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 */
017 package org.apache.commons.chain;
018
019
020 /**
021 * <p>A {@link Command} encapsulates a unit of processing work to be
022 * performed, whose purpose is to examine and/or modify the state of a
023 * transaction that is represented by a {@link Context}. Individual
024 * {@link Command}s can be assembled into a {@link Chain}, which allows
025 * them to either complete the required processing or delegate further
026 * processing to the next {@link Command} in the {@link Chain}.</p>
027 *
028 * <p>{@link Command} implementations should be designed in a thread-safe
029 * manner, suitable for inclusion in multiple {@link Chain}s that might be
030 * processed by different threads simultaneously. In general, this implies
031 * that {@link Command} classes should not maintain state information in
032 * instance variables. Instead, state information should be maintained via
033 * suitable modifications to the attributes of the {@link Context} that is
034 * passed to the <code>execute()</code> command.</p>
035 *
036 * <p>{@link Command} implementations typically retrieve and store state
037 * information in the {@link Context} instance that is passed as a parameter
038 * to the <code>execute()</code> method, using particular keys into the
039 * <code>Map</code> that can be acquired via
040 * <code>Context.getAttributes()</code>. To improve interoperability of
041 * {@link Command} implementations, a useful design pattern is to expose the
042 * key values used as JavaBeans properties of the {@link Command}
043 * implementation class itself. For example, a {@link Command} that requires
044 * an input and an output key might implement the following properties:</p>
045 *
046 * <pre>
047 * private String inputKey = "input";
048 * public String getInputKey() {
049 * return (this.inputKey);
050 * }
051 * public void setInputKey(String inputKey) {
052 * this.inputKey = inputKey;
053 * }
054 *
055 * private String outputKey = "output";
056 * public String getOutputKey() {
057 * return (this.outputKey);
058 * }
059 * public void setOutputKey(String outputKey) {
060 * this.outputKey = outputKey;
061 * }
062 * </pre>
063 *
064 * <p>And the operation of accessing the "input" information in the context
065 * would be executed by calling:</p>
066 *
067 * <pre>
068 * String input = (String) context.get(getInputKey());
069 * </pre>
070 *
071 * <p>instead of hard coding the attribute name. The use of the "Key"
072 * suffix on such property names is a useful convention to identify properties
073 * being used in this fashion, as opposed to JavaBeans properties that simply
074 * configure the internal operation of this {@link Command}.</p>
075 *
076 * @author Craig R. McClanahan
077 * @version $Revision: 480477 $ $Date: 2006-11-29 08:34:52 +0000 (Wed, 29 Nov 2006) $
078 */
079
080 public interface Command {
081
082 /**
083 * <p>Commands should return <code>CONTINUE_PROCESSING</code> if the processing
084 * of the given {@link Context} should be delegated to a subsequent
085 * {@link Command} in an enclosing {@link Chain}.</p>
086 *
087 * @since Chain 1.1
088 */
089 public static final boolean CONTINUE_PROCESSING = false;
090
091 /**
092 * <p>Commands should return <code>PROCESSING_COMPLETE</code>
093 * if the processing of the given {@link Context}
094 * has been completed.</p>
095 *
096 * @since Chain 1.1
097 */
098 public static final boolean PROCESSING_COMPLETE = true;
099 /**
100 * <p>Execute a unit of processing work to be performed. This
101 * {@link Command} may either complete the required processing
102 * and return <code>true</code>, or delegate remaining processing
103 * to the next {@link Command} in a {@link Chain} containing this
104 * {@link Command} by returning <code>false</code>
105 *
106 * @param context The {@link Context} to be processed by this
107 * {@link Command}
108 *
109 * @exception Exception general purpose exception return
110 * to indicate abnormal termination
111 * @exception IllegalArgumentException if <code>context</code>
112 * is <code>null</code>
113 *
114 * @return <code>true</code> if the processing of this {@link Context}
115 * has been completed, or <code>false</code> if the processing
116 * of this {@link Context} should be delegated to a subsequent
117 * {@link Command} in an enclosing {@link Chain}
118 */
119 boolean execute(Context context) throws Exception;
120
121
122 }