本文基于源码注释的解读,对sleep、wait、notify方法进行学习与了解,下面我们就来说一说关于notify和wait的原理?我们一起去了解并探讨一下这个问题吧!

notify和wait的原理(wait以及notify方法区别于联系)

notify和wait的原理

前言

本文基于源码注释的解读,对sleep、wait、notify方法进行学习与了解。

另外,文章中出现了“持有对象锁,释放对象锁”的描述,之所以对对象进行加锁,是因为我们讨论问题的背景是多线程。如果多个线程同时对一个变量进行操作,有可能发生意想不到的情形,因此我们需要对对象先进行加锁,再进行操作。既然有加锁这一操作,当然也就有释放锁这一操作。

sleep方法

Thread类中的sleep方法:

/** * 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;

解析sleep方法的源码注释:

调用sleep方法可以让当前正在运行的线程进入睡眠状态,即暂时停止运行指定的单位时间。并且该线程在睡眠期间不会释放对象锁。

sleep方法的目的是让当前线程暂停运行一段时间,而与对象锁相关的信息无影响,如果执行sleep方法时是处于持有对象锁的状态,那么睡眠时依然持有对象锁,如果执行sleep方法时不是处于持有对象锁的状态,睡眠期间也不会持有对象锁。

wait方法

Object类中的wait方法:

/** * Causes the current thread to wait until another thread invokes the * {@link java.lang.Object#notify()} method or the * {@link java.lang.Object#notifyAll()} method for this object. * In other words, this method behaves exactly as if it simply * performs the call {@code wait(0)}. * <p> * The current thread must own this object's monitor. The thread * releases ownership of this monitor and waits until another thread * notifies threads waiting on this object's monitor to wake up * either through a call to the {@code notify} method or the * {@code notifyAll} method. The thread then waits until it can * re-obtain ownership of the monitor and resumes execution. * <p> * As in the one argument version, interrupts and spurious wakeups are * possible, and this method should always be used in a loop: * <pre> * synchronized (obj) { * while (<condition does not hold>) * obj.wait(); * ... // Perform action appropriate to condition * } * </pre> * This method should only be called by a thread that is the owner * of this object's monitor. See the {@code notify} method for a * description of the ways in which a thread can become the owner of * a monitor. */ public final native void wait(long timeout) throws InterruptedException;

解析wait方法的源码注释:

调用wait方法可以让当前线程(即调用object.wait方法的那个线程)进入等待唤醒状态,该线程会处于等待唤醒状态直到另一个线程调用了object对象的notify方法或者notifyAll方法。该方法没有形参,相当于调用了参数为0的wait(long timeout)方法。

同时,要调用wait方法,前提是获取了这个对象的锁。在调用wait方法时,线程会释放锁并进入等待状态。在被唤醒后,该线程会一直处于等待获取锁的状态直到它重新获取到锁,然后才可以重新恢复运行状态。

注释也强调了,该方法应该只在获取了对象的锁之后才去调用,即wait方法应该放在synchronized(obj){}块中,否则运行期间会抛出IllegalMonitorStateException异常。

notify方法和notifyAll方法

既然介绍了wait方法,怎能少了notify方法和notifyAll方法。

Object类中的notify方法:

/** * Wakes up a single thread that is waiting on this object's * monitor. If any threads are waiting on this object, one of them * is chosen to be awakened. The choice is arbitrary and occurs at * the discretion of the implementation. A thread waits on an object's * monitor by calling one of the {@code wait} methods. * <p> * The awakened thread will not be able to proceed until the current * thread relinquishes the lock on this object. The awakened thread will * compete in the usual manner with any other threads that might be * actively competing to synchronize on this object; for example, the * awakened thread enjoys no reliable privilege or disadvantage in being * the next thread to lock this object. * <p> * This method should only be called by a thread that is the owner * of this object's monitor. A thread becomes the owner of the * object's monitor in one of three ways: * <ul> * <li>By executing a synchronized instance method of that object. * <li>By executing the body of a {@code synchronized} statement * that synchronizes on the object. * <li>For objects of type {@code Class,} by executing a * synchronized static method of that class. * </ul> * <p> * Only one thread at a time can own an object's monitor. * * @throws IllegalMonitorStateException if the current thread is not * the owner of this object's monitor. * @see java.lang.Object#notifyAll() * @see java.lang.Object#wait() */ public final native void notify();

解析notify方法的源码注释:

举例来解释,比方说多线程下,object对象是共享对象,那么对object对象的操作可以放在synchronized(object)同步块中,当线程A在同步块中执行object.wait方法,线程A就进入了等待状态。这时在线程Q中的同步块中执行object.notify(),就能唤醒线程A。另一种情形,假如线程ABC都执行了object.wait方法,那么当线程Q中执行了object.notify(),则只能唤醒其中一个,至于具体是哪一个,是任意的。

调用notify唤醒的线程,处于等待获取锁状态,且这个被唤醒的线程,相对于其他在等待获取锁的线程,没有任何特权,也没有任何劣势,即公平竞争。

如注释中所描述,同步块,可以是对共享对象进行同步处理,也可以直接对实例方法进行同步处理,还可以对类进行同步处理。

如果,当前调用notify方法的线程没有获取对象锁,则将抛出IllegalMonitorStateException异常。

notifyAll方法与notify方法很类似,不同之处是notify方法唤醒任意一个线程而notifyAll方法唤醒的是所有线程。

wait方法和sleep方法的对比

    正如方法定义所描述,这两个方法都是native方法,且都会抛出InterruptedException,其中sleep方法是类方法,而wait方法是实例方法。

    sleep方法是Thread类的方法,而wait方法是Object类的方法,由于一切类都是继承自Object类,因此Thread类中也有wait方法。

    wait方法和sleep方法都可以通过interrupt方法打断线程的暂停状态,从而使得线程立刻抛出IntterruptedException。

    sleep方法的作用是让当前线程暂停指定的时间,无关对象锁;而wait方法则是用于多个线程间的信息交互,与对象锁有关。

    sleep方法是类方法,而锁是对象级别的。因此sleep方法并不影响锁的相关行为。因此如果在调用sleep方法时该线程是处理持有对象锁的状态,睡眠状态中仍然持有锁。而wait方法能安全使用的前提是获取了对象锁,wait方法之所以可以用于多个线程间的信息交流,正是它会释放对象锁。

    这里引用知乎用户“孙立伟”的一段话:Thread.sleep和Object.wait都会暂停当前的线程,对于CPU资源来说,不管是哪种方式暂停的线程,都表示它暂时不再需要CPU的执行时间。操作系统会将执行时间分配给其它线程。区别是,调用wait后,需要别的线程执行notify/notifyAll才能够重新获得CPU执行时间。