java 为什么wait(),notify(),notifyAll()必须在同步方法/代码块中调用？

Java同步机制详解

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最新推荐文章于 2024-09-05 08:59:00 发布

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本文深入解析了Java中监视器的概念，解释了为什么wait(), notify() 和 notifyAll() 方法必须在同步方法或同步代码块中调用。通过实例阐述了这些方法的作用及调用时机的重要性。

在Java中，所有对象都能够被作为"监视器monitor"——指一个拥有一个独占锁，一个入口队列和一个等待队列的实体entity。所有对象的非同步方法都能够在任意时刻被任意线程调用，此时不需要考虑加锁的问题。而对于对象的同步方法来说，在任意时刻有且仅有一个拥有该对象独占锁的线程能够调用它们。例如，一个同步方法是独占的。如果在线程调用某一对象的同步方法时，对象的独占锁被其他线程拥有，那么当前线程将处于阻塞状态，并添加到对象的入口队列中。

只有在调用线程拥有某个对象的独占锁时，才能够调用该对象的wait(),notify()和notifyAll()方法。这一点通常不会被程序员注意，因为程序验证通常是在对象的同步方法或同步代码块中调用它们的。如果尝试在未获取对象锁时调用这三个方法，那么你将得到一个"java.lang.IllegalMonitorStateException:current thread not owner"。

当一个线程正在某一个对象的同步方法中运行时调用了这个对象的wait()方法，那么这个线程将释放该对象的独占锁并被放入这个对象的等待队列。注意，wait()方法强制当前线程释放对象锁。这意味着在调用某对象的wait()方法之前，当前线程必须已经获得该对象的锁。因此，线程必须在某个对象的同步方法或同步代码块中才能调用该对象的wait()方法。

当某线程调用某对象的notify()或notifyAll()方法时，任意一个(对于notify())或者所有(对于notifyAll())在该对象的等待队列中的线程，将被转移到该对象的入口队列。接着这些队列(译者注:可能只有一个)将竞争该对象的锁，最终获得锁的线程继续执行。如果没有线程在该对象的等待队列中等待获得锁，那么notify()和notifyAll()将不起任何作用。在调用对象的notify()和notifyAll()方法之前，调用线程必须已经得到该对象的锁。因此，必须在某个对象的同步方法或同步代码块中才能调用该对象的notify()或notifyAll()方法。

对于处于某对象的等待队列中的线程，只有当其他线程调用此对象的notify()或notifyAll()方法时才有机会继续执行。

调用wait()方法的原因通常是，调用线程希望某个特殊的状态(或变量)被设置之后再继续执行。调用notify()或notifyAll()方法的原因通常是，调用线程希望告诉其他等待中的线程:"特殊状态已经被设置"。这个状态作为线程间通信的通道，它必须是一个可变的共享状态(或变量)。

例如，生产者线程向缓冲区中写入数据，消费者线程从缓冲区中读取数据。消费者线程需要等待直到生产者线程完成一次写入操作。生产者线程需要等待消费者线程完成一次读取操作。假设wait(),notify(),notifyAll()方法不需要加锁就能够被调用。此时消费者线程调用wait()正在进入状态变量的等待队列(译者注:可能还未进入)。在同一时刻，生产者线程调用notify()方法打算向消费者线程通知状态改变。那么此时消费者线程将错过这个通知并一直阻塞。因此，对象的wait(),notify(),notifyAll()方法必须在该对象的同步方法或同步代码块中被互斥地调用。

下面是原文：

Why wait(), notify(), notifyAll() must be called inside a synchronized method/block?

In Java, any object can act as a monitor - that's an entity with a single lock, an entry queue, and a waiting queue. An object's method without qualifed by the keywordsynchronized can be invoked by any number of threads at any time, the lock is ignored. The synchronized method of an object, one and only one thread, who owns the lock of that object, can be permitted to run that method at any time;i.e. asynchronized method is mutually exclusive . If, at the time of invocation, another thread owns the lock, then the calling thread will be put in the Blocked state and is added to the entry queue.

The wait(), notify(), and notifyAll() methods should be called for an object only when the current thread has already locked the object's lock. This point sometimes goes unnoticed because programmers are used to calling these methods from withinsynchronized methods or blocks. Otherwise, you will get "java.lang.IllegalMonitorStateException: current thread not owner" at runtime.

When a thread running in a synchronized method of an object is calling the wait()method of the same object, that thread releases the lock of the object and is added to that object's waiting queue. As long as it's there, it sits idle. Note also that wait()forces the thread to release its lock. This means that it must own the lock of an object before calling the wait() method of that (same) object. Hence the thread must be in one of the object's synchronized methods or synchronized block before calling wait().

When a thread invokes an object's notify() or notifyAll() method, one (an arbitrary thread) or all of the threads in its waiting queue are removed from the waiting queueto the entry queue. They then actively contend for the object's lock, and the one that gets the lock goes on to execute. If no threads are waiting in the waiting queue, then notify() and notifyAll() have no effect. Before calling the notify() or notifyAll()method of an object, a thread must own the lock of the object. Hence it must be in one of the object's synchronized methods or synchronized block.

A thread in the waiting queue of an object can run again only when some other thread calls the notify() (or the notifyAll) method of the same object.

The reason to call wait() is that the thread does not want to execute a block of code until a particular state to be achieved. It wants to wait until a particular state to be achieved. The reason to call notify() or notifyAll() method is that the thread will signal others that "a particular state has been achieved". The state is a communication channel between threads and it must be shared mutable state.

For example, one thread read data from a buffer and one thread write data into buffer. The reading data thread needs to wait until the writing data thread completly write a block data into the buffer. The wirting data thread needs to wait until the reading data thread completly read the data from the buffer. If wait(), notify(), andnotifyAll() methods can be called by a ordinary method , the reading thread callswait() and the thread is being added to waiting queue . At just the same moment, the writing thread calls notify() to signal the condition changes. The reading thread misses the change and waits forever. Hence, they must be called inside a synchronized method or block which is mutually exclusive.