概念性的东西或许有些难理解，打个比方，有一个题库，里面有很多数学题目，学生来抽取题库中的题来解答，其中题库相当所有线程，题目相当于单个线程，学生相当于cpu。那么某一次抽取到题目A，开始解答，解到一半，吧唧，调用了个yield，好的，这个时候学生会把未做完的题目A放回题库，然后重新选取一个题目进行解答，那么下一次选取到哪个题目，仍然是学生自己决定，可能选到题目B、C、D...但是，也可能仍然选到题目A，这就是yield的作用。

demo如下

public class MyThreadYield implements Runnable{       @Override     public void run() {         for (int i = 10; i < 20; i++) {             System.out.PRintln(Thread.currentThread().getName() + ":priorITy-" + Thread.currentThread().getPriority() + "-------" + i);              if (i == 15) {                 Thread.yield();             }         }     }       public static void main(String[] args) throws InterruptedException {         Thread thread = new Thread(new MyThreadYield());         thread.setName("First");          Thread thread1 = new Thread(new MyThreadYield());         thread1.setName("second");          thread.start();         thread1.start();      }  } 

在java doc中我们可以看到官方的解释，yield更多的是作为调试或测试时候使用(也就是疯狂重分配，提高切换概率，模拟并发)

/**      * 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();