Executor 任务执行器

Executor：

• 是一个接口
• 用于执行提交的任务
• 解耦任务提交和执行（线程的创建及调度）

Executor的实现可以根据实际需求延展不同的逻辑：
1. 对于提交的任务同步或者异步执行，如下同步执行：

class ThreadPerTaskExecutor implements Executor {
   public void execute(Runnable r) {
     new Thread(r).start();
}

2. 另起线程执行任务，如下：

 class ThreadPerTaskExecutor implements Executor {
     public void execute(Runnable r) {
     new Thread(r).start();
 }

3. 对于执行的任务添加限制：

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);
     }
   }
 }}