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* Copyright 2004 The Apache Software Foundation
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package javax.servlet;
import java.io.IOException;
/**
* A filter is an object that performs filtering tasks on either the
* request to a resource (a servlet or static content), or on the response
* from a resource, or both.
*
* <p>Filters perform filtering in the <code>doFilter</code> method.
* Every Filter has access to a FilterConfig object from which it can obtain
* its initialization parameters, and a reference to the ServletContext which
* it can use, for example, to load resources needed for filtering tasks.
*
* <p>Filters are configured in the deployment descriptor of a web
* application.
*
* <p>Examples that have been identified for this design are:
* <ol>
* <li>Authentication Filters
* <li>Logging and Auditing Filters
* <li>Image conversion Filters
* <li>Data compression Filters
* <li>Encryption Filters
* <li>Tokenizing Filters
* <li>Filters that trigger resource access events
* <li>XSL/T filters
* <li>Mime-type chain Filter
* </ol>
*
* @since Servlet 2.3
*/
public interface Filter {
/**
* Called by the web container to indicate to a filter that it is
* being placed into service.
*
* <p>The servlet container calls the init
* method exactly once after instantiating the filter. The init
* method must complete successfully before the filter is asked to do any
* filtering work.
*
* <p>The web container cannot place the filter into service if the init
* method either
* <ol>
* <li>Throws a ServletException
* <li>Does not return within a time period defined by the web container
* </ol>
*/
public void init(FilterConfig filterConfig) throws ServletException;
/**
* The <code>doFilter</code> method of the Filter is called by the
* container each time a request/response pair is passed through the
* chain due to a client request for a resource at the end of the chain.
* The FilterChain passed in to this method allows the Filter to pass
* on the request and response to the next entity in the chain.
*
* <p>A typical implementation of this method would follow the following
* pattern:
* <ol>
* <li>Examine the request
* <li>Optionally wrap the request object with a custom implementation to
* filter content or headers for input filtering
* <li>Optionally wrap the response object with a custom implementation to
* filter content or headers for output filtering
* <li>
* <ul>
* <li><strong>Either</strong> invoke the next entity in the chain
* using the FilterChain object
* (<code>chain.doFilter()</code>),
* <li><strong>or</strong> not pass on the request/response pair to
* the next entity in the filter chain to
* block the request processing
* </ul>
* <li>Directly set headers on the response after invocation of the
* next entity in the filter chain.
* </ol>
*/
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain)
throws IOException, ServletException;
/**
* Called by the web container to indicate to a filter that it is being
* taken out of service.
*
* <p>This method is only called once all threads within the filter's
* doFilter method have exited or after a timeout period has passed.
* After the web container calls this method, it will not call the
* doFilter method again on this instance of the filter.
*
* <p>This method gives the filter an opportunity to clean up any
* resources that are being held (for example, memory, file handles,
* threads) and make sure that any persistent state is synchronized
* with the filter's current state in memory.
*/
public void destroy();
}