sleep方法

源码如下：

    /**      * 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;

可以看到sleep是一个静态的本地方法，因为是本地方法，所以并没有java代码的实现，其实是调用了底层的C库函数来实现的睡眠。

有一个long类型的参数，表示睡眠多少毫秒。

阅读注释，sleep方法的含义就是，让当前正在执行任务的线程睡眠（临时地停止执行）指定的毫秒数，这个精度和准确性是用系统时钟和调度器保证的。但是，线程并不会释放它拥有的锁。

注意该方法会抛出InterruptedException中断异常。

sleep不会释放锁代码示例：

public class ThreadsleepDemo{      private Object object = new Object();      public static void main(String[] args) {          ThreadsleepDemo threadsleepDemo = new ThreadsleepDemo();         Thread thread1 = threadsleepDemo.new SleepDemoThread();         thread1.setName("线程1");         Thread thread2 = threadsleepDemo.new SleepDemoThread();         thread2.setName("线程2");         thread1.start();         thread2.start();      }      class SleepDemoThread extends Thread{         @Override         public void run() {             synchronized (object){                 System.out.println(Thread.currentThread().getName());                 try {                     Thread.sleep(1000);                 } catch (InterruptedException e) {                     e.printStackTrace();                 }                 System.out.println(Thread.currentThread().getName());             }         }     } } 

输出结果如下：

线程1开始运行 线程1运行结束 线程2开始运行 线程2运行结束

可以多运行几次，可能会有线程1在上面或和线程2在上面，但始终都是一个行程运行完了才会运行另一个线程，中间不会插入进来一个线程运行。

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.      *      * <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.      *      * <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.      */     public static native void yield();

当前线程对调度器的一个暗示，表示愿意让出CPU执行器的当前使用权，但是调度器可以自由忽略这个提示。

Yeild是一种在可能会过度使用一个CPU的多个线程之间提升相对进度试探性尝试。它的使用应该结合详细的性能分析和基准测试来进行，确保它确实有预期的效果。

很少使用这种方法。 它可能对调试或测试有用，可能有助于根据竞态条件重现错误。 在设计并发控制结构（例如java.util.concurrent.locks包中的并行控制结构）时也可能有用。

join方法

join有三个重载的方法

join() join(long millis)     join(long millis,int nanoseconds)

主要看下第二个方法的源码