package org.apache.turbine.pipeline;

 /*

  * 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.

  */

 import java.io.IOException;

 import org.apache.turbine.util.TurbineException;

 /**

  * <p>A <b>Valve</b> is a request processing component.  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.

  *

  * @author Craig R. McClanahan

  * @author Gunnar Rjnning

  * @author Peter Donald

  * @author <a href="mailto:dlr@finemaltcoding.com">Daniel Rall</a>

  *

  * @see #invoke(PipelineData, ValveContext)

  */

 public interface Valve

 {

     /**

      * <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>context.invokeNext()</code>.

      * <li>Examine, but not modify, the properties of the resulting Response

      *     (which was created by a subsequently invoked Valve via a

      *     call to <code>context.invokeNext()</code>).

      * </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.

      * <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>invokeNext()</code> method has returned.

      * <li>Perform any actions on the output stream associated with the

      *     specified Response after the <code>invokeNext()</code> method has

      *     returned.

      * </ul>

      *

      * @param pipelineData The run-time information, including the servlet

      * request and response we are processing.

      * @param context The valve context used to invoke the next valve

      *  in the current processing pipeline

      *

      * @throws IOException Thrown by a subsequent Valve.

      * @throws TurbineException Thrown by a subsequent Valve.

      */

     public void invoke(PipelineData pipelineData, ValveContext context)

         throws IOException, TurbineException;

     /**

      * Initialize the valve before using in a pipeline.

      * @throws Exception if initialization fails

      */

     public void initialize()

         throws Exception;

 }