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/licenses/publicdomain
  34  */
  35 
  36 package 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  */
 128 public 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 }