关于多线程中sleep、join、yield的区别

好了、说了多线程，那就不得不说说多线程的sleep()、join()和yield()三个方法的区别啦

　　1、sleep（）方法

 /**
     * Causes the currently executing thread to sleep (temporarily cease
     * execution) for the specified number of milliseconds, subject to
     * the precision and accuracy of system timers and schedulers. The thread
     * does not lose ownership of any monitors.
     *  意思是说：当前正在执行的线程休眠（暂时停止执行）指定的毫秒数，具体取决于系统计时器和调度程序的精度和准确性。 该线程不会失去任何监视器的所有权。
     * @param  millis
     *         the length of time to sleep in milliseconds
     *         毫秒为单位
     * @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 static native void sleep(long millis) throws InterruptedException;

其实主要的就是他是让其他线程走，自己进行休眠，但是自己却不会释放对象锁，也就是说，如果有同步锁的时候，其他线程不能访问共享数据。

注意该方法要捕获异常 比如有两个线程同时执行(没有Synchronized)，一个线程优先级为MAX_PRIORITY，另一 个为MIN_PRIORITY，如果没有Sleep()方法，只有高优先级的线程执行完成后，低优先级 的线程才能执行；但当高优先级的线程sleep(5000)后，低优先级就有机会执行了。 总之，sleep()可以使低优先级的线程得到执行的机会，当然也可以让同优先级、高优先级的 线程有执行的机会。

　　2、yield() 方法

    /**
     * A hint to the scheduler that the current thread is willing to yield
     * its current use of a processor. The scheduler is free to ignore this
     * hint.
     *   意思是说 提示当前线程可以让处理器忽略当前线程，去处理其他线程
     * <p> Yield is a heuristic attempt to improve relative progression
     * between threads that would otherwise over-utilise a CPU. Its use
     * should be combined with detailed profiling and benchmarking to
     * ensure that it actually has the desired effect.
     * 它是一种启发式尝试，用于改善线程之间的相对进展，否则会过度利用CPU。 它的使用应与详细的分析和基准测试相结合，以确保它实际上具有所需的效果。
     * <p> It is rarely appropriate to use this method. It may be useful
     * for debugging or testing purposes, where it may help to reproduce
     * bugs due to race conditions. It may also be useful when designing
     * concurrency control constructs such as the ones in the
     * {@link java.util.concurrent.locks} package.
   　 *  使用这种方法很少是合适的。 它可能对调试或测试目的很有用，它可能有助于重现因竞争条件而产生的错误。 在设计并发控制结构（如中的那些）时，它也可能很有用
     */
    public static native void yield();

yield() 这个方法从以上注释可以看出，也是一个休眠自身线程的方法，同样不会释放自身锁的标识，区别在于它是没有参数的，即yield()方 法只是使当前线程重新回到可执行状态，

所以执行yield()的线程有可能在进入到可执行状态 后马上又被执行，另外yield()方法只能使同优先级或者高优先级的线程得到执行机会，这也 和sleep()方法不同。

3、join() 方法

这个方法比较有意思，Thread的非静态方法join()让一个线程B“加入”到另外一个线程A的尾部。在A执行完毕之前， B不能工作。

/**
     * Waits for this thread to die.
     *  等待线程死亡
     * <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);  // 调用了有参方法
    }

  /**
     * Waits at most {@code millis} milliseconds for this thread to
     * die. A timeout of {@code 0} means to wait forever.
     *  这个线程最多等多少毫秒，如果超时了，就会进行线程死锁
     * <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 {

保证当前线程停止执行，直到该线程所加入的线程完成为止。然而，如果它加入的线程没有存活，则当前线程不需要停止。