- 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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Java 进阶:如何让线程主动让出 CPU
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