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/*

 * Written by Doug Lea with assistance from members of JCP JSR-166

 * Expert Group and released to the public domain, as explained at

 * http://creativecommons.org/licenses/publicdomain

 */

package java.util.concurrent;

/**

 * An object that executes submitted {@link Runnable} tasks. This

 * interface provides a way of decoupling task submission from the

 * mechanics of how each task will be run, including details of thread

 * use, scheduling, etc.  An <tt>Executor</tt> is normally used

 * instead of explicitly creating threads. For example, rather than

 * invoking <tt>new Thread(new(RunnableTask())).start()</tt> for each

 * of a set of tasks, you might use:

 *

 * <pre>

 * Executor executor = <em>anExecutor</em>;

 * executor.execute(new RunnableTask1());

 * executor.execute(new RunnableTask2());

 * ...

 * </pre>

 *

 * However, the <tt>Executor</tt> interface does not strictly

 * require that execution be asynchronous. In the simplest case, an

 * executor can run the submitted task immediately in the caller's

 * thread:

 *

 * <pre>

 * class DirectExecutor implements Executor {

 *     public void execute(Runnable r) {

 *         r.run();

 *     }

 * }</pre>

 *

 * More typically, tasks are executed in some thread other

 * than the caller's thread.  The executor below spawns a new thread

 * for each task.

 *

 * <pre>

 * class ThreadPerTaskExecutor implements Executor {

 *     public void execute(Runnable r) {

 *         new Thread(r).start();

 *     }

 * }</pre>

 *

 * Many <tt>Executor</tt> implementations impose some sort of

 * limitation on how and when tasks are scheduled.  The executor below

 * serializes the submission of tasks to a second executor,

 * illustrating a composite executor.

 *

 * <pre>

 * class SerialExecutor implements Executor {

 *     final Queue<Runnable> tasks = new ArrayDeque<Runnable>();

 *     final Executor executor;

 *     Runnable active;

 *

 *     SerialExecutor(Executor executor) {

 *         this.executor = executor;

 *     }

 *

 *     public synchronized void execute(final Runnable r) {

 *         tasks.offer(new Runnable() {

 *             public void run() {

 *                 try {

 *                     r.run();

 *                 } finally {

 *                     scheduleNext();

 *                 }

 *             }

 *         });

 *         if (active == null) {

 *             scheduleNext();

 *         }

 *     }

 *

 *     protected synchronized void scheduleNext() {

 *         if ((active = tasks.poll()) != null) {

 *             executor.execute(active);

 *         }

 *     }

 * }</pre>

 *

 * The <tt>Executor</tt> implementations provided in this package

 * implement {@link ExecutorService}, which is a more extensive

 * interface.  The {@link ThreadPoolExecutor} class provides an

 * extensible thread pool implementation. The {@link Executors} class

 * provides convenient factory methods for these Executors.

 *

 * <p>Memory consistency effects: Actions in a thread prior to

 * submitting a {@code Runnable} object to an {@code Executor}

 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>

 * its execution begins, perhaps in another thread.

 *

 * @since 1.5

 * @author Doug Lea

 */

public interface Executor {

    /**

     * Executes the given command at some time in the future.  The command

     * may execute in a new thread, in a pooled thread, or in the calling

     * thread, at the discretion of the <tt>Executor</tt> implementation.

     *

     * @param command the runnable task

     * @throws RejectedExecutionException if this task cannot be

     * accepted for execution.

     * @throws NullPointerException if command is null

     */

    void execute(Runnable command);

}