线程池（4）Executors.newScheduledThreadPool-只执行1次

例子1：延迟3秒后，只执行1次

ScheduledExecutorService es = Executors.newScheduledThreadPool(5);
        log.info("开始时间");
        try {
            for (int i = 0; i < 20; i++) {
                Runnable syncRunnable = new Runnable() {
                    @Override
                    public void run() {
                        log.info(Thread.currentThread().getName());
                    }
                };
                es.schedule(syncRunnable, 3000, TimeUnit.MILLISECONDS);
            }
        } finally {
            es.shutdown();
        }

运行结果：

            10:41:39.589 开始时间
            10:41:42.595 pool-1-thread-1
            10:41:42.595 pool-1-thread-2
            10:41:42.595 pool-1-thread-3
            10:41:42.595 pool-1-thread-4
            10:41:42.596 pool-1-thread-2
            10:41:42.596 pool-1-thread-5
            10:41:42.596 pool-1-thread-1
            10:41:42.596 pool-1-thread-3
            10:41:42.597 pool-1-thread-4
            10:41:42.597 pool-1-thread-2
            10:41:42.597 pool-1-thread-5
            10:41:42.597 pool-1-thread-1
            10:41:42.598 pool-1-thread-3
            10:41:42.598 pool-1-thread-4
            10:41:42.598 pool-1-thread-2
            10:41:42.599 pool-1-thread-5
            10:41:42.599 pool-1-thread-1
            10:41:42.600 pool-1-thread-3
            10:41:42.600 pool-1-thread-4
            10:41:42.600 pool-1-thread-2

调用的ThreadPoolExecutor：

public ScheduledThreadPoolExecutor(int corePoolSize) {
        super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
              new DelayedWorkQueue());
    }

corePoolSize=5，maximumPoolSize=Integer.MAX_VALUE

keepAliveTime=0纳秒

allowCoreThreadTimeout=false（默认）

采用延迟队列DelayedWorkQueue

因此，

• 线程池中的线程数永远是5，永久存活
• 对于新任务，当队列未满时，插入队列；当队列已满时，默认执行AbortPolicy，即抛出异常。
• 支持线程reuse

因此，类似newFixedThreadPool

public ScheduledThreadPoolExecutor(int corePoolSize) {        super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,              new DelayedWorkQueue());    }