Java Thread.yield详解

　　这是Java中的一种线程让步方法，让Java中的线程从执行状态变成就绪状态，然后处理器再从就绪队列中挑选线程进行执行（优先级大的，被挑选的概率较大），这种转换也不确定，让或者不让都是取决与处理器，线程可能继续占有处理器。

**
     * 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.
     */
　　//暗示调度器让当前线程让出占用的处理器，但是让或者是不让，都是调度器自己决定的，有可能调用此方法过后，线程依然占用处理器
    public static native void yield();

　　看《Java并发编程艺术》这本书，里面有用到Thread.yield方法，来测试Java线程优先级，代码如下：

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.TimeUnit;

public class Priority {
    private static volatile boolean notStart=true;
    private static volatile boolean notEnd=true;

    public static void main(String[] argvs) throws Exception{
        List<Job> jobs=new ArrayList<Job>();
        for(int i=0;i<10;i++){
            int priority=i<5?Thread.MIN_PRIORITY:Thread.MAX_PRIORITY;
            Job job=new Job(priority);
            jobs.add(job);
            Thread thread=new Thread(job,"Thread:"+i);
            thread.setPriority(priority);
            thread.start();
        }
        notStart=false;
        TimeUnit.SECONDS.sleep(10);
        notEnd=false;

        for(Job job:jobs){
            System.out.println("Job Priority:"+job.priority+", Count: "+job.jobCount);
        }
    }

    static class Job implements Runnable{
        private int priority;
        private long jobCount;

        public Job(int priority){
            this.priority=priority;
        }

        @Override
        public void run() {
            while(notStart){
                Thread.yield();
            }
            while(notEnd){
                Thread.yield();
                jobCount++;
            }
        }
    }
}

　　跑出结果如下：

Job Priority:1, Count: 6087836
Job Priority:1, Count: 6573345
Job Priority:1, Count: 6024024
Job Priority:1, Count: 6606573
Job Priority:1, Count: 6901572
Job Priority:10, Count: 6118724
Job Priority:10, Count: 5968747
Job Priority:10, Count: 5939391
Job Priority:10, Count: 6206129
Job Priority:10, Count: 6187854

　发现即使线程优先级为1和为10的线程，最后获取CPU的次数是差不多的，没有明显差距，线程优先级没有生效。程序的正确性不能依赖于线程的优先级。