15. Web MVC framework
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## 15. Web MVC framework

## 15.1 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` implementation can even write directly to the response stream. Typically, a `ModelAndView` instance consists of a view name and a model `Map`, which contains bean names and corresponding objects such as a command or form, which contain reference data. View name resolution is highly configurable, through bean names, a properties file, or your own `ViewResolver` implementation. The model (the M in MVC) is based on the `Map` interface, which allows for the complete abstraction of the view technology. You can integrate directly JSP, Velocity, or any other rendering technology. The model `Map` is simply transformed into an appropriate format, such as JSP request attributes or a Velocity template model.

### 15.1.1 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 F, 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 G, 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 3.5.4, “Request, session, and global session scopes”

### 15.1.2 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.

## 15.2 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 requesting processing workflow in Spring Web MVC (high level)

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 J2EE 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>*.form</url-pattern>
        </servlet-mapping>

    </web-app>

In the preceding example, all requests ending with `.form` will be handled by the `example` `DispatcherServlet`. This 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 3.13, “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.

Context hierarchy in Spring Web MVC

Upon initialization of a `DispatcherServlet`, the framework 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 15.7, “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.

The Spring `DispatcherServlet` uses special beans to process requests and render the appropriate views. These beans are part of Spring Framework. You can configure them in the `WebApplicationContext`, just as you configure any other bean. However, for most beans, sensible defaults are provided so you initially do not need to configure them. These beans are described in the following table.

Table 15.1. Special beans in the `WebApplicationContext`

Bean type | Explanation
--- | ---
controllers | Form the `C` part of the MVC.
handler mappings | Handle the execution of a list of pre-processors and post-processors and controllers that will be executed if they match certain criteria (for example, a matching URL specified with the controller).
view resolvers | Resolves view names to views.
locale resolver | A locale resolver is a component capable of resolving the locale a client is using, in order to be able to offer internationalized views
Theme resolver | A theme resolver is capable of resolving themes your web application can use, for example, to offer personalized layouts
multipart file resolver | Contains functionality to process file uploads from HTML forms.
handler exception resolvers | Contains functionality to map exceptions to views or implement other more complex exception handling code.

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

1. 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`.

2. 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.

3. 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.

4. 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 15.8.2, “Using the MultipartResolver” for further information about multipart handling).

5. 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.

6. 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 15.2. `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`.

## 15.3 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, the PetClinic web application leverages the annotation support described in this section, in the context of simple form processing.

    @Controller
    public class HelloWorldController {

        @RequestMapping("/helloWorld")
        public ModelAndView helloWorld() {
            ModelAndView mav = new ModelAndView();
            mav.setViewName("helloWorld");
            mav.addObject("message", "Hello World!");
            return mav;
        }
    }

As you can see, the `@Controller` and `@RequestMapping` annotations allow flexible method names and signatures. In this particular example the method has no parameters and returns a `ModelAndView`, but various other (and better) strategies exist, as are explained later in this section. `ModelAndView`, `@Controller`, and `@RequestMapping` form the basis for the Spring MVC implementation. This section documents these annotations and how they are most commonly used in a Servlet environment.

### 15.3.1 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-3.0.xsd
            http://www.springframework.org/schema/context
            http://www.springframework.org/schema/context/spring-context-3.0.xsd">

        <context:component-scan base-package="org.springframework.samples.petclinic.web"/>

        // ...

    </beans>

### 15.3.2 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") or specific HTTP request parameters.

The following example 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());
        }

    }

| Working with interface-based @Controller classes
A common pitfall when working with annotated controller classes happens when applying functionality that requires creating a proxy 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 to the interface as as the mapping mechanism can only "see" the interface exposed by the proxy. As an alternative, you may choose to 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 7.6, “Proxying mechanisms”.

#### 15.3.2.1 URI Templates

To access parts of a request URL in your handling methods, use the URI templates in the `@RequestMapping` path value.

Use the `@PathVariable` method parameter annotation to indicate that a method parameter should be bound to the value of a URI template variable.

The following code snippet shows the usage of a single `@PathVariable` in a controller method:

    @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 in the request URI. For example, when a request comes in for /owners/fred, the value fred is bound to the method parameter `String ownerId`.

The matching of method parameter names to URI Template variable names can only be done if your code is compiled with debugging enabled. If you do not have debugging enabled, you must specify the name of the URI Template variable name in the @PathVariable annotation in order to bind the resolved value of the variable name to a method parameter. For example:

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

You can also use a controller method with the following signature:

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

You can use multiple @PathVariable annotations to bind to multiple URI Template variables:

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

The following code snippet shows the usage of path variables on a relative path, so that the `findPet()` method will be invoked for `/owners/42/pets/21`, for instance.

    @Controller
    @RequestMapping("/owners/{ownerId}")
    public class RelativePathUriTemplateController {

      @RequestMapping("/pets/{petId}")
      public void findPet(@PathVariable String ownerId, @PathVariable String petId, Model model) {
        // implementation omitted
      }
    }

| Tip
Method parameters that are decorated with the `@PathVariable` annotation can be of any simple type such as int, long, Date, etc. Spring automatically converts to the appropriate type and throws a `TypeMismatchException` if the type is not correct. You can further customize this conversion process by customizing the data binder. See Section 15.3.2.12, “Customizing WebDataBinder initialization”.

#### 15.3.2.2 Advanced `@RequestMapping` options

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}`).

The handler method names are taken into account for narrowing if no path was specified explicitly, according to the specified `org.springframework.web.servlet.mvc.multiaction.MethodNameResolver` (by default an `org.springframework.web.servlet.mvc.multiaction.InternalPathMethodNameResolver`). This only applies if annotation mappings do not specify a path mapping explicitly. In other words, the method name is only used for narrowing among a set of matching methods; it does not constitute a primary path mapping itself.

If you have a single default method (without explicit path mapping), then all requests without a more specific mapped method found are dispatched to it. If you have multiple such default methods, then the method name is taken into account for choosing between them.

You can narrow path mappings through parameter conditions: a sequence of "myParam=myValue" style expressions, with a request only mapped if each such parameter is found to have the given value. For example:

    @Controller
    @RequestMapping("/owners/{ownerId}")
    public class RelativePathUriTemplateController {

      @RequestMapping(value = "/pets/{petId}", params="myParam=myValue")
      public void findPet(@PathVariable String ownerId, @PathVariable String petId, Model model) {
        // implementation omitted
      }
    }

"myParam" style expressions are also supported, with such parameters having to be present in the request (allowed to have any value). Finally, "!myParam" style expressions indicate that the specified parameter is not supposed to be present in the request.

Similarly, path mappings can be narrowed down through header conditions:

    @Controller
    @RequestMapping("/owners/{ownerId}")
    public class RelativePathUriTemplateController {

    @RequestMapping(value = "/pets", method = RequestMethod.POST, headers="content-type=text/*")
      public void addPet(Pet pet, @PathVariable String ownerId) {
        // implementation omitted
      }
    }

In the above example, the `addPet()` method is only invoked when the `content-type` matches the `text/*` pattern, for example, `text/xml`.

#### 15.3.2.3 Supported handler method arguments and return types

Handler methods that are annotated with `@RequestMapping` can have very flexible signatures. Most of them can be used in arbitrary order (see below for more details).

- 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 `AnnotationMethodHandlerAdapter`'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 Section 15.3.2.1, “URI Templates”.

- `@RequestParam` annotated parameters for access to specific Servlet request parameters. Parameter values are converted to the declared method argument type. See Section 15.3.2.4, “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 `HttpMessageConverter`s. See Section 15.3.2.5, “Mapping the request body with the @RequestBody annotation”.

- `HttpEntity<?>` parameters for access to the Servlet request HTTP headers and contents. The request stream will be converted to the entity body using `HttpMessageConverter`s. See Section 15.3.2.7, “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.

- Command or form objects to bind parameters to: as bean properties or fields, with customizable type conversion, depending on `@InitBinder` methods and/or the HandlerAdapter configuration. See the `webBindingInitializer` property on `AnnotationMethodHandlerAdapter`. Such command objects along with their validation results will be exposed as model attributes by default, using the non-qualified command class name in property notation. For example, "orderAddress" for type "mypackage.OrderAddress". Specify a parameter-level `ModelAttribute` annotation for declaring a specific model attribute name.

- `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.

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 15.1. 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) { … }

The following return types are supported for handler methods:

- 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 `HttpMessageConverter`s. See Section 15.3.2.6, “Mapping the response body with the @ResponseBody annotation”.

- A `HttpEntity<?>` or `ResponseEntity<?>` object to provide access to the Servlet reponse HTTP headers and contents. The entity body will be converted to the response stream using `HttpMessageConverter`s. See Section 15.3.2.7, “Using HttpEntity<?>”.

- 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.

#### 15.3.2.4 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)`).

#### 15.3.2.5 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. `DispatcherServlet` supports annotation based processing using the `DefaultAnnotationHandlerMapping` and `AnnotationMethodHandlerAdapter`. In Spring 3.0 the `AnnotationMethodHandlerAdapter` is extended to support the `@RequestBody` and has the following `HttpMessageConverters` registered by default:

- `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.

- `MarshallingHttpMessageConverter` converts to/from an object using the `org.springframework.oxm` package.

For more information on these converters, see Message Converters.

The `MarshallingHttpMessageConverter` requires a `Marshaller` and `Unmarshaller` from the `org.springframework.oxm` package to be configured on an instance of `AnnotationMethodHandlerAdapter` in the application context. For example:

    <bean class="org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter">
        <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"/>

#### 15.3.2.6 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.

#### 15.3.2.7 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();
      // do something with request header and body

      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.

#### 15.3.2.8 Providing a link to data from the model with `@ModelAttribute`

`@ModelAttribute` has two usage scenarios in controllers. When you place it on a method parameter, `@ModelAttribute` maps a model attribute to the specific, annotated method parameter (see the `processSubmit()` method below). This is how the controller gets a reference to the object holding the data entered in the form.

You can also use `@ModelAttribute` at the method level to provide reference data for the model (see the `populatePetTypes()` method in the following example). For this usage the method signature can contain the same types as documented previously for the `@RequestMapping` annotation.

| Note
`@ModelAttribute` annotated methods are executed before the chosen `@RequestMapping` annotated handler method. They effectively pre-populate the implicit model with specific attributes, often loaded from a database. Such an attribute can then already be accessed through `@ModelAttribute` annotated handler method parameters in the chosen handler method, potentially with binding and validation applied to it.

The following code snippet shows these two usages of this annotation:

    @Controller
    @RequestMapping("/owners/{ownerId}/pets/{petId}/edit")
    @SessionAttributes("pet")
    public class EditPetForm {

        // ...

        @ModelAttribute("types")
        public Collection<PetType> populatePetTypes() {
            return this.clinic.getPetTypes();
        }

        @RequestMapping(method = RequestMethod.POST)
        public String processSubmit(
                @ModelAttribute("pet") Pet pet,
                BindingResult result, SessionStatus status) {

            new PetValidator().validate(pet, result);
            if (result.hasErrors()) {
                return "petForm";
            }
            else {
                this.clinic.storePet(pet);
                status.setComplete();
                return "redirect:owner.do?ownerId=" + pet.getOwner().getId();
            }
        }

    }

#### 15.3.2.9 Specifying attributes to store in a session with `@SessionAttributes`

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.

#### 15.3.2.10 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)  {

      //...

    }

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

#### 15.3.2.11 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)  {

      //...

    }

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

#### 15.3.2.12 Customizing `WebDataBinder` initialization

To customize request parameter binding with PropertyEditors through Spring's `WebDataBinder`, you can use either `@InitBinder`-annotated methods within your controller or externalize your configuration by providing 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.annotation.AnnotationMethodHandlerAdapter">
        <property name="cacheSeconds" value="0" />
        <property name="webBindingInitializer">
            <bean class="org.springframework.samples.petclinic.web.ClinicBindingInitializer" />
        </property>
    </bean>

## 15.4 Handler mappings

In previous versions of Spring, users were required to define `HandlerMapping`s in the web application context to map incoming web requests to appropriate handlers. With the introduction of Spring 2.5, the `DispatcherServlet` enables the `DefaultAnnotationHandlerMapping`, which looks for `@RequestMapping` annotations on `@Controllers`. Typically, you do not need to override this default mapping, unless you need to override the default property values. These properties are:

`interceptors`

  List of interceptors to use. `HandlerInterceptor`s are discussed in Section 15.4.1, “Intercepting requests - the HandlerInterceptor interface”.

`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.

`lazyInitHandlers`

  Allows lazy initialization of singleton handlers (prototype handlers are always lazy-initialized). The default value is `false`.

| Note
The `alwaysUseFullPath`, `urlDecode`, and `lazyInitHandlers` properties are only available to subclasses of `org.springframework.web.servlet.handler.AbstractUrlHandlerMapping`.

The following example shows how to override the default mapping and add an interceptor:

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

    <beans>

### 15.4.1 Intercepting requests - the `HandlerInterceptor` interface

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: one is called before the actual handler is executed; one is called after the handler is executed; and one 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.

The following example defines a handler mapping which maps all requests matching the URL patterns "/*.form" and "/*.view" to a particular controller, `editAccountFormController`. An interceptor has been added that intercepts these requests and reroutes the user to a specific page if the time is not between 9 a.m. and 6 p.m.

    <beans>
        <bean id="handlerMapping"
              class="org.springframework.web.servlet.handler.SimpleUrlHandlerMapping">
            <property name="interceptors">
                <list>
                    <ref bean="officeHoursInterceptor"/>
                </list>
            </property>
            <property name="mappings">
                <value>
                    /*.form=editAccountFormController
                    /*.view=editAccountFormController
                </value>
            </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 < 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.

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

## 15.5 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 16, 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.

### 15.5.1 Resolving views with the `ViewResolver` interface

As discussed in Section 15.3, “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 15.3. 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 15.5.4, “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.

### 15.5.2 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!

### 15.5.3 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 `POST`ed 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.

#### 15.5.3.1 `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. All model attributes are exposed as HTTP query parameters. This means that the model must contain only objects (generally Strings or objects converted to a String representation), which can be readily converted to a textual HTTP query parameter.

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.

#### 15.5.3.2 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:/my/response/controller.html` will redirect relative to the current servlet context, while a name such as `redirect:http://myhost.com/some/arbitrary/path.html` will redirect to an absolute URL. The important thing is that, as long as this redirect view name is injected into the controller like any other logical view name, the controller is not even aware that redirection is happening.

#### 15.5.3.3 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.

### 15.5.4 `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 wildcards, 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.MappingJacksonJsonView" />
        </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 `MappingJacksonJsonView` instance 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;
        }

    }

## 15.6 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 15.4.1, “Intercepting requests - the HandlerInterceptor interface” 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.

### 15.6.1 `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.

### 15.6.2 `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 15.4. `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.

### 15.6.3 `SessionLocaleResolver`

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

### 15.6.4 `LocaleChangeInterceptor`

You can enable changing of locales by adding the `LocaleChangeInterceptor` to one of the handler mappings (see Section 15.4, “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>

## 15.7 Using themes

### 15.7.1 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.

### 15.7.2 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.

### 15.7.3 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 15.5. `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.

## 15.8 Spring's multipart (fileupload) support

### 15.8.1 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 a `MultipartResolver` for use with Commons FileUpload).

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.

### 15.8.2 Using the `MultipartResolver`

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.

### 15.8.3 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 FileUpoadController {

        @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.

Finally, you will have to declare the controller and the resolver in the application context:

    <beans>
        <bean id="multipartResolver"
            class="org.springframework.web.multipart.commons.CommonsMultipartResolver"/>
        <!-- Declare explicitly, or use <context:annotation-config/> -->
        <bean id="fileUploadController" class="examples.FileUploadController"/>

    </beans>

## 15.9 Handling exceptions

### 15.9.1 `HandlerExceptionResolver`

Spring `HandlerExceptionResolvers` ease the pain of unexpected exceptions that occur while your request is handled by a controller that matched the request. `HandlerExceptionResolvers` somewhat resemble the exception mappings you can define in the web application descriptor `web.xml`. However, they provide a more flexible way to handle exceptions. 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 `SimpleMappingExceptionResolver`. This resolver 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.

By default, the `DispatcherServlet` registers the `DefaultHandlerExceptionResolver`. This resolver handles certain standard Spring MVC exceptions by setting a specific response status code:

Exception | HTTP Status Code
--- | ---
`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)
`MissingServletRequestParameterException` | 400 (Bad Request)
`NoSuchRequestHandlingMethodException` | 404 (Not Found)
`TypeMismatchException` | 400 (Bad Request)

### 15.9.2 `@ExceptionHandler`

An alternative to the `HandlerExceptionResolver` interface is the `@ExceptionHandler` annotation. You use the `@ExceptionHandler` method annotation within a controller to specify which method is invoked when an exception of a specific type is thrown during the execution of controller methods. For example:

    @Controller
    public class SimpleController {

      // other controller method omitted

      @ExceptionHandler(IOException.class)
      public String handleIOException(IOException ex, HttpServletRequest request) {
        return ClassUtils.getShortName(ex.getClass());
      }
    }

will invoke the 'handlerIOException' method when a `java.io.IOException` is thrown.

The `@ExceptionHandler` value can be set to an array of Exception types. If an exception is thrown 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 are very 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 or a `ModelAndView` object. Refer to the API documentation for more details.

## 15.10 Convention over configuration support

For a lot of projects, sticking to established conventions and having reasonable defaults is just what they (the projects) need... this theme of convention-over-configuration now has explicit support in Spring Web MVC. 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.

### 15.10.1 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 attendent 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.

### 15.10.2 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`, `List` or array object 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. Some examples will make the semantics of name generation for collections clearer...

- An `x.y.User[]` array with one or more `x.y.User` elements added will have the name `userList` generated.

- An `x.y.Foo[]` array with one 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).

### 15.10.3 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"?>
    <!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN 2.0//EN"
            "http://www.springframework.org/dtd/spring-beans-2.0.dtd">
    <beans>

        <!-- 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.

## 15.11 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>

## 15.12 Configuring Spring MVC

Spring 3 introduces a `mvc` XML configuration namespace that simplifies the setup of Spring MVC inside your web application. Instead of registering low-level beans such as AnnotationMethodHandlerAdapter, you can simply use the namespace and its higher-level constructs. This is generally preferred unless you require finer-grained control of the configuration at the bean level.

The mvc namespace consists of three tags: mvc:annotation-driven, mvc:interceptors, and mvc:view-controller. Each of these tags is documented below and in the XML schema.

### 15.12.1 mvc:annotation-driven

This tag registers the DefaultAnnotationHandlerMapping and AnnotationMethodHandlerAdapter beans that are required for Spring MVC to dispatch requests to @Controllers. The tag configures those two beans with sensible defaults based on what is present in your classpath. The defaults are:

1. Support for Spring 3's Type ConversionService in addition to JavaBeans PropertyEditors during Data Binding. A ConversionService instance produced by the `org.springframework.format.support.FormattingConversionServiceFactoryBean` is used by default. This can be overriden by setting the `conversion-service` attribute.

2. Support for formatting Number fields using the @NumberFormat annotation

3. Support for formatting Date, Calendar, Long, and Joda Time fields using the @DateTimeFormat annotation, if Joda Time 1.3 or higher is present on the classpath.

4. Support for validating @Controller inputs with @Valid, if a JSR-303 Provider is present on the classpath. The validation system can be explicitly configured by setting the `validator` attribute.

5. Support for reading and writing XML, if JAXB is present on the classpath.

6. Support for reading and writing JSON, if Jackson is present on the classpath.

A typical usage is shown below:

    <?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-3.0.xsd
            http://www.springframework.org/schema/mvc
            http://www.springframework.org/schema/mvc/spring-mvc-3.0.xsd">

        <!-- JSR-303 support will be detected on classpath and enabled automatically -->
        <mvc:annotation-driven/>

    </beans>

### 15.12.2 mvc:interceptors

This tag allows you to register custom HandlerInterceptors or WebRequestInterceptors that should be applied to all HandlerMapping beans. You can also restrict the URL paths specifc interceptors apply to.

An example of registering an interceptor applied to all URL paths:

    <mvc:interceptors>
        <bean class="org.springframework.web.servlet.i18n.LocaleChangeInterceptor" />
    </mvc:interceptors>

An example of registering an interceptor limited to a specific URL path:

    <mvc:interceptors>
        <mvc:interceptor>
            <mapping path="/secure/*"/>
            <bean class="org.example.SecurityInterceptor" />
        </mvc:interceptor>
    </mvc:interceptors>

### 15.12.3 mvc:view-controller

This tag is a shorcut 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 view-controller that forwards to a home page is shown below:

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

### 15.12.4 mvc:resources

This tag 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, the tag would be used as follows:

    <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:

    <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-seperated 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, the tag would be specified as:

    <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/"/>

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>

### 15.12.5 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 (given a lowest precedence order) of "/**". This handler will forward all requests to the default Servlet. To enable this feature using the default setup, simply include the tag in the form:

    <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:

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

## 15.13 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.

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