java - 线程的Join方法

Question

我注意到 Join 方法的有趣行为非常令人困惑，多线程可能已经过时且过时(在 java 8 的 lambda/streams 之后)，但仍然好奇地想知道我是否遗漏了某些东西，永远不会在实时项目中使用线程。

class JoinDemoThread1 implements Runnable {
    String threadName;

    @Override
    public void run() {
        for (int i = 0; i < 5; i++) {
            System.out.println("Testing in demothread1=" + i);
        }
    }
}

<小时/>

class JoinDemoThread2 implements Runnable {
    @Override
    public void run() {
        for (int i = 0; i < 5; i++) {
            System.out.println("Testing in demothread2=" + i);
        }
    }
}

<小时/>

public class JoinDemo {
    public static void main(String args[]) {
        JoinDemoThread1 joinDemoThread1 = new JoinDemoThread1();
        JoinDemoThread2 joinDemoThread2 = new JoinDemoThread2();

        Thread demoThread1 = new Thread(joinDemoThread1);
        Thread demoThread2 = new Thread(joinDemoThread2);

        demoThread1.start();
        demoThread2.start();

        // wait for threads to end
        try {
            demoThread1.join();
            demoThread2.join();
        } catch (Exception e) {
            System.out.println("Interrupted");
        }

        System.out.println("Ending Main program.");
    }
}

Answer 1

运行给定代码时的输出:

Testing in demothread1=0
Testing in demothread1=1
Testing in demothread1=2
Testing in demothread1=3
Testing in demothread1=4
Testing in demothread2=0
Testing in demothread2=1
Testing in demothread2=2
Testing in demothread2=3
Testing in demothread2=4
Ending Main program.

当然，看起来根本没有多线程发生。这是因为您的线程完成得如此之快，以至于调度程序没有理由交错线程。如果有更密集的过程，您将看到更预期的输出。

例如，稍微延迟线程时:

class JoinDemoThread2 implements Runnable {
    @Override
    public void run() {
        for (int i = 0; i < 5; i++) {
            try {
                Thread.sleep(500);
            } catch (InterruptedException ignored) {}
            System.out.println("Testing in demothread2=" + i);
        }
    }
}

输出将是:

Testing in demothread2=0
Testing in demothread1=0
Testing in demothread1=1
Testing in demothread2=1
Testing in demothread1=2
Testing in demothread2=2
Testing in demothread1=3
Testing in demothread2=3
Testing in demothread1=4
Testing in demothread2=4
Ending Main program.