Java 进阶：如何让线程主动让出 CPU

Java 进阶：如何让线程主动让出 CPU

Thread.sleep

sleep 方法可以让线程主动让出 CPU，但是并不会释放锁。

    /**
     * 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.
     */
    public static native void sleep(long millis) throws InterruptedException;

Thread.yield

yield 也可以让线程主动让出 CPU，然后和其他线程一起竞争 CPU，但是调度器也可以忽略 yield。哪些情况会用到 yield 呢？
1. 一般在 debug 和 test 中使用。
2. CPU 密集型应用主动让出 CPU 以避免过度占用 CPU，影响其他任务。

    /**
     * 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.
     *
     * 
 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.
     *
     * 
 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();

Thread.currentThread().suspend()

该方法已过时。为啥呢？suspend 挂起线程，并不会释放锁，又不像 sleep 那样一段时间后自动恢复，所以容易引起死锁。相对应的 resume 方法用于唤醒一个 suspend 的线程。

    /**
     * Suspends this thread.
     * 

     * First, the checkAccess method of this thread is called
     * with no arguments. This may result in throwing a
     * SecurityException (in the current thread).
     * 

     * If the thread is alive, it is suspended and makes no further
     * progress unless and until it is resumed.
     *
     * @exception  SecurityException  if the current thread cannot modify
     *               this thread.
     * @see #checkAccess
     * @deprecated   This method has been deprecated, as it is
     *   inherently deadlock-prone.  If the target thread holds a lock on the
     *   monitor protecting a critical system resource when it is suspended, no
     *   thread can access this resource until the target thread is resumed. If
     *   the thread that would resume the target thread attempts to lock this
     *   monitor prior to calling resume, deadlock results.  Such
     *   deadlocks typically manifest themselves as "frozen" processes.
     *   For more information, see
     *   Why
     *   are Thread.stop, Thread.suspend and Thread.resume Deprecated?.
     */
    @Deprecated
    public final void suspend() {
        checkAccess();
        suspend0();
    }

    @Deprecated
    public final void resume() {
        checkAccess();
        resume0();
    }    

Object.wait

wait 会把当前持有的锁释放掉同时阻塞住，让出 CPU。当其他线程调用 Object.notify/notifyAll 时，会被唤醒，可能得到 CPU，并且获得锁。

LockSupport.park

这就是锁了嘛，相对应的用 unpark 解锁。

Thread.stop

该方法已过时，直接停止线程，同时会释放所有锁，太过暴力，容易导致数据不一致。