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 typeExplanation
controllersForm the C part of the MVC.
handler mappingsHandle 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 resolversResolves view names to views.
locale resolverA locale resolver is a component capable of resolving the locale a client is using, in order to be able to offer internationalized views
Theme resolverA theme resolver is capable of resolving themes your web application can use, for example, to offer personalized layouts
multipart file resolverContains 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

ParameterExplanation
contextClassClass that implements WebApplicationContext, which instantiates the context used by this servlet. By default, the XmlWebApplicationContext is used.
contextConfigLocationString 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.
namespaceNamespace of the WebApplicationContext. Defaults to [servlet-name]-servlet.