Filter.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *  http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */

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.
 * <br><br>
 * 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, 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<br>
 * 1) Authentication Filters <br>
 * 2) Logging and Auditing Filters <br>
 * 3) Image conversion Filters <br>
 * 4) Data compression Filters <br>
 * 5) Encryption Filters <br>
 * 6) Tokenizing Filters <br>
 * 7) Filters that trigger resource access events <br>
 * 8) XSL/T filters <br>
 * 9) Mime-type chain Filter <br>
 *
 * @since Servlet 2.3
 */

public interface Filter {

    /**
     * Called by the web container to indicate to a filter that it is being placed into
     * service. 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. <br><br>
     * <p/>
     * The web container cannot place the filter into service if the init method either<br>
     * 1.Throws a ServletException <br>
     * 2.Does not return within a time period defined by the web container
     * @param filterConfig Configuration for this filter
     * @throws ServletException if the filter cannot initialize itself
     */
    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:- <br>
     * 1. Examine the request<br>
     * 2. Optionally wrap the request object with a custom implementation to
     * filter content or headers for input filtering <br>
     * 3. Optionally wrap the response object with a custom implementation to
     * filter content or headers for output filtering <br>
     * 4. a) <strong>Either</strong> invoke the next entity in the chain using the FilterChain object (<code>chain.doFilter()</code>), <br>
     * * 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>
     * * 5. Directly set headers on the response after invocation of the next entity in the filter chain.
     * @param request servlet request to filter
     * @param response servlet response to filter
     * @param chain what to do next
     * @throws java.io.IOException if there's an IO problem
     * @throws ServletException if there is a non-IO problem
     */
    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. 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. <br><br>
     * <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();


}