Java中线程顺序执行

现有线程threadone、threadtwo和threadthree，想要的运行顺序为threadone->threadtwo->threadthree，应该如何处理？这里需要用到一个简单的线程方法join().

join()方法的说明：join方法挂起当前调用线程，直到被调用线程完成后在继续执行(join() method suspends the execution of the calling thread until the object called finishes its execution).

下面看一个例子：

/**
 * Thread one for test.
 */
public class ThreadOne implements Runnable {
    @Override
    public void run() {
        String threadName = Thread.currentThread().getName();
        System.out.println(threadName + " start...");


        System.out.println(threadName + " end.");
    }
}

/**
 * Thread two for test.
 */
public class ThreadTwo implements Runnable {
    @Override
    public void run() {
        String threadName = Thread.currentThread().getName();
        System.out.println(threadName + " start...");


        System.out.println(threadName + " end.");
    }
}

/**
 * Thread three for test.
 */
public class ThreadThree implements Runnable {
    @Override
    public void run() {
        String threadName = Thread.currentThread().getName();
        System.out.println(threadName + " start...");


        System.out.println(threadName + " end.");
    }
}

public class JoinMainTest {
    public static void main(String[] args) {
        String threadName = Thread.currentThread().getName();
        System.out.println(threadName + " start...");

        Thread firstThread = new Thread(new ThreadOne());
        Thread secondThread = new Thread(new ThreadTwo());
        Thread thirdThread = new Thread(new ThreadThree());

        // 1. no order thread run
        firstThread.start();
        secondThread.start();
        thirdThread.start();

        System.out.println(threadName + " end.");
    }
}

上面得到的结果如下：

/**
*
main start...
Thread-0 start...
main end.
Thread-0 end.
Thread-1 start...
Thread-1 end.
Thread-2 start...
Thread-2 end.
*/

public class JoinMainTest {
    public static void main(String[] args) {
        String threadName = Thread.currentThread().getName();
        System.out.println(threadName + " start...");

        Thread firstThread = new Thread(new ThreadOne());
        Thread secondThread = new Thread(new ThreadTwo());
        Thread thirdThread = new Thread(new ThreadThree());
    
        // 2. thread run in order
        try {
            firstThread.start();
            firstThread.join();
            secondThread.start();
            secondThread.join();
            thirdThread.start();
            thirdThread.join();
        } catch (Exception ex) {
            System.out.println("thread join exception.");
        }
        System.out.println(threadName + " end.");
    }
}

这里得到的结果就是顺序执行的了：

/**
*
main start...
Thread-0 start...
Thread-0 end.
Thread-1 start...
Thread-1 end.
Thread-2 start...
Thread-2 end.
main end.
*/

再来看一下join()的源码，join()调用的就是join(0).可以看到，其实join就是wait，直到线程执行完成。

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;
        }
    }
}