如何让多个线程顺序执行

最新推荐文章于 2025-01-08 12:45:00 发布

原创最新推荐文章于 2025-01-08 12:45:00 发布 · 364 阅读

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#多线程 #java

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先说一下为什么会有这样的要求，如果是简单的要求按顺序执行代码，直接一行行写下来就行了。但是接口调用我们一般都放在工作线程里面，而且有时候需要拿一个接口返回的参数去请求另一个接口，这就需要控制线程按某种顺序执行。

方法1：join()

public class A1 extends Thread {
    @Override
    public void run() {
        try {
            System.out.println("--- thread A1 start ---");
            sleep((long) (Math.random() * 30));
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("--- thread A1 end ---");
    }
}

public class A2 extends Thread {
    @Override
    public void run() {
        try {
            System.out.println("--- thread A2 start ---");
            sleep((long) (Math.random() * 30));
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("--- thread A2 end ---");
    }
}

public class A3 extends Thread {
    @Override
    public void run() {
        try {
            System.out.println("--- thread A3 start ---");
            sleep((long) (Math.random() * 30));
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("--- thread A3 end ---");
    }
}

public class OrderTest {
    public static void main(String[] args) {
        try {
            A1 a1 = new A1();
            a1.start();
            a1.join();
            A2 a2 = new A2();
            a2.start();
            a2.join();
            A3 a3 = new A3();
            a3.start();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}
// 控制台输出：
--- thread A1 start ---
--- thread A1 end ---
--- thread A2 start ---
--- thread A2 end ---
--- thread A3 start ---
--- thread A3 end ---

a1.start()使a1进入就绪状态，a1和主线程都可能抢到CPU执行权，a1.join()使 a1 线程独占CPU，主线程要等a1执行完才能执行a2的初始化和a2.start()等后续代码。

另外，把a2的初始化和a2.join()放在a3的run()里面的第一行（即执行a3的逻辑代码前），把a1的初始化和a1.join()放在a2的run()里面的第一行，也能让a1、a2、a3顺序执行，效果和上述代码相同。

方法2：wait() & notify()

public class B1 extends Thread{
    @Override
    public void run() {
        synchronized (this){
            try {
                System.out.println("--- thread B1 start ---");
                sleep((long) (Math.random() * 30));
                System.out.println("--- thread B1 end ---");
                notify();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

public class B2 extends Thread {
    @Override
    public void run() {
        B1 b1 = new B1();
        b1.start();
        synchronized (b1){
            try {
                b1.wait();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
        synchronized (this){
            try {
                System.out.println("--- thread B2 start ---");
                sleep((long) (Math.random() * 30));
                System.out.println("--- thread B2 end ---");
                notify();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

public class B3 extends Thread {
    @Override
    public void run() {
        B2 b2 = new B2();
        b2.start();
        synchronized (b2){
            try {
                b2.wait();
                System.out.println("--- thread B3 start ---");
                sleep((long) (Math.random() * 30));
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("--- thread B3 end ---");
        }
    }
}

public class OrderTest {
    public static void main(String[] args) {
        new B3().start();
    }
}
// 控制台输出：
--- thread B1 start ---
--- thread B1 end ---
--- thread B2 start ---
--- thread B2 end ---
--- thread B3 start ---
--- thread B3 end ---

需要注意的是，线程B2中调用b1.wait()的时候不是让b1对象等待，而是让获取到b1对象同步锁的当前线程等待。这里面的「当前线程」指的就是线程B2了。

线程B1中的notify()，唤醒的是在当前对象上等待的线程（其实这里的notify()相当于this.notiify()）。

B2线程的run()里面，执行第一个synchronized代码块的时候，当前线程wait了，当被唤醒的时候继续执行后续代码（即第二个synchronized代码块）；B3被唤醒后执行b2.wait()后面的代码：System.out.println("--- thread B3 start ---"); 等。

方法3：线程池

public class OrderTest {
    public static void main(String[] args) {
        A1 a1 = new A1();
        A2 a2 = new A2();
        A3 a3 = new A3();
        ExecutorService single = Executors.newSingleThreadExecutor();
        single.submit(a1);
        single.submit(a2);
        single.submit(a3);
        single.shutdown();
    }
}
// 控制台输出：
--- thread A1 start ---
--- thread A1 end ---
--- thread A2 start ---
--- thread A2 end ---
--- thread A3 start ---
--- thread A3 end ---