Developing Web Applications for WebLogic Server
A filter is a Java class that is invoked in response to a request for a resource in a Web application. Resources include Java Servlets, JavaServer pages (JSP), and static resources such as HTML pages or images. A filter intercepts the request and can examine and m odify the response and request objects or execute other tasks.
Filters are an advanced J2EE feature primarily intended for situations where the developer cannot change the coding of an existing resource and needs to modify the behavior of that resource. Generally, it is more efficient to modify the code to change the behavior of the resource itself rather than using filters to modify the resource. In some situations, using filters can add unnecessary complexity to an application and degrade performance.
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.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.
To write a filter class, implement the javax.servlet.Filter interface. You must implement the following methods of this interface:
Several other methods are available on the
FilterConfig object for accessing the name of the filter, the
ServletContext and the filter's initialization attributes. For more information see the J2EE Javadocs from Sun Microsystems for
javax.servlet.FilterConfig.To access the next item in the chain (either another filter or the original resource, if that is the next item in the chain), call the
You configure filters as part of a Web application, using the application's
web.xml deployment descriptor. In the deployment descriptor, you specify the filter and then map the filter to a URL pattern or to a specific servlet in the Web application. You can specify any number of filters.
web.xmldeployment descriptor in a text editor or use the Administration Console. For more information, see Web Application Developer Tools. The
web.xmlfile is located in the
WEB-INFdirectory of your Web application.
<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
<servlet>elements. For example:
<description>This is my filter</description>
myFilterto requests that contain
myFilterfilter to a servlet called
filter-mappingelement contains a
url-patternthat matches the request are added to the chain in the order they appear in the
filter-mappingelement contains a
servlet-namethat matches the request are added to the chain after the filters that match a URL pattern.
You can use filters to post-process the output of a servlet by appending data to the output generated by the servlet. However, in order to capture the output of the servlet, you must create a wrapper for the response. (You cannot use the original response object, because the output buffer of the servlet is automatically flushed and sent to the client when the servlet completes executing and before control is returned to the last filter in the chain.) When you create such a wrapper, WebLogic Server must manipulate an additional copy of the output in memory, which can degrade performance.
For more information on wrapping the response or request objects, see the J2EE Javadocs from Sun Microsystems for