Filter


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3   * contributor license agreements.  See the NOTICE file distributed with
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9   *     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      /**
60      * The <code>doFilter</code> method of the Filter is called by the container
61      * each time a request/response pair is passed through the chain due
62      * to a client request for a resource at the end of the chain. The FilterChain passed in to this
63      * method allows the Filter to pass on the request and response to the next entity in the
64      * chain.<p>
65      * A typical implementation of this method would follow the following pattern:- <br>
66      * 1. Examine the request<br>
67      * 2. Optionally wrap the request object with a custom implementation to
68      * filter content or headers for input filtering <br>
69      * 3. Optionally wrap the response object with a custom implementation to
70      * filter content or headers for output filtering <br>
71      * 4. a) <strong>Either</strong> invoke the next entity in the chain using the FilterChain object (<code>chain.doFilter()</code>), <br>   
72      ** 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>
73      ** 5. Directly set headers on the response after invocation of the next entity in the filter chain.
74      **/
75      public void doFilter ( ServletRequest   request, ServletResponse   response, FilterChain   chain ) 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  
91  }
92  
93
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