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 * 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
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 *     http://www.apache.org/licenses/LICENSE-2.0
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 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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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.


Filters perform filtering in the doFilter method.
Every Filter has access to a FilterConfig object from which it can obtain
its initialization parameters, and a reference to the ServletContext which
it can use, for example, to load resources needed for filtering tasks.

Filters are configured in the deployment descriptor of a web
application.

Examples that have been identified for this design are:

    

  1. Authentication Filters

  2. Logging and Auditing Filters

  3. Image conversion Filters

  4. Data compression Filters

  5. Encryption Filters

  6. Tokenizing Filters

  7. Filters that trigger resource access events

  8. XSL/T filters

  9. Mime-type chain Filter



Since:Servlet 2.3
/**
 * <p>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.</p>
 * 
 * <p>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, and 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:
 * <ol>
 * <li>Authentication Filters
 * <li>Logging and Auditing Filters
 * <li>Image conversion Filters
 * <li>Data compression Filters
 * <li>Encryption Filters
 * <li>Tokenizing Filters
 * <li>Filters that trigger resource access events
 * <li>XSL/T filters
 * <li>Mime-type chain Filter
 * </ol>
 *
 * @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.


The web container cannot place the filter into service if the init
method either


    

  1. Throws a ServletException

  2. Does not return within a time period defined by the web container



Params:
  • filterConfig – a FilterConfig object containing the
                    filter's configuration and initialization parameters
Throws:
  • ServletException – if an exception has occurred that interferes with
                         the filter's normal operation
Implementation Requirements:
The default implementation takes no action.
/** 
     * <p>Called by the web container
     * to indicate to a filter that it is being placed into service.</p>
     *
     * <p>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.</p>
     * 
     * <p>The web container cannot place the filter into service if the init
     * method either</p>
     * <ol>
     * <li>Throws a ServletException
     * <li>Does not return within a time period defined by the web container
     * </ol>
     * 
     * @implSpec
     * The default implementation takes no action.
     *
     * @param filterConfig a <code>FilterConfig</code> object containing the
     *                     filter's configuration and initialization parameters 
     * @throws ServletException if an exception has occurred that interferes with
     *                          the filter's normal operation
     */
    default public void init(FilterConfig filterConfig) throws ServletException {}
	
	
    
The doFilter 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.

A typical implementation of this method would follow the following
pattern:

    

  1. Examine the request

  2. Optionally wrap the request object with a custom implementation to
filter content or headers for input filtering

  3. Optionally wrap the response object with a custom implementation to
filter content or headers for output filtering

  4. 

      

    • Either invoke the next entity in the chain
using the FilterChain object
(chain.doFilter()),

    • or not pass on the request/response pair to
the next entity in the filter chain to
block the request processing

    

  5. Directly set headers on the response after invocation of the
next entity in the filter chain.



Params:
  • request – the ServletRequest object contains the client's request
  • response – the ServletResponse object contains the filter's response
  • chain – the FilterChain for invoking the next filter or the resource
Throws:
  • IOException – if an I/O related error has occurred during the processing
  • ServletException – if an exception occurs that interferes with the
                         filter's normal operation
See Also:
/**
     * 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:
     * <ol>
     * <li>Examine the request
     * <li>Optionally wrap the request object with a custom implementation to
     * filter content or headers for input filtering
     * <li>Optionally wrap the response object with a custom implementation to
     * filter content or headers for output filtering
     * <li>
     * <ul>
     * <li><strong>Either</strong> invoke the next entity in the chain
     * using the FilterChain object
     * (<code>chain.doFilter()</code>),
     * <li><strong>or</strong> not pass on the request/response pair to
     * the next entity in the filter chain to
     * block the request processing
     * </ul>
     * <li>Directly set headers on the response after invocation of the
     * next entity in the filter chain.
     * </ol>
     *
     * @param request the <code>ServletRequest</code> object contains the client's request
     * @param response the <code>ServletResponse</code> object contains the filter's response
     * @param chain the <code>FilterChain</code> for invoking the next filter or the resource
     * @throws IOException if an I/O related error has occurred during the processing
     * @throws ServletException if an exception occurs that interferes with the
     *                          filter's normal operation
     *
     * @see UnavailableException
     */
    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.


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.


Implementation Requirements:
The default implementation takes no action.
/**
     * <p>Called by the web container 
     * to indicate to a filter that it is being
     * taken out of service.</p>
     *
     * <p>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.</p>
     *
     * <p>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.</p>
     * 
     * @implSpec
     * The default implementation takes no action.
     */
    default public void destroy() {}
}