servletapi/jsr154/src/share/javax/servlet/Filter.java

/*
* 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
*
*     http://www.apache.org/licenses/LICENSE-2.0
*
* 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;

import java.io.IOException;

	/** 
	* 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.
        * <br><br>
	* Filters perform filtering in the <code>doFilter</code> method. Every Filter has access to 
	** a FilterConfig object from which it can obtain its initialization parameters, a
	** reference to the ServletContext which it can use, for example, to load resources
	** needed for filtering tasks.
	** <p>
	** Filters are configured in the deployment descriptor of a web application
	** <p>
	** Examples that have been identified for this design are<br>
	** 1) Authentication Filters <br>
	** 2) Logging and Auditing Filters <br>
	** 3) Image conversion Filters <br>
    	** 4) Data compression Filters <br>
	** 5) Encryption Filters <br>
	** 6) Tokenizing Filters <br>
	** 7) Filters that trigger resource access events <br>
	** 8) XSL/T filters <br>
	** 9) Mime-type chain Filter <br>
	 * @since	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. <br><br>

     	* The web container cannot place the filter into service if the init method either<br>
        * 1.Throws a ServletException <br>
        * 2.Does not return within a time period defined by the web container 
	*/
	public void init(FilterConfig filterConfig) throws ServletException;
	
	
	/**
	* The <code>doFilter</code> 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.<p>
	* A typical implementation of this method would follow the following pattern:- <br>
	* 1. Examine the request<br>
	* 2. Optionally wrap the request object with a custom implementation to
	* filter content or headers for input filtering <br>
	* 3. Optionally wrap the response object with a custom implementation to
	* filter content or headers for output filtering <br>
	* 4. a) <strong>Either</strong> invoke the next entity in the chain using the FilterChain object (<code>chain.doFilter()</code>), <br>   
	** 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>
	** 5. Directly set headers on the response after invocation of the next entity in the filter chain.
	**/
    public void doFilter ( ServletRequest request, ServletResponse response, FilterChain chain ) throws IOException, ServletException;

	/**
	* 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. <br><br>
	* 
     	* 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();


}