Java併發編程初級篇(四)：等待子線程終止

咱們先看一個例子，在這個例子中你會發現主線程結束後，過了一段時間兩個子線程才結束。

定義實現Runnable接口的線程類，模擬執行必定時間後結束。

public class MyRunnable implements Runnable{
    @Override
    public void run() {
        System.out.printf("%s: I am start working.\n", Thread.currentThread().getName());
        try {
            TimeUnit.SECONDS.sleep((long) (Math.random() * 10 + 1));
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.printf("%s: I am end working.\n", Thread.currentThread().getName());
    }
}

定義主方法，啓動兩個子線程：

public class Main {
    public static void main(String[] args) {
        Thread thread1 = new Thread(new MyRunnable());
        Thread thread2 = new Thread(new MyRunnable());

        System.out.printf("%s: start two threads.\n", Thread.currentThread().getName());
        thread1.start();
        thread2.start();
        System.out.printf("%s: end.\n", Thread.currentThread().getName());
    }
}

查看控制檯日誌，你會發現主線程先於兩個子線程而結束。

main: start two threads.
main: end.
Thread-1: I am start working.
Thread-0: I am start working.
Thread-1: I am end working.
Thread-0: I am end working.

當你想讓主線程等待全部子線程執行結束後，再執行一段代碼才結束，應該怎麼作呢。Java提供了join()方法。咱們重寫主方法，在主方法中調用子線程的join()方法，便可讓主線程進入WAITING狀態並等待子線程終止後繼續執行。

public class Main {
    public static void main(String[] args) {
        Thread thread1 = new Thread(new MyRunnable());
        Thread thread2 = new Thread(new MyRunnable());

        System.out.printf("%s: start two threads.\n", Thread.currentThread().getName());
        thread1.start();
        thread2.start();

        try {
            thread2.join();
            thread1.join();
            System.out.printf("%s: child runnable both ends.\n", Thread.currentThread().getName());
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.printf("%s: end.\n", Thread.currentThread().getName());
    }
}

查看控制檯日誌，你會發現主線程在等待兩個子線程終止以後才繼續執行。

main: start two threads.
Thread-0: I am start working.
Thread-1: I am start working.
Thread-0: I am end working.
Thread-1: I am end working.
main: child runnable both ends.
main: end.