线程中的join()

http://blog.itpub.net/31555134/viewspace-2221319/

一直对join()方法不是很理解，在A线程中， B线程调用了join()方法，然后在内部实际是wait()方法，但是在效果上是A线程一直在等待，对wait()方法的调用不是很理解。(希望有朋友能够给我答疑非常感谢)

join()方法的作用是使所属的线程对象x正常执行run()方法中的任务，而使当前线程z进行无限期的阻塞，等待线程x销毁后再继续执行线程z后面的代码。方法join具有使线程排队运行的作用，有些类似同步的运行效果。

下面是join()方法源码：

    /**
     * Waits for this thread to die.
     *
     * <p> An invocation of this method behaves in exactly the same
     * way as the invocation
     *
     * <blockquote>
     * {@linkplain #join(long) join}{@code (0)}
     * </blockquote>
     *
     * @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 void join() throws InterruptedException {
        join(0);
    }

    /**
     * Waits at most {@code millis} milliseconds for this thread to
     * die. A timeout of {@code 0} means to wait forever. // 0意味着永远等待不是等待0秒   millis意味着只等待millis,超过这个时间则停止等待
     *
     * <p> This implementation uses a loop of {@code this.wait} calls
     * conditioned on {@code this.isAlive}. As a thread terminates the  //当线程终止时， 会自动调用notifyall()方法
     * {@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;
            }
        }
    }

当然了， join()方法放在start()方法后面才会有效果。

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