Filter.java

/*
 * Copyright (c) 1997, 2023 Oracle and/or its affiliates and others.
 * All rights reserved.
 * 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 jakarta.servlet;

import java.io.IOException;

/**
 * <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 {

    /**
     * <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. The container will ensure that actions
     * performed in the <code>init</code> method will be visible to any threads that subsequently call the
     * <code>doFilter</code> method according to the rules in JSR-133 (i.e. there is a 'happens before' relationship between
     * <code>init</code> and <code>doFilter</code>).
     * </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 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:
     * <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
     */
    void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException;

    /**
     * <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 void destroy() {
    }
}