Java 進階：如何讓線程主動讓出 CPU__Java

Java 進階如何讓線程主動讓出 CPU Threadsleep Threadyield ThreadcurrentThreadsuspend Objectwait LockSupportpark Threadstop

Java 進階：如何讓線程主動讓出 CPU Thread.sleep

sleep 方法可以讓線程主動讓出 CPU，但是並不會釋放鎖。

    /**     * 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 也可以讓線程主動讓出 CPU，然後和其他線程一起競爭 CPU，但是調度器也可以忽略 yield。哪些情況會用到 yield 呢。
1. 一般在 debug 和 test 中使用。
2. CPU 密集型應用主動讓出 CPU 以避免過度佔用 CPU，影響其他任務。

    /**     * 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()

該方法已淘汰。為啥呢。suspend 掛起線程，並不會釋放鎖，又不像 sleep 那樣一段時間後自動回復，所以容易引起死結。相對應的 resume 方法用於喚醒一個 suspend 的線程。

    /**     * 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 會把當前持有的鎖釋放掉同時阻塞住，讓出 CPU。當其他線程調用 Object.notify/notifyAll 時，會被喚醒，可能得到 CPU，並且獲得鎖。 LockSupport.park

這就是鎖了嘛，相對應的用 unpark 解鎖。 Thread.stop

該方法已淘汰，直接停止線程，同時會釋放所有鎖，太過暴力，容易導致資料不一致。