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1 /*
2  * Copyright 2003-2004 The Apache Software Foundation
3  *
4  * Licensed under the Apache License, Version 2.0 (the "License");
5  * you may not use this file except in compliance with the License.
6  * You may obtain a copy of the License at
7  *
8  * http://www.apache.org/licenses/LICENSE-2.0
9  *
10  * Unless required by applicable law or agreed to in writing, software
11  * distributed under the License is distributed on an "AS IS" BASIS,
12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13  * See the License for the specific language governing permissions and
14  * limitations under the License.
15  */
16 package org.apache.commons.chain;
17
18
19 /**
20  * <p>A {@link Chain} represents a configured list of
21  * {@link Command}s that will be executed in order to perform processing
22  * on a specified {@link Context}. Each included {@link Command} will be
23  * executed in turn, until either one of them returns <code>true</code>,
24  * one of the executed {@link Command}s throws an exception,
25  * or the end of the chain has been reached. The {@link Chain} itself will
26  * return the return value of the last {@link Command} that was executed
27  * (if no exception was thrown), or rethrow the thrown exception.</p>
28  *
29  * <p>Note that {@link Chain} extends {@link Command}, so that the two can
30  * be used interchangeably when a {@link Command} is expected. This makes it
31  * easy to assemble workflows in a hierarchical manner by combining subchains
32  * into an overall processing chain.</p>
33  *
34  * <p>To protect applications from evolution of this interface, specialized
35  * implementations of {@link Chain} should generally be created by extending
36  * the provided base class {@link org.apache.commons.chain.impl.ChainBase})
37  * rather than directly implementing this interface.</p>
38  *
39  * <p>{@link Chain} implementations should be designed in a thread-safe
40  * manner, suitable for execution on multiple threads simultaneously. In
41  * general, this implies that the state information identifying which
42  * {@link Command} is currently being executed should be maintained in a
43  * local variable inside the <code>execute()</code> method, rather than
44  * in an instance variable. The {@link Command}s in a {@link Chain} may be
45  * configured (via calls to <code>addCommand()</code>) at any time before
46  * the <code>execute()</code> method of the {@link Chain} is first called.
47  * After that, the configuration of the {@link Chain} is frozen.</p>
48  *
49  * @author Craig R. McClanahan
50  * @version $Revision: 1.6 $ $Date: 2004/02/25 00:01:07 $
51  */
52
53 public interface Chain extends Command {
54
55
56     /**
57      * <p>Add a {@link Command} to the list of {@link Command}s that will
58      * be called in turn when this {@link Chain}'s <code>execute()</code>
59      * method is called. Once <code>execute()</code> has been called
60      * at least once, it is no longer possible to add additional
61      * {@link Command}s; instead, an exception will be thrown.</p>
62      *
63      * @param command The {@link Command} to be added
64      *
65      * @exception IllegalArgumentException if <code>command</code>
66      * is <code>null</code>
67      * @exception IllegalStateException if this {@link Chain} has already
68      * been executed at least once, so no further configuration is allowed
69      */
70     void addCommand(Command command);
71
72
73     /**
74      * <p>Execute the processing represented by this {@link Chain} according
75      * to the following algorithm.</p>
76      * <ul>
77      * <li>If there are no configured {@link Command}s in the {@link Chain},
78      * return <code>false</code>.</li>
79      * <li>Call the <code>execute()</code> method of each {@link Command}
80      * configured on this chain, in the order they were added via calls
81      * to the <code>addCommand()</code> method, until the end of the
82      * configured {@link Command}s is encountered, or until one of
83      * the executed {@link Command}s returns <code>true</code>
84      * or throws an exception.</li>
85      * <li>Walk backwards through the {@link Command}s whose
86      * <code>execute()</code> methods, starting with the last one that
87      * was executed. If this {@link Command} instance is also a
88      * {@link Filter}, call its <code>postprocess()</code> method,
89      * discarding any exception that is thrown.</li>
90      * <li>If the last {@link Command} whose <code>execute()</code> method
91      * was called threw an exception, rethrow that exception.</li>
92      * <li>Otherwise, return the value returned by the <code>execute()</code>
93      * method of the last {@link Command} that was executed. This will be
94      * <code>true</code> if the last {@link Command} indicated that
95      * processing of this {@link Context} has been completed, or
96      * <code>false</code> if none of the called {@link Command}s
97      * returned <code>true</code>.</li>
98      * </ul>
99      *
100      * @param context The {@link Context} to be processed by this
101      * {@link Chain}
102      *
103      * @exception Exception if thrown by one of the {@link Command}s
104      * in this {@link Chain} but not handled by a <code>postprocess()</code>
105      * method of a {@link Filter}
106      * @exception IllegalArgumentException if <code>context</code>
107      * is <code>null</code>
108      *
109      * @return <code>true</code> if the processing of this {@link Context}
110      * has been completed, or <code>false</code> if the processing
111      * of this {@link Context} should be delegated to a subsequent
112      * {@link Command} in an enclosing {@link Chain}
113      */
114     boolean execute(Context context) throws Exception JavaDoc;
115
116
117 }
118
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