Another way to alter a request or response is through the use of filters. A filter, as it is defined in the Java Servlet Specification v2.3, implements the javax.servlet.Filter interface. You use a filter to create a wrapper for the request and response in order to modify the data within it. You can also use a filter to examine the headers in the request and to specify the next resource to call.
A series of filters are managed by a filter chain. Once a filter completes execution, it makes a call to the filter chain. The filter chain is responsible for determining the next operation, which could be invoking another filter, halting the request execution, throwing an exception, or calling the resource that passed the request to the first filter in the chain.
The place, during the request-handling process, where a filter is invoked depends on the requested file type (JSP or JHTML).
Nucleus-based web applications use one filter, PageFilter, by default. For information on how to implement PageFilter, see Starting the Request-Handling Pipeline. You can learn about adding other filters to Nucleus-based web applications in Adding New Request-Handling Resources to web.xml.
