As discussed in Section 15.3, “Implementing Controllers”, all controllers in the Spring Web MVC framework return a ModelAndView instance. Views in Spring are addressed by a 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.

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 viewname, 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. As you can see, you can identify a parent view, from which all views in the properties file sort of 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.

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 an order. 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, a 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. If they do not, it throws an Exception.

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!

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 InternalResourceViewResolver / InternalResourceView, which issues an internal forward or include, through the Servlet API's RequestDispatcher.forward(..) or RequestDispatcher.include(). For other view technologies, such as Velocity, XSLT, and so on, the view itself produces the content on the response stream.

It is sometimes desirable to issue an HTTP redirect back to the client, before the view is rendered. This is desirable for example when one controller has been called with POSTed data, and the response is actually a delegation to another controller (for example on a successful form submission). In this case, a normal internal forward will mean 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 do a redirect before displaying the result is that this will eliminate the possibility of the user doing a double submission of form data. The browser will have sent the initial POST, will have seen a redirect back and done a subsequent GET because of that, and thus as far as it is concerned, the current page does not reflect the result of a POST, but rather of a GET, so there is no way the user can accidentally re-POST the same data by doing a refresh. The refresh forces a GET of the result page, not a resend of the initial POST data.

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:

	Note
	One issue with the Accept header is that is impossible to change it in a web browser, in 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.

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. The Accept header may include wildcards, for example text/*, in which case a View whose Context-Type was text/xml is a compatible match.

To support the resolution of a view based on a file extension, you use the ContentNegotiatingViewResolver bean property MediaTypes to specify a mapping of file extensions to media types. For more information on the algorithm 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"/>
    </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>
</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 this 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. Alternatively, client requests can be made without a file extension and setting the Accept header 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, then 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;
	  }

  }