javax.servlet.Filter Maven / Gradle / Ivy

/**
 *
 * Copyright 2003-2004 The Apache Software Foundation
 *
 *  Licensed 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.
 */

//
// This source code implements specifications defined by the Java
// Community Process. In order to remain compliant with the specification
// DO NOT add / change / or delete method signatures!
//

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.
 * 


 * Filters perform filtering in the doFilter 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.
 * 
 * Filters are configured in the deployment descriptor of a web application
 * 

 * Examples that have been identified for this design are

 * 1) Authentication Filters 

 * 2) Logging and Auditing Filters 

 * 3) Image conversion Filters 

 * 4) Data compression Filters 

 * 5) Encryption Filters 

 * 6) Tokenizing Filters 

 * 7) Filters that trigger resource access events 

 * 8) XSL/T filters 

 * 9) Mime-type chain Filter 

 *
 * @since Servlet 2.3
 *
 * @version $Rev: 46019 $ $Date: 2004-09-14 04:56:06 -0500 (Tue, 14 Sep 2004) $
 */
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. 


     * The web container cannot place the filter into service if the init method either

     * 1.Throws a ServletException 

     * 2.Does not return within a time period defined by the web container
     */
    public void init(FilterConfig filterConfig) throws ServletException;


    /**
     * The doFilter 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.
     * A typical implementation of this method would follow the following pattern:- 

     * 1. Examine the request

     * 2. Optionally wrap the request object with a custom implementation to
     * filter content or headers for input filtering 

     * 3. Optionally wrap the response object with a custom implementation to
     * filter content or headers for output filtering 

     * 4. a) Either invoke the next entity in the chain using the FilterChain object (chain.doFilter()), 

     * 4. b) or not pass on the request/response pair to the next entity in the filter chain to block the request processing

     * 5. Directly set headers on the response after invocation of the next entity in ther filter chain.
     */
    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. 


     *
     * 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();
}