java/jakarta/servlet/Filter.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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 jakarta.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.
     * <p>
     * The web container cannot place the filter into service if the init method either:
     * <ul>
     * <li>Throws a ServletException</li>
     * <li>Does not return within a time period defined by the web container</li>
     * </ul>
     * The default implementation is a NO-OP.
     *
     * @param filterConfig The configuration information associated with the filter instance being initialised
     *
     * @throws ServletException if the initialisation fails
     */
    default 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.
     *
     * @param request  The request to process
     * @param response The response associated with the request
     * @param chain    Provides access to the next filter in the chain for this filter to pass the request and response
     *                     to for further processing
     *
     * @throws IOException      if an I/O error occurs during this filter's processing of the request
     * @throws ServletException if the processing fails for any other reason
     */
    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. The default implementation is a NO-OP.
     */
    default void destroy() {
    }
}