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 * 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},
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.
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()} 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()}) at any time before
46 * the {@code execute()} method of the {@link Chain} is first called.
47 * After that, the configuration of the {@link Chain} is frozen.</p>
48 *
49 * @param <C> Type of the context associated with this chain
50 *
51 * @author Craig R. McClanahan
52 * @version $Revision$ $Date$
53 */
54 public interface Chain<C extends Context> extends Command<C> {
55
56 /**
57 * 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()}
59 * method is called. Once {@code execute()} 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.
62 *
63 * @param <CMD> the {@link Command} type to be added in the {@link Chain}
64 * @param command The {@link Command} to be added
65 *
66 * @throws IllegalArgumentException if {@code command}
67 * is {@code null}
68 * @throws IllegalStateException if this {@link Chain} has already
69 * been executed at least once, so no further configuration
70 * is allowed
71 */
72 <CMD extends Command<C>> void addCommand(CMD command);
73
74 /**
75 * Execute the processing represented by this {@link Chain} according
76 * to the following algorithm.
77 * <ul>
78 * <li>If there are no configured {@link Command}s in the {@link Chain},
79 * return {@code false}.</li>
80 * <li>Call the {@code execute()} method of each {@link Command}
81 * configured on this chain, in the order they were added via calls
82 * to the {@code addCommand()} 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}
85 * or throws an exception.</li>
86 * <li>Walk backwards through the {@link Command}s whose
87 * {@code execute()} 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()} method,
90 * discarding any exception that is thrown.</li>
91 * <li>If the last {@link Command} whose {@code execute()} method
92 * was called threw an exception, rethrow that exception.</li>
93 * <li>Otherwise, return the value returned by the {@code execute()}
94 * method of the last {@link Command} that was executed. This will be
95 * {@code true} if the last {@link Command} indicated that
96 * processing of this {@link Context} has been completed, or
97 * {@code false} if none of the called {@link Command}s
98 * returned {@code true}.</li>
99 * </ul>
100 *
101 * @param context The {@link Context} to be processed by this
102 * {@link Chain}
103 *
104 * @return {@code true} if the processing of this {@link Context}
105 * has been completed, or {@code false} if the processing
106 * of this {@link Context} should be delegated to a
107 * subsequent {@link Command} in an enclosing {@link Chain}
108 *
109 * @throws Exception if thrown by one of the {@link Command}s
110 * in this {@link Chain} but not handled by a
111 * {@code postprocess()} method of a {@link Filter}
112 * @throws IllegalArgumentException if {@code context}
113 * is {@code null}
114 */
115 @Override
116 boolean execute(C context) throws Exception;
117 }