java多线程三 wait sleep join yield

一，wait

是object类的方法，并不是线程的方法，使用此方法会使当前线程进入阻塞状态，过程中线程会释放cpu资源和锁资源,献策韩国你进入等待池，可以通过notify或者notifyall将线程唤醒，是线程重新进入就绪状态，在wait，notify和notifyall方法使用过程中一定要注意在同步方法或同步代码块中使用，因为在同步中才会获得锁，才能释放锁，java API强制规定，否则将报出java.lang.IllegalMonitorStateException异常。

二，sleep

sleep使Thread的方法，使强制使线程进入睡眠状态Thread.sleep(time)。在此过程中该线程只会释放掉cpu资源并不会释放锁资源，也就意味着目前对象实例的抢占到刚释放的cpu资源的线程不能获得同步资源锁，但是可以去抢占没有被占用的同步资源锁。

三，join

join方法的源代码

    /**
     * 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);            //join()等同于join(0)
    }
    /**
     * 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);           //join(0)等同于wait(0)，即wait无限时间直到被notify
            }
        } else {
            while (isAlive()) {
                long delay = millis - now;
                if (delay <= 0) {
                    break;
                }
                wait(delay);
                now = System.currentTimeMillis() - base;
            }
        }
    }

可以看出join是一个同步方法，当主线程调用t的jion方法时会像去获得t的锁，然后主线程调用wait方法进入等待池，直到t线程运行结束main线程才能继续执行。

四，yield

线程让步，是将该线程，让出cpu资源进入可执行状态，然后与同优先级的线程再一并竞争抢占资源，也就是说并不意味着该线程一定会处于可执行状态，当其再一次抢到资源，会立即又转变成执行状态。