sleep() 的作用是让当前线程休眠，即当前线程会从“运行状态”进入到“休眠(阻塞)状态”。sleep()会指定休眠时间，线程休眠的时间会大于/等于该休眠时间；在线程重新被唤醒时，它会由“阻塞状态”变成“就绪状态”，从而等待cpu的调度执行。
示例：

public class SleepLockTest {

    private static Object obj = new Object();


    static class ThreadC extends Thread{
        public ThreadC(String name){
            super(name);
        }

        public void run(){
            //获取obj对象的同步锁
            synchronized (obj){
                try{
                    for(int i=0; i<5; i++){
                        System.out.printf("%s: %d\n", this.getName(), i);
                        if( i%2 == 0){
                            ThreadC.sleep(1000);
                            System.out.println("Start sleep…");
                        }
                    }
                }catch (InterruptedException e){
                    e.printStackTrace();
                }
            }
        }

    }

    public static void main(String[] args) {
        ThreadC t1 = new ThreadC("t1");
        ThreadC t2 = new ThreadC("t2");

        t1.start();
        t2.start();
    }
}

运行结果：

t1: 0
Start sleep…
t1: 1
t1: 2
Start sleep…
t1: 3
t1: 4
Start sleep…
t2: 0
Start sleep…
t2: 1
t2: 2
Start sleep…
t2: 3
t2: 4
Start sleep…

结果说明：
(01) sleep()则不会释放锁