head first Thread.join()

友情推荐：

1. 线程池原理
2. 深入Thread.sleep
3. 多线程中断机制

不使用Thread.join() 测试线程

先上代码：

/**
 * Created by Zero on 2017/8/23.
 */
public class TestJoin implements Runnable {
    public static int a = 0;

    @Override
    public void run() {
        for (int i = 0; i < 5; i++) {
            a = a + 1;
        }
    }

    public static void main(String[] args) throws InterruptedException {
        TestJoin j = new TestJoin();
        Thread thread = new Thread(j);
        thread.start();
        System.out.println(a);
    }
}

以上示例会输出5吗？可能性不大，有可能永远输出为0，之前在线程池原理的那篇就提到过，线程的启动和销毁都需要时间，此处因为thread还没启动好，或者正在为它分配资源准备运行，就已经执行完输出了。

怎样才能确保每次都能输出5呢？现在有请我们的主角join方法闪亮登场，代码如下：

/**
 * Created by apple on 2017/8/23.
 */
public class TestJoin implements Runnable {
    public static int a = 0;

    @Override
    public void run() {
        for (int i = 0; i < 5; i++) {
            a = a + 1;
        }
    }

    public static void main(String[] args) throws InterruptedException {
        TestJoin j = new TestJoin();
        Thread thread = new Thread(j);
        thread.start();
        /**
         * 测试join方法的作用，与下面的threadAgain线程作对比。
         */
        thread.join();
        System.out.println(a);
        a = 0;
        Thread threadAgain = new Thread(j);
        threadAgain.start();
        System.out.println(a);
    }
}

输出的结果将是5和0。

Thread.join()作用

Thread.join()，之前看资料的时候，有些人说可以理解成“将两个线程合并成一个线程”，我是觉得这样说是很不科学的，虽然这样通俗易懂，但这确实是两个不同的线程，只是在调用Thread.join()后，会先执行完Thread线程后再去执行当前线程，即上述的在主线程中执行到thread.join();后，先去执行thread，直到thread执行完后再去执行主线程。

测试Thread.join(long millis)

/**
 * Created by apple on 2017/8/23.
 */
public class TestJoin implements Runnable {
    public static int a = 0;

    @Override
    public void run() {
        for (int i = 0; i < 5; i++) {
            a = a + 1;
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public static void main(String[] args) throws InterruptedException {
        TestJoin j = new TestJoin();
        Thread thread = new Thread(j);
        thread.start();
        /**
         * 测试join方法的作用
         */
        thread.join(3000);
        System.out.println("thread线程结果为："+a);
        a = 0;
        Thread threadAgain = new Thread(j);
        threadAgain.start();
        System.out.println("threadAgain线程结果为："+a);
    }
}

输出：

thread线程结果为：3
threadAgain线程结果为：0

先上一段源码再来分析：

/**
     * Waits at most {@code millis} milliseconds for this thread to
     * die. A timeout of {@code 0} means to wait forever.
     *
     * <p> This implementation uses a loop of {@code this.wait} calls
     * conditioned on {@code this.isAlive}. As a thread terminates the
     * {@code this.notifyAll} method is invoked. It is recommended that
     * applications not use {@code wait}, {@code notify}, or
     * {@code notifyAll} on {@code Thread} instances.
     *
     * @param  millis
     *         the time to wait in milliseconds
     *
     * @throws  IllegalArgumentException
     *          if the value of {@code millis} is negative
     *
     * @throws  InterruptedException
     *          if any thread has interrupted the current thread. The
     *          <i>interrupted status</i> of the current thread is
     *          cleared when this exception is thrown.
     */
    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;
            }
        }
    }

源码爸爸说了，孩子，我给你millis这么长的时间，能不能完成任务那是你的事情了，能提前完成，咱就提前走下去，不能完成，过期不候，自己看着办吧。

默认情况下，Thread.join()即Thread.join(0)，当为0的时候，那才叫真爱呢，线程会一直等下去，知道执行结束为止，才会继续朝下执行。

isAlive()：用来测试线程是否处于活动状态，相当于 run 是否还在执行。