01: /*
02: * Licensed to the Apache Software Foundation (ASF) under one or more
03: * contributor license agreements. See the NOTICE file distributed with
04: * this work for additional information regarding copyright ownership.
05: * The ASF licenses this file to You under the Apache License, Version 2.0
06: * (the "License"); you may not use this file except in compliance with
07: * the License. You may obtain a copy of the License at
08: *
09: * http://www.apache.org/licenses/LICENSE-2.0
10: *
11: * Unless required by applicable law or agreed to in writing, software
12: * distributed under the License is distributed on an "AS IS" BASIS,
13: * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14: * See the License for the specific language governing permissions and
15: * limitations under the License.
16: */
17: package javax.servlet;
18:
19: import java.io.IOException;
20:
21: /**
22: * 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.
23: * <br><br>
24: * Filters perform filtering in the <code>doFilter</code> method. Every Filter has access to
25: ** a FilterConfig object from which it can obtain its initialization parameters, a
26: ** reference to the ServletContext which it can use, for example, to load resources
27: ** needed for filtering tasks.
28: ** <p>
29: ** Filters are configured in the deployment descriptor of a web application
30: ** <p>
31: ** Examples that have been identified for this design are<br>
32: ** 1) Authentication Filters <br>
33: ** 2) Logging and Auditing Filters <br>
34: ** 3) Image conversion Filters <br>
35: ** 4) Data compression Filters <br>
36: ** 5) Encryption Filters <br>
37: ** 6) Tokenizing Filters <br>
38: ** 7) Filters that trigger resource access events <br>
39: ** 8) XSL/T filters <br>
40: ** 9) Mime-type chain Filter <br>
41: * @since Servlet 2.3
42: */
43:
44: public interface Filter {
45:
46: /**
47: * Called by the web container to indicate to a filter that it is being placed into
48: * service. The servlet container calls the init method exactly once after instantiating the
49: * filter. The init method must complete successfully before the filter is asked to do any
50: * filtering work. <br><br>
51:
52: * The web container cannot place the filter into service if the init method either<br>
53: * 1.Throws a ServletException <br>
54: * 2.Does not return within a time period defined by the web container
55: */
56: public void init(FilterConfig filterConfig) throws ServletException;
57:
58: /**
59: * The <code>doFilter</code> method of the Filter is called by the container
60: * each time a request/response pair is passed through the chain due
61: * to a client request for a resource at the end of the chain. The FilterChain passed in to this
62: * method allows the Filter to pass on the request and response to the next entity in the
63: * chain.<p>
64: * A typical implementation of this method would follow the following pattern:- <br>
65: * 1. Examine the request<br>
66: * 2. Optionally wrap the request object with a custom implementation to
67: * filter content or headers for input filtering <br>
68: * 3. Optionally wrap the response object with a custom implementation to
69: * filter content or headers for output filtering <br>
70: * 4. a) <strong>Either</strong> invoke the next entity in the chain using the FilterChain object (<code>chain.doFilter()</code>), <br>
71: ** 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>
72: ** 5. Directly set headers on the response after invocation of the next entity in the filter chain.
73: **/
74: public void doFilter(ServletRequest request,
75: ServletResponse response, FilterChain chain)
76: throws IOException, ServletException;
77:
78: /**
79: * Called by the web container to indicate to a filter that it is being taken out of service. This
80: * method is only called once all threads within the filter's doFilter method have exited or after
81: * a timeout period has passed. After the web container calls this method, it will not call the
82: * doFilter method again on this instance of the filter. <br><br>
83: *
84: * This method gives the filter an opportunity to clean up any resources that are being held (for
85: * example, memory, file handles, threads) and make sure that any persistent state is synchronized
86: * with the filter's current state in memory.
87: */
88:
89: public void destroy();
90:
91: }
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