多线程：保证三个线程依次按顺序执行？newSingleThreadExecutor！！！

newSingleThreadExecutor 这个线程池，保证线程里面的任务依次执行，这让我发现了新大陆，

立马实践了一下，发现不负所望；

public class TestJoin {
    public static void main(String[] args) throws InterruptedException {
        final Thread t1 = new Thread(new Runnable() {
            public void run() {
                System.out.println(Thread.currentThread().getName() + " run 1");
            }
        }, "T1");
        final Thread t2 = new Thread(new Runnable() {
            public void run() {
                System.out.println(Thread.currentThread().getName() + " run 2");
                try {
                    t1.join(10);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }, "T2");
        final Thread t3 = new Thread(new Runnable() {
            public void run() {
                System.out.println(Thread.currentThread().getName() + " run 3");
                try {
                    t2.join(10);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }, "T3");
        // method1  第一反应的解决方法，事实证明不可靠
        //t1.start();
        //t2.start();
        //t3.start();

//        method 2 使用 单个任务的线程池来实现。保证线程的依次执行
        ExecutorService executor = Executors.newSingleThreadExecutor();
        executor.submit(t1);
        executor.submit(t2);
        executor.submit(t3);
        executor.shutdown();
    }
}

---

在这里发现结果每次都是 t1执行，t2执行，t3执行，这里达到目的了之后，不禁回想，这个Single线程池为啥这么好用，不由得就像把线程池狠狠的看一遍，然后就去翻看了源代码

/**
     * Creates an Executor that uses a single worker thread operating
     * off an unbounded queue. (Note however that if this single
     * thread terminates due to a failure during execution prior to
     * shutdown, a new one will take its place if needed to execute
     * subsequent tasks.)  Tasks are guaranteed to execute
     * sequentially, and no more than one task will be active at any
     * given time. Unlike the otherwise equivalent
     * {@code newFixedThreadPool(1)} the returned executor is
     * guaranteed not to be reconfigurable to use additional threads.
     *
     * @return the newly created single-threaded Executor
     */
    public static ExecutorService newSingleThreadExecutor() {
        return new FinalizableDelegatedExecutorService
            (new ThreadPoolExecutor(1, 1,
                                    0L, TimeUnit.MILLISECONDS,
                                    new LinkedBlockingQueue()));
    }

这个实例化的意思我尝试翻译了一下，意思就是创建一个只有一个线程的线程池来操作不限数量的队列，也就是把线程放进了一个队列中，队列我们都知道是FIFO的（LinkedBlockingQueue）。SingleThreadExecutor的工作线程只有一个，其他队列中的线程都处于休眠，也就是sleep状态，当这个worker线程做完事了，也就是run方法运行结束，就又从队列中拿出一个休眠线程（sleep）出来唤醒（notify），这样依次把队列中的所有线程处理完毕，这样并没有结束，如果线程池中没有待处理的线程，线程池一直会等待，等待下一次任务的提交，除非把线程池给shutdown掉，这样线程池的生命周期才算完毕。