如何控制线程的执行顺序

错误示例：

public class Test {
    public static void main(String[] args){
        thread1.start();
        thread2.start();
        thread3.start();
    }

    static Thread thread1 = new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("thread1");
        }
    });

    static Thread thread2 = new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("thread2");
        }
    });

    static Thread thread3 = new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("thread3");
        }
    });
}

执行结果：

thread2
thread1
thread3

原因：start()方法只是让子线程处于就绪状态，最终执行状态是不可控的

方案一：

public class Test {
    public static void main(String[] args) throws InterruptedException {
        thread1.start();
        thread1.join();//主线程放弃cpu执行权，让给子线程执行
        thread2.start();
        thread2.join();
        thread3.start();
        thread3.join();
    }

    static Thread thread1 = new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("thread1");
        }
    });

    static Thread thread2 = new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("thread2");
        }
    });

    static Thread thread3 = new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("thread3");
        }
    });
}

结果：

thread1
thread2
thread3

join方法的原理就是调用相应线程的wait方法进行等待操作的，例如A线程中调用了B线程的join方法，则相当于在A线程中调用了B线程的wait方法，当B线程执行完（或者到达等待时间），B线程会自动调用自身的notifyAll方法唤醒A线程，从而达到同步的目的。

源码：

public final synchronized void join(long millis)
    throws InterruptedException {
        long base = System.currentTimeMillis();
        long now = 0;

        if (millis < 0) {
            throw new IllegalArgumentException("timeout value is negative");
        }

        if (millis == 0) {
            while (isAlive()) {
                wait(0);
            }
        } else {
            while (isAlive()) {
                long delay = millis - now;
                if (delay <= 0) {
                    break;
                }
                wait(delay);
                now = System.currentTimeMillis() - base;
            }
        }
    }

wait(0)是当前主线程进入等待

方案二：

public class Test {
    public static void main(String[] args) throws InterruptedException {
        executorService.submit(thread1);
        executorService.submit(thread2);
        executorService.submit(thread3);
    }

    static ExecutorService executorService = Executors.newSingleThreadExecutor();

    static Thread thread1 = new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("thread1");
        }
    });

    static Thread thread2 = new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("thread2");
        }
    });

    static Thread thread3 = new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println("thread3");
        }
    });
}

执行结果：

thread1
thread2
thread3

单线程化线程池(newSingleThreadExecutor)的优点，串行执行所有任务