Most applications have some requirements, such as security and logging, that are applicable across all application requests. To add such functionality separately to each application service would be time-consuming, error-prone, and difficult to maintain. Even implementing these services within a front controller would still require code changes to add and remove services. The sequence diagram in Figure 1 below shows how each Web resource is responsible for calling such services individually.

Figure 1. Before Intercepting Filter

The Intercepting Filter pattern wraps existing application resources with a filter that intercepts the reception of a request and the transmission of a response. An intercepting filter can pre-process or redirect application requests, and can post-process or replace the content of application responses. Intercepting filters can also be stacked one on top of the other to add a chain of separate, declaratively-deployable services to existing Web resources with no changes to source code. Figure 2 below shows a chain of two intercepting filters intercepting requests to two Web resources that they wrap.

Figure 2. After Intercepting Filter

See the Core J2EETM Patterns

The Servlet Filter interface in the Java^TM 2 Platform, Enterprise Edition (J2EE) platform is a direct implementation of the Intercepting Filter pattern. The sample application defines two servlet filters: one that mediates customer signon, and another that enforces response encoding.

• The user signon filter.

  Sample application class SignOnFilter is an intercepting filter that intercepts all requests to the pet store front controller. The SignOnFilter servlet filter detects requests for pages that require user signon, and redirects such requests to the user signon page if no user is currently signed on. An exhaustive explanation of this process appears in User Signon and Customer Registration Module (part of the online document Sample Application Design and Implementation [ BT-SADI02 ]).

• The encoding filter.

  Sample application class EncodingFilter is an intercepting filter that modifies responses from the pet store front controller, ensuring that the encoding of the response is always set consistently.

  • The code sample below from class EncodingFilter shows how the servlet filter sets the response encoding of each response, and then passes the request to the next filter down the chain. The filter chain mechanism makes such filters composable, so multiple intercepting filters can wrap a single resource.

                    
    public void doFilter(ServletRequest srequest,
                         ServletResponse sresponse,
                         FilterChain chain)
       throws IOException, ServletException {
                
       HttpServletRequest request = (HttpServletRequest)srequest;
       request.setCharacterEncoding(targetEncoding);
    
       // move on to the next
       chain.doFilter(srequest,sresponse);  
    }
                  

  • The excerpt below from the pet store web application description web.xml shows how the encoding filter is configured to wrap all requests matching the pattern " /*" (relative to the application context root).

                    
    
      <!-- Encoding Filter Mapping Start-->
      <filter-mapping>
        <filter-name>EncodingFilter</filter-name>
        <url-pattern>/*</url-pattern>
      </filter-mapping>