1/*
2 * Written by Doug Lea with assistance from members of JCP JSR-166
3 * Expert Group and released to the public domain, as explained at
4 * http://creativecommons.org/publicdomain/zero/1.0/
5 */
6
7package java.util.concurrent;
8
9/**
10 * An object that executes submitted {@link Runnable} tasks. This
11 * interface provides a way of decoupling task submission from the
12 * mechanics of how each task will be run, including details of thread
13 * use, scheduling, etc.  An {@code Executor} is normally used
14 * instead of explicitly creating threads. For example, rather than
15 * invoking {@code new Thread(new(RunnableTask())).start()} for each
16 * of a set of tasks, you might use:
17 *
18 * <pre>
19 * Executor executor = <em>anExecutor</em>;
20 * executor.execute(new RunnableTask1());
21 * executor.execute(new RunnableTask2());
22 * ...
23 * </pre>
24 *
25 * However, the {@code Executor} interface does not strictly
26 * require that execution be asynchronous. In the simplest case, an
27 * executor can run the submitted task immediately in the caller's
28 * thread:
29 *
30 *  <pre> {@code
31 * class DirectExecutor implements Executor {
32 *   public void execute(Runnable r) {
33 *     r.run();
34 *   }
35 * }}</pre>
36 *
37 * More typically, tasks are executed in some thread other
38 * than the caller's thread.  The executor below spawns a new thread
39 * for each task.
40 *
41 *  <pre> {@code
42 * class ThreadPerTaskExecutor implements Executor {
43 *   public void execute(Runnable r) {
44 *     new Thread(r).start();
45 *   }
46 * }}</pre>
47 *
48 * Many {@code Executor} implementations impose some sort of
49 * limitation on how and when tasks are scheduled.  The executor below
50 * serializes the submission of tasks to a second executor,
51 * illustrating a composite executor.
52 *
53 *  <pre> {@code
54 * class SerialExecutor implements Executor {
55 *   final Queue<Runnable> tasks = new ArrayDeque<Runnable>();
56 *   final Executor executor;
57 *   Runnable active;
58 *
59 *   SerialExecutor(Executor executor) {
60 *     this.executor = executor;
61 *   }
62 *
63 *   public synchronized void execute(final Runnable r) {
64 *     tasks.offer(new Runnable() {
65 *       public void run() {
66 *         try {
67 *           r.run();
68 *         } finally {
69 *           scheduleNext();
70 *         }
71 *       }
72 *     });
73 *     if (active == null) {
74 *       scheduleNext();
75 *     }
76 *   }
77 *
78 *   protected synchronized void scheduleNext() {
79 *     if ((active = tasks.poll()) != null) {
80 *       executor.execute(active);
81 *     }
82 *   }
83 * }}</pre>
84 *
85 * The {@code Executor} implementations provided in this package
86 * implement {@link ExecutorService}, which is a more extensive
87 * interface.  The {@link ThreadPoolExecutor} class provides an
88 * extensible thread pool implementation. The {@link Executors} class
89 * provides convenient factory methods for these Executors.
90 *
91 * <p>Memory consistency effects: Actions in a thread prior to
92 * submitting a {@code Runnable} object to an {@code Executor}
93 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
94 * its execution begins, perhaps in another thread.
95 *
96 * @since 1.5
97 * @author Doug Lea
98 */
99public interface Executor {
100
101    /**
102     * Executes the given command at some time in the future.  The command
103     * may execute in a new thread, in a pooled thread, or in the calling
104     * thread, at the discretion of the {@code Executor} implementation.
105     *
106     * @param command the runnable task
107     * @throws RejectedExecutionException if this task cannot be
108     * accepted for execution
109     * @throws NullPointerException if command is null
110     */
111    void execute(Runnable command);
112}
113