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    package javax.servlet;
021    
022    import java.io.IOException;
023    
024            /** 
025            * 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.
026            * <br><br>
027            * Filters perform filtering in the <code>doFilter</code> method. Every Filter has access to 
028            ** a FilterConfig object from which it can obtain its initialization parameters, a
029            ** reference to the ServletContext which it can use, for example, to load resources
030            ** needed for filtering tasks.
031            ** <p>
032            ** Filters are configured in the deployment descriptor of a web application
033            ** <p>
034            ** Examples that have been identified for this design are<br>
035            ** 1) Authentication Filters <br>
036            ** 2) Logging and Auditing Filters <br>
037            ** 3) Image conversion Filters <br>
038            ** 4) Data compression Filters <br>
039            ** 5) Encryption Filters <br>
040            ** 6) Tokenizing Filters <br>
041            ** 7) Filters that trigger resource access events <br>
042            ** 8) XSL/T filters <br>
043            ** 9) Mime-type chain Filter <br>
044             * @since       Servlet 2.3
045            */
046    
047    public interface Filter {
048    
049            /** 
050            * Called by the web container to indicate to a filter that it is being placed into
051            * service. The servlet container calls the init method exactly once after instantiating the
052            * filter. The init method must complete successfully before the filter is asked to do any
053            * filtering work. <br><br>
054    
055            * The web container cannot place the filter into service if the init method either<br>
056            * 1.Throws a ServletException <br>
057            * 2.Does not return within a time period defined by the web container 
058            */
059            public void init(FilterConfig filterConfig) throws ServletException;
060            
061            
062            /**
063            * The <code>doFilter</code> method of the Filter is called by the container
064            * each time a request/response pair is passed through the chain due
065            * to a client request for a resource at the end of the chain. The FilterChain passed in to this
066            * method allows the Filter to pass on the request and response to the next entity in the
067            * chain.<p>
068            * A typical implementation of this method would follow the following pattern:- <br>
069            * 1. Examine the request<br>
070            * 2. Optionally wrap the request object with a custom implementation to
071            * filter content or headers for input filtering <br>
072            * 3. Optionally wrap the response object with a custom implementation to
073            * filter content or headers for output filtering <br>
074            * 4. a) <strong>Either</strong> invoke the next entity in the chain using the FilterChain object (<code>chain.doFilter()</code>), <br>   
075            ** 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>
076            ** 5. Directly set headers on the response after invocation of the next entity in the filter chain.
077            **/
078        public void doFilter ( ServletRequest request, ServletResponse response, FilterChain chain ) throws IOException, ServletException;
079    
080            /**
081            * Called by the web container to indicate to a filter that it is being taken out of service. This 
082            * method is only called once all threads within the filter's doFilter method have exited or after
083            * a timeout period has passed. After the web container calls this method, it will not call the
084            * doFilter method again on this instance of the filter. <br><br>
085            * 
086            * This method gives the filter an opportunity to clean up any resources that are being held (for
087            * example, memory, file handles, threads) and make sure that any persistent state is synchronized
088            * with the filter's current state in memory.
089            */
090    
091            public void destroy();
092    
093    
094    }
095