01 /*
02 * Copyright 2004 The Apache Software Foundation
03 *
04 * Licensed under the Apache License, Version 2.0 (the "License");
05 * you may not use this file except in compliance with the License.
06 * You may obtain a copy of the License at
07 *
08 * http://www.apache.org/licenses/LICENSE-2.0
09 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16 package javax.servlet;
17
18 import java.io.IOException;
19
20 /**
21 * 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.
22 * <br><br>
23 * Filters perform filtering in the <code>doFilter</code> method. Every Filter has access to
24 ** a FilterConfig object from which it can obtain its initialization parameters, a
25 ** reference to the ServletContext which it can use, for example, to load resources
26 ** needed for filtering tasks.
27 ** <p>
28 ** Filters are configured in the deployment descriptor of a web application
29 ** <p>
30 ** Examples that have been identified for this design are<br>
31 ** 1) Authentication Filters <br>
32 ** 2) Logging and Auditing Filters <br>
33 ** 3) Image conversion Filters <br>
34 ** 4) Data compression Filters <br>
35 ** 5) Encryption Filters <br>
36 ** 6) Tokenizing Filters <br>
37 ** 7) Filters that trigger resource access events <br>
38 ** 8) XSL/T filters <br>
39 ** 9) Mime-type chain Filter <br>
40 * @since Servlet 2.3
41 */
42
43 public interface Filter {
44
45 /**
46 * Called by the web container to indicate to a filter that it is being placed into
47 * service. The servlet container calls the init method exactly once after instantiating the
48 * filter. The init method must complete successfully before the filter is asked to do any
49 * filtering work. <br><br>
50
51 * The web container cannot place the filter into service if the init method either<br>
52 * 1.Throws a ServletException <br>
53 * 2.Does not return within a time period defined by the web container
54 */
55 public void init(FilterConfig filterConfig) throws ServletException;
56
57 /**
58 * The <code>doFilter</code> method of the Filter is called by the container
59 * each time a request/response pair is passed through the chain due
60 * to a client request for a resource at the end of the chain. The FilterChain passed in to this
61 * method allows the Filter to pass on the request and response to the next entity in the
62 * chain.<p>
63 * A typical implementation of this method would follow the following pattern:- <br>
64 * 1. Examine the request<br>
65 * 2. Optionally wrap the request object with a custom implementation to
66 * filter content or headers for input filtering <br>
67 * 3. Optionally wrap the response object with a custom implementation to
68 * filter content or headers for output filtering <br>
69 * 4. a) <strong>Either</strong> invoke the next entity in the chain using the FilterChain object (<code>chain.doFilter()</code>), <br>
70 ** 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>
71 ** 5. Directly set headers on the response after invocation of the next entity in the filter chain.
72 **/
73 public void doFilter(ServletRequest request,
74 ServletResponse response, FilterChain chain)
75 throws IOException, ServletException;
76
77 /**
78 * Called by the web container to indicate to a filter that it is being taken out of service. This
79 * method is only called once all threads within the filter's doFilter method have exited or after
80 * a timeout period has passed. After the web container calls this method, it will not call the
81 * doFilter method again on this instance of the filter. <br><br>
82 *
83 * This method gives the filter an opportunity to clean up any resources that are being held (for
84 * example, memory, file handles, threads) and make sure that any persistent state is synchronized
85 * with the filter's current state in memory.
86 */
87
88 public void destroy();
89
90 }
|