You define filters in the context of a Web application. A filter intercepts a request for a specific named resource or a group of resources (based on a URL pattern) and executes the code in the filter. For each resource or group of resources, you can specify a single filter or multiple filters that are invoked in a specific order, called a chain.

When a filter intercepts a request, it has access to the javax.servlet.ServletRequest and javax.servlet.ServletResponse objects that provide access to the HTTP request and response, and a javax.servlet.FilterChain object. The FilterChain object contains a list of filters that can be invoked sequentially. When a filter has completed its work, the filter can either call the next filter in the chain, block the request, throw an exception, or invoke the originally requested resource.

After the original resource is invoked, control is passed back to the filter at the bottom of the list in the chain. This filter can then examine and modify the response headers and data, block the request, throw an exception, or invoke the next filter up from the bottom of the chain. This process continues in reverse order up through the chain of filters.

Filters can be useful for the following functions:

• Implementing a logging function

• Implementing user-written security functionality

• Debugging

• Encryption

• Data compression

• Modifying the response sent to the client. (However, post processing the response can degrade the performance of your application.)

1. Open the web.xml deployment descriptor in a text editor or an XML editor. See Web Application Developer Tools. The web.xml file is located in the WEB-INF directory of your Web application.
2. Add a filter declaration. The filter element declares a filter, defines a name for the filter, and specifies the Java class that executes the filter. The filter element must directly follow the context-param element and directly precede the listener and servlet elements. For example:

   <context-param>Param</context-param>
   <filter>
     <icon>
       <small-icon>MySmallIcon.gif</small-icon>
       <large-icon>MyLargeIcon.gif</large-icon>
     </icon>
     <filter-name>myFilter</filter-name>
     <display-name>My Filter</display-name>
     <description>This is my filter</description>
     <filter-class>examples.myFilterClass</filter-class>
   </filter>
   <listener>Listener</listener>
   <servlet>Servlet</servlet>
   

   The icon, description, and display-name elements are optional.

3. Specify one or more initialization attributes inside a filter element. For example:

   <filter>
     <icon>
       <small-icon>MySmallIcon.gif</small-icon>
       <large-icon>MyLargeIcon.gif</large-icon>
     </icon>
     <filter-name>myFilter</filter-name>
     <display-name>My Filter</display-name>
     <description>This is my filter</description>
     <filter-class>examples.myFilterClass</filter-class>
     <init-param>
       <param-name>myInitParam</param-name>
       <param-value>myInitParamValue</param-value>
     </init-param>
   </filter>
   

   Your Filter class can read the initialization attributes using the FilterConfig.getInitParameter() or FilterConfig.getInitParameters() methods.

4. Add filter mappings. The filter-mapping element specifies which filter to execute based on a URL pattern or servlet name. The filter-mapping element must immediately follow the filter element(s).

   • To create a filter mapping using a URL pattern, specify the name of the filter and a URL pattern. URL pattern matching is performed according to the rules specified in the Servlet 4.0 specification at https://jcp.org/en/jsr/detail?id=369. For example, the following filter-mapping maps myFilter to requests that contain /myPattern/.

     <filter-mapping>
       <filter-name>myFilter</filter-name>
       <url-pattern>/myPattern/*</url-pattern>
     </filter-mapping>
     

   • To create a filter mapping for a specific servlet, map the filter to the name of a servlet that is registered in the Web application. For example, the following code maps the myFilter filter to a servlet called myServlet:

     <filter-mapping>
       <filter-name>myFilter</filter-name>
       <servlet-hame>myServlet</servlet-name>
     </filter-mapping>
     

5. To create a chain of filters, specify multiple filter mappings. See Configuring a Chain of Filters.

WebLogic Server creates a chain of filters by creating a list of all the filter mappings that match an incoming HTTP request. The ordering of the list is determined by the following sequence:

1. Filters where the filter-mapping element contains a url-pattern that matches the request are added to the chain in the order they appear in the web.xml deployment descriptor.

2. Filters where the filter-mapping element contains a servlet-name that matches the request are added to the chain after the filters that match a URL pattern.

3. The last item in the chain is always the originally requested resource.

In your filter class, use the FilterChain.doFilter() method to invoke the next item in the chain.