001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *      http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.juneau.rest.processor;
018
019import java.io.*;
020
021import org.apache.juneau.http.response.*;
022import org.apache.juneau.rest.*;
023import org.apache.juneau.rest.annotation.*;
024
025import jakarta.servlet.http.*;
026
027/**
028 * Defines the interface for processors that convert POJOs to appropriate HTTP responses.
029 *
030 * <p>
031 * The REST Server API uses the concept of registered response processors for converting objects returned by REST
032 * methods or set through {@link RestResponse#setContent(Object)} into appropriate HTTP responses.
033 *
034 * <p>
035 * Response processors can be associated with REST resources via the following:
036 * <ul>
037 *    <li class='ja'>{@link Rest#responseProcessors}
038 *    <li class='jm'>{@link org.apache.juneau.rest.RestContext.Builder#responseProcessors()}
039 * </ul>
040 *
041 * <p>
042 * Response processors can be used to process POJOs that cannot normally be handled through Juneau serializers, or
043 * because it's simply easier to define response processors for special cases.
044 *
045 * <p>
046 * The following example shows how to create a response processor to handle special <c>Foo</c> objects outside the
047 * normal Juneau architecture.
048 * <p class='bjava'>
049 *    <ja>@Rest</ja>(
050 *       path=<js>"/example"</js>,
051 *       responseProcessors=FooProcessor.<jk>class</jk>
052 *    )
053 *    <jk>public class</jk> Example <jk>extends</jk> BasicRestServlet {
054 *
055 *       <ja>@RestGet</ja>(<js>"/"</js>)
056 *       <jk>public</jk> Foo test1() {
057 *          <jk>return new</jk> Foo(<js>"123"</js>);
058 *       }
059 *
060 *       <jk>public static class</jk> FooProcessor <jk>implements</jk> ResponseProcessor {
061 *          <ja>@Override</ja>
062 *          <jk>public int</jk> process(RestOpSession <jv>opSession</jv>) {
063 *
064 *             RestResponse <jv>res</jv> = <jv>opSession</jv>.getRestResponse();
065 *             Foo <jv>foo</jv> = <jv>res</jv>.getOutput(Foo.<jk>class</jk>);
066 *
067 *             <jk>if</jk> (<jv>foo</jv> == <jk>null</jk>)
068 *                <jk>return</jk> <jsf>NEXT</jsf>;  <jc>// Let the next processor handle it.</jc>
069 *
070 *             <jc>// Set some headers and content.</jc>
071 *             <jv>res</jv>.setHeader(<js>"Foo-ID"</js>, <jv>foo</jv>.getId());
072 *             <jv>res</jv>.getWriter().write(<js>"foo.id="</js> + <jv>foo</jv>.getId());
073 *
074 *             <jk>return</jk> <jsf>FINISHED</jsf>;  <jc>// We handled it.</jc>
075 *          }
076 *       }
077 *    }
078 * </p>
079 *
080 * <h5 class='section'>See Also:</h5><ul>
081 *    <li class='link'><a class="doclink" href="https://juneau.apache.org/docs/topics/ResponseProcessors">Response Processors</a>
082 * </ul>
083 */
084public interface ResponseProcessor {
085
086   /**
087    * Return code indicating to proceed to the next response processor in the chain.
088    */
089   int NEXT = 0;
090
091   /**
092    * Return code indicating that processing is complete and to exit the chain.
093    */
094   int FINISHED = 1;
095
096   /**
097    * Return code indicating to restart processing the chain from the beginning.
098    */
099   int RESTART = 2;
100
101   /**
102    * Process this response if possible.
103    *
104    * @param opSession The HTTP call.
105    * @return One of the following codes:
106    *    <ul>
107    *       <li><c>0</c> - The processor could not handle the request.
108    *       <li><c>1</c> - The processor was able to fully handle the request.
109    *       <li><c>2</c> - The processor was able to partially handle the request by replacing the output.
110    *          The response processors should start over.
111    *    </ul>
112    * @throws IOException
113    *    If low-level exception occurred on output stream.
114    *    <br>Results in a {@link HttpServletResponse#SC_INTERNAL_SERVER_ERROR} error.
115    * @throws BasicHttpException
116    *    If some other exception occurred.
117    *    <br>Can be used to provide an appropriate HTTP response code and message.
118    */
119   int process(RestOpSession opSession) throws IOException, BasicHttpException;
120}