java/org/apache/catalina/Valve.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 org.apache.catalina;

import java.io.IOException;

import javax.servlet.ServletException;

import org.apache.catalina.connector.Request;
import org.apache.catalina.connector.Response;

/**
 * A <b>Valve</b> is a request processing component associated with a particular Container. A series of Valves are
 * generally associated with each other into a Pipeline. The detailed contract for a Valve is included in the
 * description of the <code>invoke()</code> method below.
 * <p>
 * <b>HISTORICAL NOTE</b>: The "Valve" name was assigned to this concept because a valve is what you use in a real world
 * pipeline to control and/or modify flows through it.
 */
public interface Valve {


    // -------------------------------------------------------------- Properties

    /**
     * Returns the next Valve in this pipeline, or <code>null</code> if this is the last Valve in the pipeline.
     *
     * @return the next Valve in the pipeline containing this Valve, or <code>null</code> if this is the last Valve in
     *             the pipeline.
     */
    Valve getNext();


    /**
     * Set the next Valve in the pipeline containing this Valve.
     *
     * @param valve The new next valve, or <code>null</code> if none
     */
    void setNext(Valve valve);


    // ---------------------------------------------------------- Public Methods


    /**
     * Execute a periodic task, such as reloading, etc. This method will be invoked inside the classloading context of
     * this container. Unexpected throwables will be caught and logged.
     */
    void backgroundProcess();


    /**
     * <p>
     * Perform request processing as required by this Valve.
     * </p>
     * <p>
     * An individual Valve <b>MAY</b> perform the following actions, in the specified order:
     * </p>
     * <ul>
     * <li>Examine and/or modify the properties of the specified Request and Response.
     * <li>Examine the properties of the specified Request, completely generate the corresponding Response, and return
     * control to the caller.
     * <li>Examine the properties of the specified Request and Response, wrap either or both of these objects to
     * supplement their functionality, and pass them on.
     * <li>If the corresponding Response was not generated (and control was not returned), call the next Valve in the
     * pipeline (if there is one) by executing <code>getNext().invoke()</code>.
     * <li>Examine, but not modify, the properties of the resulting Response (which was created by a subsequently
     * invoked Valve or Container).
     * </ul>
     * <p>
     * A Valve <b>MUST NOT</b> do any of the following things:
     * </p>
     * <ul>
     * <li>Change request properties that have already been used to direct the flow of processing control for this
     * request (for instance, trying to change the virtual host to which a Request should be sent from a pipeline
     * attached to a Host or Context in the standard implementation).
     * <li>Create a completed Response <strong>AND</strong> pass this Request and Response on to the next Valve in the
     * pipeline.
     * <li>Consume bytes from the input stream associated with the Request, unless it is completely generating the
     * response, or wrapping the request before passing it on.
     * <li>Modify the HTTP headers included with the Response after the <code>getNext().invoke()</code> method has
     * returned.
     * <li>Perform any actions on the output stream associated with the specified Response after the
     * <code>getNext().invoke()</code> method has returned.
     * </ul>
     *
     * @param request  The servlet request to be processed
     * @param response The servlet response to be created
     *
     * @exception IOException      if an input/output error occurs, or is thrown by a subsequently invoked Valve,
     *                                 Filter, or Servlet
     * @exception ServletException if a servlet error occurs, or is thrown by a subsequently invoked Valve, Filter, or
     *                                 Servlet
     */
    void invoke(Request request, Response response) throws IOException, ServletException;


    boolean isAsyncSupported();
}