java多线程如何实现顺序访问

1、通过join()方法指定顺序

     当然，thread的join方法实现多线程顺序执行有两种方式，一种是在子线程内部调用join()方法，另一种是直接在主线程调用join()方法；写法上是主线程上调用join()更直观，谁先执行谁后执行一目了然。子线程内部调用的话，相对的要注意调用的顺序。

1.子线程内部调用join()

public class ThreadJoinDemo {
    public static void main(String[] args) throws InterruptedException {
        final Thread thread1 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("打开冰箱！");
            }
        });

        final Thread thread2 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    thread1.join();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("拿出一瓶牛奶！");
            }
        });

        final Thread thread3 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    thread2.join();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("关上冰箱！");
            }
        });

        //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（还有一个原因就是之前说的多线程的生命周期中有就绪状态，需要等待cpu调度，start并不意味着运行状态）。
        thread3.start();
        thread2.start();
        thread1.start();
    }
}

2.在主线程中通过join()方法指定顺序

public class ThreadMainJoinDemo {
    public static void main(String[] args) throws InterruptedException {
        final Thread thread1 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("打开冰箱！");
            }
        });

        final Thread thread2 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("拿出一瓶牛奶！");
            }
        });

        final Thread thread3 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("关上冰箱！");
            }
        });

        thread1.start();
        thread1.join();
        thread2.start();
        thread2.join();
        thread3.start();
    }
}

2、使用jdk5推出的线程池Executors创建单例的方法

public class ThreadPoolDemo {

   static ExecutorService executorService = Executors.newSingleThreadExecutor();

    public static void main(String[] args) {
        final Thread thread1 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("打开冰箱！");
            }
        });

        final Thread thread2 =new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("拿出一瓶牛奶！");
            }
        });

        final Thread thread3 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("关上冰箱！");
            }
        });
        executorService.submit(thread1);
        executorService.submit(thread2);
        executorService.submit(thread3);
        executorService.shutdown();        //使用完毕记得关闭线程池
    }

}

3、CountDownLatch（倒计数）类

     CountDownLatch类的特点就是可以初始化设置一个state的值，每次调用countDown()方法进行减一，如果state没有减小到0就会被await()方法一直阻塞。利用CountDownLatch方法可以实现和join()方法子线程运用一样的妙处。当然，CountDownLatch类的缺点是state不能重置，只能初始化一次，意思就是减小到0后，就没用了。。。不能加回去。

public class ThreadCountDownLatchDemo {

    private static CountDownLatch countDownLatch1 = new CountDownLatch(1);

    private static CountDownLatch countDownLatch2 = new CountDownLatch(1);

    public static void main(String[] args) {
        final Thread thread1 = new Thread(new Runnable() {
            @Override
            public void run() {
                System.out.println("打开冰箱！");
                countDownLatch1.countDown();
            }
        });

        final Thread thread2 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    countDownLatch1.await();
                    System.out.println("拿出一瓶牛奶！");
                    countDownLatch2.countDown();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        });

        final Thread thread3 = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    countDownLatch2.await();
                    System.out.println("关上冰箱！");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        });

        //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();
        thread1.start();
        thread2.start();
    }
}