Java多线程(2)：线程加入/join()

线程加入

join()方法，等待其他线程终止。在当前线程(主线程)中调用另一个线程(子线程)的join()方法，则当前线程转入阻塞状态，直到另一个线程运行结束，当前线程再由阻塞转为就绪状态。

也就是主线程中，子线程调用了join()方法后面的代码，只有等到子线程结束了才能执行。

在很多情况下，主线程生成并起动了子线程，如果子线程里要进行大量的耗时的运算，主线程往往将于子线程之前结束。

如下例子：

class JoinRunnable implements Runnable {
    @Override
    public void run() {
        System.out.println("Child thread start.");
        try {
            for (int i = 1; i <= 5; i++) {
                System.out.println("Child thread runing [" + i + "].");
                Thread.sleep((int) (Math.random() * 100));
            }
        } catch (InterruptedException e) {
            System.out.println("Child thread interrupted.");
        }
        System.out.println("Child thread end.");
    }
}

public class JoinDemo {
    // main是主线程
    public static void main(String args[]) {
        System.out.println("Main thread start.");
        new Thread(new JoinRunnable()).start();
        System.out.println("Main thread end.");
    }
}

执行结果：

Main thread start.
Main thread end.
Child thread start.
Child thread runing [1].
Child thread runing [2].
Child thread runing [3].
Child thread runing [4].
Child thread runing [5].
Child thread end.

如果主线程需要等待子线程执行完成之后再结束，这个时候就要用到join()方法了。

如下例子：

class JoinRunnable implements Runnable {
    @Override
    public void run() {
        System.out.println("Child thread start.");
        try {
            for (int i = 1; i <= 5; i++) {
                System.out.println("Child thread runing [" + i + "].");
                Thread.sleep((int) (Math.random() * 100));
            }
        } catch (InterruptedException e) {
            System.out.println("Child thread interrupted.");
        }
        System.out.println("Child thread end.");
    }
}

public class JoinDemo {
    // main是主线程
    public static void main(String args[]) {
        System.out.println("Main thread start.");
        Thread t = new Thread(new JoinRunnable());
        t.start();
        try {
            t.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("Main thread end.");
    }
}

执行结果：

Main thread start.
Child thread start.
Child thread runing [1].
Child thread runing [2].
Child thread runing [3].
Child thread runing [4].
Child thread runing [5].
Child thread end.
Main thread end.