16.5 Views and resolving them

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. The section entitled Chapter 17, View technologies has details of how to integrate and use a number of disparate view technologies.

The two interfaces which 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.

16.5.1 Resolving views - the ViewResolver interface

As discussed in the section entitled Section 16.3, “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. We'll list most of them and then provide a couple of examples.

Table 16.4. View resolvers

AbstractCachingViewResolverAn abstract view resolver which takes care of caching views. Often views need preparation before they can be used, extending this view resolver provides caching of views.
XmlViewResolverAn 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.
ResourceBundleViewResolverAn implementation of ViewResolver that uses bean definitions in a ResourceBundle, specified by the bundle basename. The bundle is typically defined in a properties file, located in the classpath. The default file name is views.properties.
UrlBasedViewResolverA simple implementation of the ViewResolver interface that effects the direct resolution of symbolic view names to URLs, without an explicit mapping definition. This is appropriate if your symbolic names match the names of your view resources in a straightforward manner, without the need for arbitrary mappings.
InternalResourceViewResolverA convenience subclass of UrlBasedViewResolver that supports InternalResourceView (i.e. Servlets and JSPs), and subclasses such as JstlView and TilesView. The view class for all views generated by this resolver can be specified via setViewClass(..). See the Javadocs for the UrlBasedViewResolver class for details.
VelocityViewResolver / FreeMarkerViewResolverA convenience subclass of UrlBasedViewResolver that supports VelocityView (i.e. Velocity templates) or FreeMarkerView respectively and custom subclasses of them.

As an example, when using JSP for 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"
    <property name="viewClass" value="org.springframework.web.servlet.view.JstlView"/>
    <property name="prefix" value="/WEB-INF/jsp/"/>
    <property name="suffix" value=".jsp"/>

When returning test as a viewname, this view resolver will hand the request over to the RequestDispatcher that will send the request to /WEB-INF/jsp/test.jsp.

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

<bean id="viewResolver"
    <property name="basename" value="views"/>
    <property name="defaultParentView" value="parentView"/>

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.

A note on caching - subclasses of AbstractCachingViewResolver cache view instances they have resolved. This greatly improves performance when using certain view technologies. It's possible to turn off the cache, by setting the cache property to false. Furthermore, if you have the requirement to be able to refresh a certain view at runtime (for example when a Velocity template has been modified), you can use the removeFromCache(String viewName, Locale loc) method.

16.5.2 Chaining ViewResolvers

Spring supports more than just one view resolver. This allows you to chain resolvers and, for example, override specific views in certain circumstances. Chaining view resolvers is pretty straightforward - just add more than one resolver to your application context and, if necessary, set the order property to specify an order. Remember, the higher the order property, the later the view resolver will be 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 (which 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 id="excelViewResolver" class="org.springframework.web.servlet.view.XmlViewResolver">
  <property name="order" value="1"/>
  <property name="location" value="/WEB-INF/views.xml"/>

<!-- in views.xml -->

  <bean name="report" class="org.springframework.example.ReportExcelView"/>

If a specific view resolver does not result in a view, Spring will inspect the context to see if other view resolvers are configured. If there are additional view resolvers, it will continue to inspect them. If not, it will throw an Exception.

You have to keep something else in mind - the contract of a view resolver mentions that a view resolver can return null to indicate the view could not be found. Not all view resolvers do this however! This is 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 - this can only be done once. The same holds for the VelocityViewResolver and some others. Check the Javadoc for the view resolver to see if you're dealing with a view resolver that does not report non-existing views. As a result of this, putting an InternalResourceViewResolver in the chain in a place other than the last, will result in the chain not being fully inspected, since the InternalResourceViewResolver will always return a view!

16.5.3 Redirecting to views

As has been mentioned, a controller normally returns a logical view name, which a view resolver resolves to a particular view technology. For view technologies such as JSPs that are actually processed via the Servlet/JSP engine, this is normally handled via InternalResourceViewResolver / InternalResourceView which will ultimately end up issuing an internal forward or include, via the Servlet API's RequestDispatcher.forward(..) or RequestDispatcher.include(). For other view technologies, such as Velocity, XSLT, etc., 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 would just force a GET of the result page, not a resend of the initial POST data. RedirectView

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

The RedirectView simply ends up issuing an HttpServletResponse.sendRedirect() call, which will come back to the client browser as an HTTP redirect. All model attributes are simply exposed as HTTP query parameters. This does mean that the model must contain only objects (generally Strings or convertible to Strings) which can be readily converted to a string-form HTTP query parameter.

If using RedirectView and the view is created by the controller itself, it is preferable for 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 redirect: prefix

While the use of RedirectView works fine, if the controller itself is creating the RedirectView, there is no getting around 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... it should generally think only in terms of view names that have been injected into it.

The special redirect: prefix allows this to be achieved. If a view name is returned which has the prefix redirect:, then 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 deal just 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. The forward: prefix

It is also possible to use a special forward: prefix for view names that will ultimately be resolved by UrlBasedViewResolver and subclasses. All this does is create an InternalResourceView (which ultimately does a RequestDispatcher.forward()) around the rest of the view name, which is considered a URL. Therefore, there is never any use in using this prefix when using InternalResourceViewResolver / InternalResourceView anyway (for JSPs for example), but it's of potential use when you are primarily using another view technology, but still want to force a forward to happen to 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 prefix is just injected into the controller, the controller does not have to be aware that anything special is happening in terms of handling the response.