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   * Copyright (c) 1997-2010 Oracle and/or its affiliates. All rights reserved.
   * The contents of this file are subject to the terms of either the GNU
   * General Public License Version 2 only ("GPL") or the Common Development
   * and Distribution License("CDDL") (collectively, the "License").  You
   * may not use this file except in compliance with the License.  You can
  * obtain a copy of the License at
  * or packager/legal/LICENSE.txt.  See the License for the specific
  * language governing permissions and limitations under the License.
  * When distributing the software, include this License Header Notice in each
  * file and include the License file at packager/legal/LICENSE.txt.
  * GPL Classpath Exception:
  * Oracle designates this particular file as subject to the "Classpath"
  * exception as provided by Oracle in the GPL Version 2 section of the License
  * file that accompanied this code.
  * Modifications:
  * If applicable, add the following below the License Header, with the fields
  * enclosed by brackets [] replaced by your own identifying information:
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  * only the GPL Version 2, indicate your decision by adding "[Contributor]
  * elects to include this software in this distribution under the [CDDL or GPL
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  * its licensees as provided above.  However, if you add GPL Version 2 code
  * and therefore, elected the GPL Version 2 license, then the option applies
  * only if the new code is made subject to such option by the copyright
  * holder.
  * This file incorporates work covered by the following copyright and
  * permission notice:
  * Copyright 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
  * 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;
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, and 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

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

  1. Throws a ServletException
  2. Does not return within a time period defined by the web container
    public void init(FilterConfig filterConfigthrows 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
    • Either invoke the next entity in the chain using the FilterChain object (chain.doFilter()),
    • or not pass on the request/response pair to the next entity in the filter chain to block the request processing
  4. Directly set headers on the response after invocation of the next entity in the filter chain.
    public void doFilter(ServletRequest requestServletResponse response,
                         FilterChain chain)
            throws IOExceptionServletException;

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();
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