/*

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

/*

 * This file is available under and governed by the GNU General Public

 * License version 2 only, as published by the Free Software Foundation.

 * However, the following notice accompanied the original version of this

 * file:

 *

 * 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/publicdomain/zero/1.0/

 */

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 {@code Executor} is normally used

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

 * invoking {@code new Thread(new RunnableTask()).start()} for each

 * of a set of tasks, you might use:

 *

 * <pre> {@code

 * Executor executor = anExecutor();

 * executor.execute(new RunnableTask1());

 * executor.execute(new RunnableTask2());

 * ...}</pre>

 *

 * However, the {@code Executor} 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> {@code

 * 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> {@code

 * class ThreadPerTaskExecutor implements Executor {

 *   public void execute(Runnable r) {

 *     new Thread(r).start();

 *   }

 * }}</pre>

 *

 * Many {@code Executor} 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> {@code

 * class SerialExecutor implements Executor {

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

 *   final Executor executor;

 *   Runnable active;

 *

 *   SerialExecutor(Executor executor) {

 *     this.executor = executor;

 *   }

 *

 *   public synchronized void execute(Runnable r) {

 *     tasks.add(() -> {

 *       try {

 *         r.run();

 *       } finally {

 *         scheduleNext();

 *       }

 *     });

 *     if (active == null) {

 *       scheduleNext();

 *     }

 *   }

 *

 *   protected synchronized void scheduleNext() {

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

 *       executor.execute(active);

 *     }

 *   }

 * }}</pre>

 *

 * The {@code Executor} 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 {@code Executor} 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);

}