/* * 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: * *
 * Executor executor = anExecutor;
 * executor.execute(new RunnableTask1());
 * executor.execute(new RunnableTask2());
 * ...
 * 
* * 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: * *
 {@code
 * class DirectExecutor implements Executor {
 *   public void execute(Runnable r) {
 *     r.run();
 *   }
 * }}
* * More typically, tasks are executed in some thread other * than the caller's thread. The executor below spawns a new thread * for each task. * *
 {@code
 * class ThreadPerTaskExecutor implements Executor {
 *   public void execute(Runnable r) {
 *     new Thread(r).start();
 *   }
 * }}
* * 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. * *
 {@code
 * class SerialExecutor implements Executor {
 *   final Queue tasks = new ArrayDeque();
 *   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);
 *     }
 *   }
 * }}
* * 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. * *

Memory consistency effects: Actions in a thread prior to * submitting a {@code Runnable} object to an {@code Executor} * happen-before * 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); }