked in the specified order for the presence of the resource for any given request. For example, to enable the serving of resources from both the web application root and from a known path of /META-INF/public-web-resources/ in any jar on the classpath use:

                              @EnableWebMvc                                            @Configuration                            public              class              WebConfig              extends              WebMvcConfigurerAdapter {                              @Override                            public              void              addResourceHandlers(ResourceHandlerRegistry registry) {     registry.addResourceHandler("/resources/**")       .addResourceLocations("/",              "classpath:/META-INF/public-web-resources/");   }  }

And in XML:

              <mvc:resources              mapping="/resources/**"              location="/, classpath:/META-INF/public-web-resources/"              />

When serving resources that may change when a new version of the application is deployed, it is recommended that you incorporate a version string into the mapping pattern used to request the resources, so that you may force clients to request the newly deployed version of your application's resources. Such a version string can be parameterized and accessed using SpEL so that it may be easily managed in a single place when deploying new versions.

As an example, let's consider an application that uses a performance-optimized custom build (as recommended) of the Dojo JavaScript library in production, and that the build is generally deployed within the web application at a path of /public-resources/dojo/dojo.js. Since different parts of Dojo may be incorporated into the custom build for each new version of the application, the client web browsers need to be forced to re-download that custom-built dojo.js resource any time a new version of the application is deployed. A simple way to achieve this would be to manage the version of the application in a properties file, such as:

application.version=1.0.0

and then to make the properties file's values accessible to SpEL as a bean using the util:properties tag:

              <util:properties              id="applicationProps"              location="/WEB-INF/spring/application.properties"              />

With the application version now accessible via SpEL, we can incorporate this into the use of the resources tag:

              <mvc:resources              mapping="/resources-#{applicationProps['application.version']}/**"              location="/public-resources/"              />

In Java, you can use the @PropertySouce annotation and then inject the Environment abstraction for access to all defined properties:

                              @Configuration                                            @EnableWebMvc                                            @PropertySource("/WEB-INF/spring/application.properties")                            public              class              WebConfig              extends              WebMvcConfigurerAdapter {                              @Inject                            Environment env;                              @Override                            public              void              addResourceHandlers(ResourceHandlerRegistry registry) {     registry.addResourceHandler("/resources-"              + env.getProperty("application.version") +              "/**")       .addResourceLocations("/public-resources/");   }  }

and finally, to request the resource with the proper URL, we can take advantage of the Spring JSP tags:

              <spring:eval              expression="@applicationProps['application.version']"              var="applicationVersion"              />              <spring:url              value="/resources-{applicationVersion}"              var="resourceUrl"              >              <spring:param              name="applicationVersion"              value="${applicationVersion}"              />              </spring:url>              <script              src="${resourceUrl}/dojo/dojo.js"              type="text/javascript"              >              </script>

17.15.7 mvc:default-servlet-handler

This tag allows for mapping the DispatcherServlet to "/" (thus overriding the mapping of the container's default Servlet), while still allowing static resource requests to be handled by the container's default Servlet. It configures a DefaultServletHttpRequestHandler with a URL mapping of "/**" and the lowest priority relative to other URL mappings.

This handler will forward all requests to the default Servlet. Therefore it is important that it remains last in the order of all other URL HandlerMappings. That will be the case if you use <mvc:annotation-driven> or alternatively if you are setting up your own customized HandlerMapping instance be sure to set its order property to a value lower than that of the DefaultServletHttpRequestHandler, which is Integer.MAX_VALUE.

To enable the feature using the default setup use:

                              @Configuration                                            @EnableWebMvc                            public              class              WebConfig              extends              WebMvcConfigurerAdapter {                              @Override                            public              void              configureDefaultServletHandling(DefaultServletHandlerConfigurer configurer) {     configurer.enable();   }  }

Or in XML:

              <mvc:default-servlet-handler/>

The caveat to overriding the "/" Servlet mapping is that the RequestDispatcher for the default Servlet must be retrieved by name rather than by path. The DefaultServletHttpRequestHandler will attempt to auto-detect the default Servlet for the container at startup time, using a list of known names for most of the major Servlet containers (including Tomcat, Jetty, GlassFish, JBoss, Resin, WebLogic, and WebSphere). If the default Servlet has been custom configured with a different name, or if a different Servlet container is being used where the default Servlet name is unknown, then the default Servlet's name must be explicitly provided as in the following example:

                              @Configuration                                            @EnableWebMvc                            public              class              WebConfig              extends              WebMvcConfigurerAdapter {                              @Override                            public              void              configureDefaultServletHandling(DefaultServletHandlerConfigurer configurer) {     configurer.enable("myCustomDefaultServlet");   }  }

Or in XML:

              <mvc:default-servlet-handler              default-servlet-name="myCustomDefaultServlet"              />

17.15.8 More Spring Web MVC Resources

See the following links and pointers for more resources about Spring Web MVC:

There are many excellent articles and tutorials that show how to build web applications with Spring MVC. Read them at the Spring Documentation page.
"Expert Spring Web MVC and Web Flow" by Seth Ladd and others (published by Apress) is an excellent hard copy source of Spring Web MVC goodness.

17.15.9 Advanced Customizations with MVC Java Config

As you can see from the above examples, MVC Java config and the MVC namespace provide higher level constructs that do not require deep knowledge of the underlying beans created for you. Instead it helps you to focus on your application needs. However, at some point you may need more fine-grained control or you may simply wish to understand the underlying configuration.

The first step towards more fine-grained control is to see the underlying beans created for you. In MVC Java config you can see the Javadoc and the @Bean methods in WebMvcConfigurationSupport. The configuration in this class is automatically imported through the @EnableWebMvc annotation. In fact if you open @EnableWebMvc you can see the @Import statement.

The next step towards more fine-grained control is to customize a property on one of the beans created in WebMvcConfigurationSupport or perhaps to provide your own instance. This requires two things -- remove the @EnableWebMvc annotation in order to prevent the import and then extend directly from WebMvcConfigurationSupport. Here is an example:

                              @Configuration                            public              class              WebConfig              extends              WebMvcConfigurationSupport {                              @Override                            public              void              addInterceptors(InterceptorRegistry registry){        }                              @Override                                            @Bean                            public              RequestMappingHandlerAdapter requestMappingHandlerAdapter() {                  }  }

Note that modifying beans in this way does not prevent you from using any of the higher-level constructs shown earlier in this section.

17.15.10 Advanced Customizations with the MVC Namespace

Fine-grained control over the configuration created for you is a bit harder with the MVC namespace.

If you do need to do that, rather than replicating the configuration it provides, consider configuring a BeanPostProcessor that detects the bean you want to customize by type and then modifying its properties as necessary. For example:

                              @Component                            public              class              MyPostProcessor              implements              BeanPostProcessor {              public              Object postProcessBeforeInitialization(Object bean, String name)              throws              BeansException {              if              (bean              instanceof              RequestMappingHandlerAdapter) {            }   }  }

Note that MyPostProcessor needs to be included in an <component scan /> in order for it to be detected or if you prefer you can declare it explicitly with an XML bean declaration.

Design Pattern Used by Struts 2 for Implementing Interceptors

Source: https://docs.spring.io/spring-framework/docs/3.2.x/spring-framework-reference/html/mvc.html

Design Pattern Used by Struts 2 for Implementing Interceptors

17. Web MVC framework

17.1 Introduction to Spring Web MVC framework

17.1.1 Features of Spring Web MVC

17.1.2 Pluggability of other MVC implementations

17.2 The DispatcherServlet

17.2.1 Special Bean Types In the WebApplicationContext

17.2.2 Default DispatcherServlet Configuration

17.2.3 DispatcherServlet Processing Sequence

17.3 Implementing Controllers

17.3.1 Defining a controller with @Controller

17.3.2 Mapping Requests With @RequestMapping

New Support Classes for @RequestMapping methods in Spring MVC 3.1

URI Template Patterns

URI Template Patterns with Regular Expressions

Path Patterns

Patterns with Placeholders

Matrix Variables

Consumable Media Types

Producible Media Types

Request Parameters and Header Values

17.3.3 Defining @RequestMapping handler methods

Supported method argument types

Supported method return types

Binding request parameters to method parameters with @RequestParam

Mapping the request body with the @RequestBody annotation

Mapping the response body with the @ResponseBody annotation

Using HttpEntity<?>

Using @ModelAttribute on a method

Using @ModelAttribute on a method argument

Using @SessionAttributes to store model attributes in the HTTP session between requests

Specifying redirect and flash attributes

Working with "application/x-www-form-urlencoded" data

Mapping cookie values with the @CookieValue annotation

Mapping request header attributes with the @RequestHeader annotation

Method Parameters And Type Conversion

Customizing WebDataBinder initialization

Customizing data binding with @InitBinder

Configuring a custom WebBindingInitializer

Customizing data binding with externalized @InitBinder methods

Support for the 'Last-Modified' Response Header To Facilitate Content Caching

17.3.4 Asynchronous Request Processing

Exception Handling for Async Requests

Intercepting Async Requests

Configuration for Async Request Processing

Servlet 3 Async Config

Spring MVC Async Config

17.4 Handler mappings

17.4.1 Intercepting requests with a HandlerInterceptor

17.5 Resolving views

17.5.1 Resolving views with the ViewResolver interface

17.5.2 Chaining ViewResolvers

17.5.3 Redirecting to views

RedirectView

The redirect: prefix

The forward: prefix

17.5.4ContentNegotiatingViewResolver

17.6 Using flash attributes

17.7 Building URIs

17.8 Using locales

17.8.1AcceptHeaderLocaleResolver

17.8.2CookieLocaleResolver

17.8.3SessionLocaleResolver

17.8.4LocaleChangeInterceptor

17.9 Using themes

17.9.1 Overview of themes

17.9.2 Defining themes

17.9.3 Theme resolvers

17.10 Spring's multipart (file upload) support

17.10.1 Introduction

17.10.2 Using a MultipartResolver with Commons FileUpload

17.10.3 Using a MultipartResolver with Servlet 3.0

17.10.4 Handling a file upload in a form

17.10.5 Handling a file upload request from programmatic clients

17.11 Handling exceptions

17.11.1HandlerExceptionResolver

17.11.2@ExceptionHandler

17.11.3 Handling Standard Spring MVC Exceptions

17.11.4 Annotating Business Exceptions With @ResponseStatus

17.11.5 Customizing the Default Servlet Container Error Page

17.2 The `DispatcherServlet`

17.2.1 Special Bean Types In the `WebApplicationContext`

17.3.1 Defining a controller with `@Controller`

17.3.2 Mapping Requests With `@RequestMapping`

New Support Classes for `@RequestMapping` methods in Spring MVC 3.1

17.3.3 Defining `@RequestMapping` handler methods

Binding request parameters to method parameters with `@RequestParam`

Mapping the response body with the `@ResponseBody` annotation

Using `HttpEntity<?>`

Using `@ModelAttribute` on a method

Using `@ModelAttribute` on a method argument

Using `@SessionAttributes` to store model attributes in the HTTP session between requests

Working with `"application/x-www-form-urlencoded"` data

Customizing `WebDataBinder` initialization

Customizing data binding with `@InitBinder`

Configuring a custom `WebBindingInitializer`

Customizing data binding with externalized `@InitBinder` methods

17.4.1 Intercepting requests with a `HandlerInterceptor`

17.5.1 Resolving views with the `ViewResolver` interface

`RedirectView`

The `redirect:` prefix

The `forward:` prefix

17.5.4`ContentNegotiatingViewResolver`

17.7 Building `URI`s

17.8.1`AcceptHeaderLocaleResolver`

17.8.2`CookieLocaleResolver`

17.8.3`SessionLocaleResolver`

17.8.4`LocaleChangeInterceptor`

17.10.2 Using a `MultipartResolver` with Commons FileUpload

17.10.3 Using a `MultipartResolver` with Servlet 3.0

17.11.1`HandlerExceptionResolver`

17.11.2`@ExceptionHandler`

17.11.4 Annotating Business Exceptions With `@ResponseStatus`

17.12.1 The Controller `ControllerClassNameHandlerMapping`

17.12.2 The Model `ModelMap` (`ModelAndView`)

17.12.3 The View - `RequestToViewNameTranslator`