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Executor Java Doc 全解

这个接口是用来执行外部提交的Runnable任务的。它解耦了任务提交和任务执行方式，以及线程创建和调度的细节。 Executor一般用来替代显示地创建线程

举个例子，

以前你是这样： Thread(new(RunnableTask())).start()

现在你最好这样

Executor executor = anExecutor;
executor.execute(new RunnableTask1());
executor.execute(new RunnableTask2());
...
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不过，Executor接口并没有严格要求一定要 异步执行Runnable任务 。可以参考下面的例子，你也可以直接在调用者的线程中直接执行Runnable方法:

class DirectExecutor implements Executor {
   public void execute(Runnable r) {
     r.run();
   }
 }
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但是通常情况，最好还是在另一个线程去执行runnable：

class ThreadPerTaskExecutor implements Executor {
  public void execute(Runnable r) {
    new Thread(r).start();
  }
}
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许多Executor的实现都会加上一些功能限制，例如Runnable任务如何调度。如下所示，下面的Executor将提交的任务传递给一个二级Executor:

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);
    }
  }
}
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Executor还有个子类ExecutorService，它提供更多的接口。ThreadPoolExecutor类提供了一个线程池实现。Executors类提供了很多方便的工厂方法。

Memory consistency effects: Actions in a thread prior to submitting a Runnable object to an Executor happen-before its execution begins, perhaps in another thread.