    001package org.apache.turbine.pipeline;
    002
    003
    004/*
    005 * Licensed to the Apache Software Foundation (ASF) under one
    006 * or more contributor license agreements.  See the NOTICE file
    007 * distributed with this work for additional information
    008 * regarding copyright ownership.  The ASF licenses this file
    009 * to you under the Apache License, Version 2.0 (the
    010 * "License"); you may not use this file except in compliance
    011 * with the License.  You may obtain a copy of the License at
    012 *
    013 *   http://www.apache.org/licenses/LICENSE-2.0
    014 *
    015 * Unless required by applicable law or agreed to in writing,
    016 * software distributed under the License is distributed on an
    017 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
    018 * KIND, either express or implied.  See the License for the
    019 * specific language governing permissions and limitations
    020 * under the License.
    021 */
    022
    023
    024import java.io.IOException;
    025
    026import org.apache.turbine.util.TurbineException;
    027
    028/**
    029 * <p>A <b>Valve</b> is a request processing component.  A series of
    030 * Valves are generally associated with each other into a Pipeline.
    031 * The detailed contract for a Valve is included in the description of
    032 * the <code>invoke()</code> method below.</p>
    033 *
    034 * <b>HISTORICAL NOTE</b>:  The "Valve" name was assigned to this concept
    035 * because a valve is what you use in a real world pipeline to control and/or
    036 * modify flows through it.
    037 *
    038 * @author Craig R. McClanahan
    039 * @author Gunnar Rjnning
    040 * @author Peter Donald
    041 * @author <a href="mailto:dlr@finemaltcoding.com">Daniel Rall</a>
    042 *
    043 * @see #invoke(PipelineData, ValveContext)
    044 */
    045public interface Valve
    046{
    047    /**
    048     * <p>Perform request processing as required by this Valve.</p>
    049     *
    050     * <p>An individual Valve <b>MAY</b> perform the following actions, in
    051     * the specified order:</p>
    052     * <ul>
    053     * <li>Examine and/or modify the properties of the specified Request and
    054     *     Response.
    055     * <li>Examine the properties of the specified Request, completely generate
    056     *     the corresponding Response, and return control to the caller.
    057     * <li>Examine the properties of the specified Request and Response, wrap
    058     *     either or both of these objects to supplement their functionality,
    059     *     and pass them on.
    060     * <li>If the corresponding Response was not generated (and control was not
    061     *     returned, call the next Valve in the pipeline (if there is one) by
    062     *     executing <code>context.invokeNext()</code>.
    063     * <li>Examine, but not modify, the properties of the resulting Response
    064     *     (which was created by a subsequently invoked Valve via a
    065     *     call to <code>context.invokeNext()</code>).
    066     * </ul>
    067     *
    068     * <p>A Valve <b>MUST NOT</b> do any of the following things:</p>
    069     * <ul>
    070     * <li>Change request properties that have already been used to direct
    071     *     the flow of processing control for this request.
    072     * <li>Create a completed Response <strong>AND</strong> pass this
    073     *     Request and Response on to the next Valve in the pipeline.
    074     * <li>Consume bytes from the input stream associated with the Request,
    075     *     unless it is completely generating the response, or wrapping the
    076     *     request before passing it on.
    077     * <li>Modify the HTTP headers included with the Response after the
    078     *     <code>invokeNext()</code> method has returned.
    079     * <li>Perform any actions on the output stream associated with the
    080     *     specified Response after the <code>invokeNext()</code> method has
    081     *     returned.
    082     * </ul>
    083     *
    084     * @param pipelineData The run-time information, including the servlet
    085     * request and response we are processing.
    086     * @param context The valve context used to invoke the next valve
    087     *  in the current processing pipeline
    088     *
    089     * @throws IOException Thrown by a subsequent Valve.
    090     * @throws TurbineException Thrown by a subsequent Valve.
    091     */
    092    public void invoke(PipelineData pipelineData, ValveContext context)
    093        throws IOException, TurbineException;
    094
    095    /**
    096     * Initialize the valve before using in a pipeline.
    097     * @throws Exception if initialization fails
    098     */
    099    public void initialize()
    100        throws Exception;
    101}