多线程顺序运行的 4 种方法

1、在子线程中通过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。

举例：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。

public class ThreadJoinDemo {
    public static void main(String[] args) {
// 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();
    }
    static final Thread thread1 = new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("打开冰箱！！！");
        }
    });
    static 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！！！");
        }
    });
    static 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！！！");
        }
    });
}
// 运行结果：
打开冰箱！！！
拿出一瓶牛奶！！！
关上冰箱！！！

2、在主线程中通过join()方法指定顺序

简单说一下子线程与主线程的区别，子线程指的是发生在Thread内部的代码，主线程指的是发生在main函数中的代码。

我们可以在main函数中通过join()方法让主线程阻塞等待以达到指定顺序执行的目的。

public class ThreadJoinDemo {
    public static void main(String[] args) throws InterruptedException {
        thread1.start();
        thread1.join();
        thread2.start();
        thread2.join();
        thread3.start();
    }
    static final Thread thread1 = new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("打开冰箱！！！");
        }
    });
    static final Thread thread2 = new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("拿出一瓶牛奶！！！");
        }
    });
    static final Thread thread3 = new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("关上冰箱！！！");
        }
    });
}
// 输出结果：
打开冰箱！！！
拿出一瓶牛奶！！！
关上冰箱！！！

3、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。

public class ThreadCountDownLatchDemo {
</span><span style="color: rgba(0, 0, 255, 1)">private</span> <span style="color: rgba(0, 0, 255, 1)">static</span> CountDownLatch countDownLatch1 = <span style="color: rgba(0, 0, 255, 1)">new</span> CountDownLatch(1<span style="color: rgba(0, 0, 0, 1)">);

</span><span style="color: rgba(0, 0, 255, 1)">private</span> <span style="color: rgba(0, 0, 255, 1)">static</span> CountDownLatch countDownLatch2 = <span style="color: rgba(0, 0, 255, 1)">new</span> CountDownLatch(1<span style="color: rgba(0, 0, 0, 1)">);

</span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">static</span> <span style="color: rgba(0, 0, 255, 1)">void</span><span style="color: rgba(0, 0, 0, 1)"> main(String[] args) {
    </span><span style="color: rgba(0, 0, 255, 1)">final</span> Thread thread1 = <span style="color: rgba(0, 0, 255, 1)">new</span> Thread(<span style="color: rgba(0, 0, 255, 1)">new</span><span style="color: rgba(0, 0, 0, 1)"> Runnable() {
        @Override
        </span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span><span style="color: rgba(0, 0, 0, 1)"> run() {
            System.out.println(</span>"打开冰箱！"<span style="color: rgba(0, 0, 0, 1)">);
            <span style="color: rgba(255, 0, 0, 1)">countDownLatch1.countDown();</span>
        }
    });

    </span><span style="color: rgba(0, 0, 255, 1)">final</span> Thread thread2 = <span style="color: rgba(0, 0, 255, 1)">new</span> Thread(<span style="color: rgba(0, 0, 255, 1)">new</span><span style="color: rgba(0, 0, 0, 1)"> Runnable() {
        @Override
        </span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span><span style="color: rgba(0, 0, 0, 1)"> run() {
            </span><span style="color: rgba(0, 0, 255, 1)">try</span><span style="color: rgba(0, 0, 0, 1)"> {
                <span style="color: rgba(255, 0, 0, 1)">countDownLatch1.await();</span>
                System.out.println(</span>"拿出一瓶牛奶！"<span style="color: rgba(0, 0, 0, 1)">);
                <span style="color: rgba(255, 0, 0, 1)">countDownLatch2.countDown();</span>
            } </span><span style="color: rgba(0, 0, 255, 1)">catch</span><span style="color: rgba(0, 0, 0, 1)"> (InterruptedException e) {
                e.printStackTrace();
            }
        }
    });

    </span><span style="color: rgba(0, 0, 255, 1)">final</span> Thread thread3 = <span style="color: rgba(0, 0, 255, 1)">new</span> Thread(<span style="color: rgba(0, 0, 255, 1)">new</span><span style="color: rgba(0, 0, 0, 1)"> Runnable() {
        @Override
        </span><span style="color: rgba(0, 0, 255, 1)">public</span> <span style="color: rgba(0, 0, 255, 1)">void</span><span style="color: rgba(0, 0, 0, 1)"> run() {
            </span><span style="color: rgba(0, 0, 255, 1)">try</span><span style="color: rgba(0, 0, 0, 1)"> {
                <span style="color: rgba(255, 0, 0, 1)">countDownLatch2.await();</span>
                System.out.println(</span>"关上冰箱！"<span style="color: rgba(0, 0, 0, 1)">);
            } </span><span style="color: rgba(0, 0, 255, 1)">catch</span><span style="color: rgba(0, 0, 0, 1)"> (InterruptedException e) {
                e.printStackTrace();
            }
        }
    });

    </span><span style="color: rgba(0, 128, 0, 1)">//</span><span style="color: rgba(0, 128, 0, 1)">下面三行代码顺序可随意调整，程序运行结果不受影响</span>