1 /*
2 * Licensed to the Apache Software Foundation (ASF) under one or more
3 * contributor license agreements. See the NOTICE file distributed with
4 * this work for additional information regarding copyright ownership.
5 * The ASF licenses this file to You under the Apache License, Version 2.0
6 * (the "License"); you may not use this file except in compliance with
7 * the License. You may obtain a copy of the License at
8 *
9 * http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 */
17 package org.apache.commons.chain;
18
19
20 /**
21 * <p>A {@link Chain} represents a configured list of
22 * {@link Command}s that will be executed in order to perform processing
23 * on a specified {@link Context}. Each included {@link Command} will be
24 * executed in turn, until either one of them returns <code>true</code>,
25 * one of the executed {@link Command}s throws an exception,
26 * or the end of the chain has been reached. The {@link Chain} itself will
27 * return the return value of the last {@link Command} that was executed
28 * (if no exception was thrown), or rethrow the thrown exception.</p>
29 *
30 * <p>Note that {@link Chain} extends {@link Command}, so that the two can
31 * be used interchangeably when a {@link Command} is expected. This makes it
32 * easy to assemble workflows in a hierarchical manner by combining subchains
33 * into an overall processing chain.</p>
34 *
35 * <p>To protect applications from evolution of this interface, specialized
36 * implementations of {@link Chain} should generally be created by extending
37 * the provided base class {@link org.apache.commons.chain.impl.ChainBase})
38 * rather than directly implementing this interface.</p>
39 *
40 * <p>{@link Chain} implementations should be designed in a thread-safe
41 * manner, suitable for execution on multiple threads simultaneously. In
42 * general, this implies that the state information identifying which
43 * {@link Command} is currently being executed should be maintained in a
44 * local variable inside the <code>execute()</code> method, rather than
45 * in an instance variable. The {@link Command}s in a {@link Chain} may be
46 * configured (via calls to <code>addCommand()</code>) at any time before
47 * the <code>execute()</code> method of the {@link Chain} is first called.
48 * After that, the configuration of the {@link Chain} is frozen.</p>
49 *
50 * @author Craig R. McClanahan
51 * @version $Revision: 480477 $ $Date: 2006-11-29 08:34:52 +0000 (Wed, 29 Nov 2006) $
52 */
53
54 public interface Chain extends Command {
55
56
57 /**
58 * <p>Add a {@link Command} to the list of {@link Command}s that will
59 * be called in turn when this {@link Chain}'s <code>execute()</code>
60 * method is called. Once <code>execute()</code> has been called
61 * at least once, it is no longer possible to add additional
62 * {@link Command}s; instead, an exception will be thrown.</p>
63 *
64 * @param command The {@link Command} to be added
65 *
66 * @exception IllegalArgumentException if <code>command</code>
67 * is <code>null</code>
68 * @exception IllegalStateException if this {@link Chain} has already
69 * been executed at least once, so no further configuration is allowed
70 */
71 void addCommand(Command command);
72
73
74 /**
75 * <p>Execute the processing represented by this {@link Chain} according
76 * to the following algorithm.</p>
77 * <ul>
78 * <li>If there are no configured {@link Command}s in the {@link Chain},
79 * return <code>false</code>.</li>
80 * <li>Call the <code>execute()</code> method of each {@link Command}
81 * configured on this chain, in the order they were added via calls
82 * to the <code>addCommand()</code> method, until the end of the
83 * configured {@link Command}s is encountered, or until one of
84 * the executed {@link Command}s returns <code>true</code>
85 * or throws an exception.</li>
86 * <li>Walk backwards through the {@link Command}s whose
87 * <code>execute()</code> methods, starting with the last one that
88 * was executed. If this {@link Command} instance is also a
89 * {@link Filter}, call its <code>postprocess()</code> method,
90 * discarding any exception that is thrown.</li>
91 * <li>If the last {@link Command} whose <code>execute()</code> method
92 * was called threw an exception, rethrow that exception.</li>
93 * <li>Otherwise, return the value returned by the <code>execute()</code>
94 * method of the last {@link Command} that was executed. This will be
95 * <code>true</code> if the last {@link Command} indicated that
96 * processing of this {@link Context} has been completed, or
97 * <code>false</code> if none of the called {@link Command}s
98 * returned <code>true</code>.</li>
99 * </ul>
100 *
101 * @param context The {@link Context} to be processed by this
102 * {@link Chain}
103 *
104 * @exception Exception if thrown by one of the {@link Command}s
105 * in this {@link Chain} but not handled by a <code>postprocess()</code>
106 * method of a {@link Filter}
107 * @exception IllegalArgumentException if <code>context</code>
108 * is <code>null</code>
109 *
110 * @return <code>true</code> if the processing of this {@link Context}
111 * has been completed, or <code>false</code> if the processing
112 * of this {@link Context} should be delegated to a subsequent
113 * {@link Command} in an enclosing {@link Chain}
114 */
115 boolean execute(Context context) throws Exception;
116
117
118 }