javax.servlet.Filter

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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();
}