方法源码
/**
* 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.
*
* @param millis
* the length of time to sleep 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 static native void sleep(long millis) throws InterruptedException;
/**
* Causes the currently executing thread to sleep (temporarily cease
* execution) for the specified number of milliseconds plus the specified
* number of nanoseconds, subject to the precision and accuracy of system
* timers and schedulers. The thread does not lose ownership of any
* monitors.
*
* @param millis
* the length of time to sleep in milliseconds
*
* @param nanos
* {@code 0-999999} additional nanoseconds to sleep
*
* @throws IllegalArgumentException
* if the value of {@code millis} is negative, or the value of
* {@code nanos} is not in the range {@code 0-999999}
*
* @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 void sleep(long millis, int nanos)
native关键字：Java Native Interface (Java本地接口)
sleep(long millis)方法睡眠一定的毫秒数，sleep方法都不回放弃monitor锁，这个非常重要- JDK1.5之后引入了一个枚举TimeUnit，对sleep进行了封装，推荐使用此方法来实现sleep。
与sleep类似的是yield方法，也是Thread类提供的一个方法，提醒调用器当前的线程愿意放弃当前获取的CPU资源，如果CPU的资源不紧张的话则此方法相当于失效。若是紧张的话则会提供给其他线程进行使用。yield的方法将线程从RUNNING状态切换到RUNNABLE状态。不过不推荐使用。
sleep和yield的区别：
- sleep会导致当前线程暂停指定的时间，没有CPU时间片的消耗。
- yield只是对CPU调度器的一个提示，如果CPU调用器没有忽略这个提示，它会导致线程上下文的切换
- sleep会使线程短暂block，会在给定的时间内释放CPU资源。
- yield会使RUNNING状态Thread进入RUNNABLE状态，前提是CPU没有忽略这个提醒。
- sleep几乎百分百地完成了给定时间的休眠，而yield的提示并不能一定担保。
- 一个线程sleep另一个线程调用interrupt会捕捉到中断信号，而yield则不会。
Thread.interrupt中断信号在sleep中会被设置为true，而yield不会产生影响。