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/*
 * 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. 

 * 

 * 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
 */
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:
     * 

     * Throws a ServletException
     * Does not return within a time period defined by the web
     *     container
     * 
     *
     * @param filterConfig The configuration information associated with the
     *                     filter instance being initialised
     *
     * @throws ServletException if the initialisation fails
     */
    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 the filter chain.
     *
     * @param request  The request to process
     * @param response The response associated with the request
     * @param chain    Provides access to the next filter in the chain for this
     *                 filter to pass the request and response to for further
     *                 processing
     *
     * @throws IOException if an I/O error occurs during this filter's
     *                     processing of the request
     * @throws ServletException if the processing fails for any other reason
     */
    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();
}