001    /*
002     * Licensed to the Apache Software Foundation (ASF) under one
003     * or more contributor license agreements.  See the NOTICE file
004     * distributed with this work for additional information
005     * regarding copyright ownership.  The ASF licenses this file
006     * to you under the Apache License, Version 2.0 (the
007     * "License"); you may not use this file except in compliance
008     * with the License.  You may obtain a copy of the License at
009     *
010     *  http://www.apache.org/licenses/LICENSE-2.0
011     *
012     * Unless required by applicable law or agreed to in writing,
013     * software distributed under the License is distributed on an
014     * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
015     * KIND, either express or implied.  See the License for the
016     * specific language governing permissions and limitations
017     * under the License.
018     */
019    
020    //
021    // This source code implements specifications defined by the Java
022    // Community Process. In order to remain compliant with the specification
023    // DO NOT add / change / or delete method signatures!
024    //
025    
026    package javax.servlet;
027    
028    import java.io.IOException;
029    
030    /**
031     * 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.
032     * <br><br>
033     * Filters perform filtering in the <code>doFilter</code> method. Every Filter has access to
034     * a FilterConfig object from which it can obtain its initialization parameters, a
035     * reference to the ServletContext which it can use, for example, to load resources
036     * needed for filtering tasks.
037     * <p>
038     * Filters are configured in the deployment descriptor of a web application
039     * <p>
040     * Examples that have been identified for this design are<br>
041     * 1) Authentication Filters <br>
042     * 2) Logging and Auditing Filters <br>
043     * 3) Image conversion Filters <br>
044     * 4) Data compression Filters <br>
045     * 5) Encryption Filters <br>
046     * 6) Tokenizing Filters <br>
047     * 7) Filters that trigger resource access events <br>
048     * 8) XSL/T filters <br>
049     * 9) Mime-type chain Filter <br>
050     *
051     * @since Servlet 2.3
052     *
053     * @version $Rev: 467553 $ $Date: 2006-10-25 06:01:51 +0200 (Mi, 25. Okt 2006) $
054     */
055    public interface Filter {
056        /**
057         * Called by the web container to indicate to a filter that it is being placed into
058         * service. The servlet container calls the init method exactly once after instantiating the
059         * filter. The init method must complete successfully before the filter is asked to do any
060         * filtering work. <br><br>
061         * The web container cannot place the filter into service if the init method either<br>
062         * 1.Throws a ServletException <br>
063         * 2.Does not return within a time period defined by the web container
064         */
065        public void init(FilterConfig filterConfig) throws ServletException;
066    
067    
068        /**
069         * The <code>doFilter</code> method of the Filter is called by the container
070         * each time a request/response pair is passed through the chain due
071         * to a client request for a resource at the end of the chain. The FilterChain passed in to this
072         * method allows the Filter to pass on the request and response to the next entity in the
073         * chain.<p>
074         * A typical implementation of this method would follow the following pattern:- <br>
075         * 1. Examine the request<br>
076         * 2. Optionally wrap the request object with a custom implementation to
077         * filter content or headers for input filtering <br>
078         * 3. Optionally wrap the response object with a custom implementation to
079         * filter content or headers for output filtering <br>
080         * 4. a) <strong>Either</strong> invoke the next entity in the chain using the FilterChain object (<code>chain.doFilter()</code>), <br>
081         * 4. b) <strong>or</strong> not pass on the request/response pair to the next entity in the filter chain to block the request processing<br>
082         * 5. Directly set headers on the response after invocation of the next entity in ther filter chain.
083         */
084        public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException;
085    
086        /**
087         * Called by the web container to indicate to a filter that it is being taken out of service. This
088         * method is only called once all threads within the filter's doFilter method have exited or after
089         * a timeout period has passed. After the web container calls this method, it will not call the
090         * doFilter method again on this instance of the filter. <br><br>
091         *
092         * This method gives the filter an opportunity to clean up any resources that are being held (for
093         * example, memory, file handles, threads) and make sure that any persistent state is synchronized
094         * with the filter's current state in memory.
095         */
096        public void destroy();
097    }
098