Home | History | Annotate | Download | only in concurrent 
      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 
     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 {@code Executor} is normally used
     43  * instead of explicitly creating threads. For example, rather than
     44  * invoking {@code new Thread(new RunnableTask()).start()} for each
     45  * of a set of tasks, you might use:
     46  *
     47  * <pre> {@code
     48  * Executor executor = anExecutor();
     49  * executor.execute(new RunnableTask1());
     50  * executor.execute(new RunnableTask2());
     51  * ...}</pre>
     52  *
     53  * However, the {@code Executor} interface does not strictly require
     54  * that execution be asynchronous. In the simplest case, an executor
     55  * can run the submitted task immediately in the caller's thread:
     56  *
     57  * <pre> {@code
     58  * class DirectExecutor implements Executor {
     59  *   public void execute(Runnable r) {
     60  *     r.run();
     61  *   }
     62  * }}</pre>
     63  *
     64  * More typically, tasks are executed in some thread other than the
     65  * caller's thread.  The executor below spawns a new thread for each
     66  * task.
     67  *
     68  * <pre> {@code
     69  * class ThreadPerTaskExecutor implements Executor {
     70  *   public void execute(Runnable r) {
     71  *     new Thread(r).start();
     72  *   }
     73  * }}</pre>
     74  *
     75  * Many {@code Executor} implementations impose some sort of
     76  * limitation on how and when tasks are scheduled.  The executor below
     77  * serializes the submission of tasks to a second executor,
     78  * illustrating a composite executor.
     79  *
     80  * <pre> {@code
     81  * class SerialExecutor implements Executor {
     82  *   final Queue<Runnable> tasks = new ArrayDeque<>();
     83  *   final Executor executor;
     84  *   Runnable active;
     85  *
     86  *   SerialExecutor(Executor executor) {
     87  *     this.executor = executor;
     88  *   }
     89  *
     90  *   public synchronized void execute(final Runnable r) {
     91  *     tasks.add(new Runnable() {
     92  *       public void run() {
     93  *         try {
     94  *           r.run();
     95  *         } finally {
     96  *           scheduleNext();
     97  *         }
     98  *       }
     99  *     });
    100  *     if (active == null) {
    101  *       scheduleNext();
    102  *     }
    103  *   }
    104  *
    105  *   protected synchronized void scheduleNext() {
    106  *     if ((active = tasks.poll()) != null) {
    107  *       executor.execute(active);
    108  *     }
    109  *   }
    110  * }}</pre>
    111  *
    112  * The {@code Executor} implementations provided in this package
    113  * implement {@link ExecutorService}, which is a more extensive
    114  * interface.  The {@link ThreadPoolExecutor} class provides an
    115  * extensible thread pool implementation. The {@link Executors} class
    116  * provides convenient factory methods for these Executors.
    117  *
    118  * <p>Memory consistency effects: Actions in a thread prior to
    119  * submitting a {@code Runnable} object to an {@code Executor}
    120  * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
    121  * its execution begins, perhaps in another thread.
    122  *
    123  * @since 1.5
    124  * @author Doug Lea
    125  */
    126 public interface Executor {
    127 
    128     /**
    129      * Executes the given command at some time in the future.  The command
    130      * may execute in a new thread, in a pooled thread, or in the calling
    131      * thread, at the discretion of the {@code Executor} implementation.
    132      *
    133      * @param command the runnable task
    134      * @throws RejectedExecutionException if this task cannot be
    135      * accepted for execution
    136      * @throws NullPointerException if command is null
    137      */
    138     void execute(Runnable command);
    139 }
    140