Java 进阶：如何让线程主动让出 CPU

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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
  • Threadsleep
  • Threadyield
  • ThreadcurrentThreadsuspend
  • Objectwait
  • LockSupportpark
  • Threadstop

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. * * <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(); 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. * <p> * First, the <code>checkAccess</code> method of this thread is called * with no arguments. This may result in throwing a * <code>SecurityException </code>(in the current thread). * <p> * 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 <code>resume</code>, deadlock results. Such * deadlocks typically manifest themselves as "frozen" processes. * For more information, see * <a href="{@docRoot}/../technotes/guides/concurrency/threadPrimitiveDeprecation.html">Why * are Thread.stop, Thread.suspend and Thread.resume Deprecated?</a>. */ @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。

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