多线程下保证线程执行的顺序

通过倒数计时器CountDownLatch实现。
CountDownLatch通过计数器提供了更灵活的控制，只要检测到计数器为0当前线程就可以往下执行而不用管相应的thread是否执行完毕


    private static CountDownLatch countDownLatch2 = new CountDownLatch(1);
    private static CountDownLatch countDownLatch3 = new CountDownLatch(1);

    @Test
    public void test1() throws InterruptedException {
    
    
        Thread thread1 = new Thread(() -> {
    
    
            System.out.println(Thread.currentThread().getName() + "打开冰箱");
            countDownLatch2.countDown();
        }, "线程1");

        Thread thread2 = new Thread(() -> {
    
    
            try {
    
    
                countDownLatch2.await();
                System.out.println(Thread.currentThread().getName() + "拿出可乐" );
                countDownLatch3.countDown();
            } catch (InterruptedException e) {
    
    
                e.printStackTrace();
            }
        }, "线程2");

        Thread thread3 = new Thread(() -> {
    
    
            try {
    
    
                countDownLatch3.await();
                System.out.println(Thread.currentThread().getName() + "关上冰箱" );
            } catch (InterruptedException e) {
    
    
                e.printStackTrace();
            }
        }, "线程3");

        //下面三行代码顺序可随意调整，程序运行结果不受影响
        thread3.start();
        thread2.start();
        thread1.start();
    }

在主线程中通过join()方法指定顺序
通过join()方法使当前线程“阻塞”，等待指定线程执行完毕后继续执行。
在线程thread2中，加上一句thread1.join()，其意义在于，当前线程2运行到此行代码时会进入阻塞状态，直到线程thread1执行完毕后，线程thread2才会继续运行，这就保证了线程thread1与线程thread2的运行顺序。
底层通过jdk synchronized wait notify 实现


    @Test
    public void test2() throws InterruptedException {
    
    
        Thread thread1 = new Thread(() -> {
    
    
            System.out.println(Thread.currentThread().getName() + "打开冰箱");
        }, "线程1");
        Thread thread2 = new Thread(() -> {
    
    
            System.out.println(Thread.currentThread().getName() + "拿出可乐" );
        }, "线程2");
        Thread thread3 = new Thread(() -> {
    
    
            System.out.println(Thread.currentThread().getName() + "关上冰箱" );
        }, "线程3");
        //等待thread1执行完毕后，再执行thread2，最后执行thread3
        thread1.start();
        thread1.join();
        thread2.start();
        thread2.join();
        thread3.start();
    }

在子线程中通过join()方法指定顺序

    @Test
    public void test3() throws InterruptedException {
    
    
        Thread thread1 = new Thread(() -> {
    
    
            System.out.println(Thread.currentThread().getName() + "打开冰箱");
        }, "线程1");

        Thread thread2 = new Thread(() -> {
    
    
            try {
    
    
                thread1.join();
            } catch (InterruptedException e) {
    
    
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + "拿出可乐" );
        }, "线程2");

        Thread thread3 = new Thread(() -> {
    
    
            try {
    
    
                thread2.join();
            } catch (InterruptedException e) {
    
    
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + "关上冰箱" );
        }, "线程3");

        //下面三行代码顺序可随意调整，程序运行结果不受影响，因为我们在子线程中通过“join()方法”已经指定了运行顺序。
        thread3.start();
        thread2.start();
        thread1.start();
    }

通过创建单一化线程池newSingleThreadExecutor()实现


	static ExecutorService executorService = Executors.newSingleThreadExecutor();

    @Test
    public void test4() {
    
    
        Thread thread1 = new Thread(() -> {
    
    
            System.out.println(Thread.currentThread().getName() + "打开冰箱");
        });

        Thread thread2 = new Thread(() -> {
    
    
            try {
    
    
                thread1.join();
            } catch (InterruptedException e) {
    
    
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + "拿出可乐" );
        });

        Thread thread3 = new Thread(() -> {
    
    
            try {
    
    
                thread2.join();
            } catch (InterruptedException e) {
    
    
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + "关上冰箱" );
        });
        
        executorService.submit(thread1);
        executorService.submit(thread2);
        executorService.submit(thread3);
        //使用完毕记得关闭线程池
        executorService.shutdown();        
    }

多线程下保证线程执行的顺序

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