Executor接口的定义

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:
 *
 Executor用于执行提交的Runnable任务。Executor提供了每个线程如何运行，线程的具体使用调度
 机制的解耦的一种方式。一般用Executor执行线程，而不是显示地创建线程。比如
 调用new Thread(new(RunnableTask())).start()执行线程任务。我们可以用以下方式执行线程：
 * <pre>
 * Executor executor = [i]anExecutor[/i];
 * 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:
 *
Executor不需要严格意义上地异步执行线程。在一些简单的例子中，executor可以
立即在调用线程中，运行提交的任务。
 * <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.
 *
 大多数情况下，任务是在一些线程中执行，而不是调用者线程。下面的执行器
 为每个任务创建一个新的线程。
 * <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.
 *一些执行器用于实现任务何时如何调度，下面executor用于执行串行任务
 *  <pre> {@code
 * 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.
 *
ExecutorService提供了Executor实现，是一个广泛使用的接口。ThreadPoolExecutor为
一个可扩展线程池的实现。Executors提供了线程池执行器的便利的工厂方法。
 * <p>Memory consistency effects: Actions in a thread prior to
 * submitting a {@code Runnable} object to an {@code Executor}
 * [url=package-summary.html#MemoryVisibility]<i>happen-before</i>[/url]
 * 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.
     *
     在将来的某个时间执行给个的线程命令。这个命令也许在一个新线程中执行，
     或一个池线程中，或调用线程中，或任凭Executor实现任意执行。
     * @param command the runnable task
     * @throws RejectedExecutionException if this task cannot be
     * accepted for execution.如果线程必能不执行器执行，这抛出拒绝执行异常
     * @throws NullPointerException if command is null
     */
    void execute(Runnable command);
}