线程中的yield()

属于本地方法

    /**
     * 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();

 当前线程愿意放弃当前的CPU，但是调度程序可以忽略这一意愿，所以当线程执行了yield()方法后不一定就从running进入到ruunable状态。