1   /*
2    * Licensed to the Apache Software Foundation (ASF) under one
3    * or more contributor license agreements.  See the NOTICE file
4    * distributed with this work for additional information
5    * regarding copyright ownership.  The ASF licenses this file
6    * to you under the Apache License, Version 2.0 (the
7    * "License"); you may not use this file except in compliance
8    * with the License.  You may obtain a copy of the License at
9    *
10   *  http://www.apache.org/licenses/LICENSE-2.0
11   *
12   * Unless required by applicable law or agreed to in writing,
13   * software distributed under the License is distributed on an
14   * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
15   * KIND, either express or implied.  See the License for the
16   * specific language governing permissions and limitations
17   * under the License.
18   */
19  
20  package javax.servlet;
21  
22  import java.io.IOException;
23  
24  	/** 
25  	* 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.
26          * <br><br>
27  	* Filters perform filtering in the <code>doFilter</code> method. Every Filter has access to 
28  	** a FilterConfig object from which it can obtain its initialization parameters, a
29  	** reference to the ServletContext which it can use, for example, to load resources
30  	** needed for filtering tasks.
31  	** <p>
32  	** Filters are configured in the deployment descriptor of a web application
33  	** <p>
34  	** Examples that have been identified for this design are<br>
35  	** 1) Authentication Filters <br>
36  	** 2) Logging and Auditing Filters <br>
37  	** 3) Image conversion Filters <br>
38      	** 4) Data compression Filters <br>
39  	** 5) Encryption Filters <br>
40  	** 6) Tokenizing Filters <br>
41  	** 7) Filters that trigger resource access events <br>
42  	** 8) XSL/T filters <br>
43  	** 9) Mime-type chain Filter <br>
44  	 * @since	Servlet 2.3
45  	*/
46  
47  public interface Filter {
48  
49  	/** 
50  	* Called by the web container to indicate to a filter that it is being placed into
51  	* service. The servlet container calls the init method exactly once after instantiating the
52  	* filter. The init method must complete successfully before the filter is asked to do any
53  	* filtering work. <br><br>
54  
55       	* The web container cannot place the filter into service if the init method either<br>
56          * 1.Throws a ServletException <br>
57          * 2.Does not return within a time period defined by the web container 
58  	*/
59  	public void init(FilterConfig filterConfig) throws ServletException;
60  	
61  	
62  	/**
63  	* The <code>doFilter</code> method of the Filter is called by the container
64  	* each time a request/response pair is passed through the chain due
65  	* to a client request for a resource at the end of the chain. The FilterChain passed in to this
66  	* method allows the Filter to pass on the request and response to the next entity in the
67  	* chain.<p>
68  	* A typical implementation of this method would follow the following pattern:- <br>
69  	* 1. Examine the request<br>
70  	* 2. Optionally wrap the request object with a custom implementation to
71  	* filter content or headers for input filtering <br>
72  	* 3. Optionally wrap the response object with a custom implementation to
73  	* filter content or headers for output filtering <br>
74  	* 4. a) <strong>Either</strong> invoke the next entity in the chain using the FilterChain object (<code>chain.doFilter()</code>), <br>   
75  	** 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>
76  	** 5. Directly set headers on the response after invocation of the next entity in the filter chain.
77  	**/
78      public void doFilter ( ServletRequest request, ServletResponse response, FilterChain chain ) throws IOException, ServletException;
79  
80  	/**
81  	* Called by the web container to indicate to a filter that it is being taken out of service. This 
82  	* method is only called once all threads within the filter's doFilter method have exited or after
83  	* a timeout period has passed. After the web container calls this method, it will not call the
84  	* doFilter method again on this instance of the filter. <br><br>
85  	* 
86       	* This method gives the filter an opportunity to clean up any resources that are being held (for
87  	* example, memory, file handles, threads) and make sure that any persistent state is synchronized
88  	* with the filter's current state in memory.
89  	*/
90  
91  	public void destroy();
92  
93  
94  }
95