Executor.java revision 51b1b6997fd3f980076b8081f7f1165ccc2a4008 
1/*
2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
3 *
4 * This code is free software; you can redistribute it and/or modify it
5 * under the terms of the GNU General Public License version 2 only, as
6 * published by the Free Software Foundation.  Oracle designates this
7 * particular file as subject to the "Classpath" exception as provided
8 * by Oracle in the LICENSE file that accompanied this code.
9 *
10 * This code is distributed in the hope that it will be useful, but WITHOUT
11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
13 * version 2 for more details (a copy is included in the LICENSE file that
14 * accompanied this code).
15 *
16 * You should have received a copy of the GNU General Public License version
17 * 2 along with this work; if not, write to the Free Software Foundation,
18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
19 *
20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
21 * or visit www.oracle.com if you need additional information or have any
22 * questions.
23 */
24
25/*
26 * This file is available under and governed by the GNU General Public
27 * License version 2 only, as published by the Free Software Foundation.
28 * However, the following notice accompanied the original version of this
29 * file:
30 *
31 * Written by Doug Lea with assistance from members of JCP JSR-166
32 * Expert Group and released to the public domain, as explained at
33 * http://creativecommons.org/publicdomain/zero/1.0/
34 */
35
36package java.util.concurrent;
37
38/**
39 * An object that executes submitted {@link Runnable} tasks. This
40 * interface provides a way of decoupling task submission from the
41 * mechanics of how each task will be run, including details of thread
42 * use, scheduling, etc.  An <tt>Executor</tt> is normally used
43 * instead of explicitly creating threads. For example, rather than
44 * invoking <tt>new Thread(new(RunnableTask())).start()</tt> for each
45 * of a set of tasks, you might use:
46 *
47 * <pre>
48 * Executor executor = <em>anExecutor</em>;
49 * executor.execute(new RunnableTask1());
50 * executor.execute(new RunnableTask2());
51 * ...
52 * </pre>
53 *
54 * However, the <tt>Executor</tt> interface does not strictly
55 * require that execution be asynchronous. In the simplest case, an
56 * executor can run the submitted task immediately in the caller's
57 * thread:
58 *
59 * <pre>
60 * class DirectExecutor implements Executor {
61 *     public void execute(Runnable r) {
62 *         r.run();
63 *     }
64 * }</pre>
65 *
66 * More typically, tasks are executed in some thread other
67 * than the caller's thread.  The executor below spawns a new thread
68 * for each task.
69 *
70 * <pre>
71 * class ThreadPerTaskExecutor implements Executor {
72 *     public void execute(Runnable r) {
73 *         new Thread(r).start();
74 *     }
75 * }</pre>
76 *
77 * Many <tt>Executor</tt> implementations impose some sort of
78 * limitation on how and when tasks are scheduled.  The executor below
79 * serializes the submission of tasks to a second executor,
80 * illustrating a composite executor.
81 *
82 *  <pre> {@code
83 * class SerialExecutor implements Executor {
84 *   final Queue<Runnable> tasks = new ArrayDeque<Runnable>();
85 *   final Executor executor;
86 *   Runnable active;
87 *
88 *   SerialExecutor(Executor executor) {
89 *     this.executor = executor;
90 *   }
91 *
92 *   public synchronized void execute(final Runnable r) {
93 *     tasks.offer(new Runnable() {
94 *       public void run() {
95 *         try {
96 *           r.run();
97 *         } finally {
98 *           scheduleNext();
99 *         }
100 *       }
101 *     });
102 *     if (active == null) {
103 *       scheduleNext();
104 *     }
105 *   }
106 *
107 *   protected synchronized void scheduleNext() {
108 *     if ((active = tasks.poll()) != null) {
109 *       executor.execute(active);
110 *     }
111 *   }
112 * }}</pre>
113 *
114 * The <tt>Executor</tt> implementations provided in this package
115 * implement {@link ExecutorService}, which is a more extensive
116 * interface.  The {@link ThreadPoolExecutor} class provides an
117 * extensible thread pool implementation. The {@link Executors} class
118 * provides convenient factory methods for these Executors.
119 *
120 * <p>Memory consistency effects: Actions in a thread prior to
121 * submitting a {@code Runnable} object to an {@code Executor}
122 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
123 * its execution begins, perhaps in another thread.
124 *
125 * @since 1.5
126 * @author Doug Lea
127 */
128public interface Executor {
129
130    /**
131     * Executes the given command at some time in the future.  The command
132     * may execute in a new thread, in a pooled thread, or in the calling
133     * thread, at the discretion of the <tt>Executor</tt> implementation.
134     *
135     * @param command the runnable task
136     * @throws RejectedExecutionException if this task cannot be
137     * accepted for execution.
138     * @throws NullPointerException if command is null
139     */
140    void execute(Runnable command);
141}
142