诡异的异常IllegalMonitorStateException

诡异的异常IllegalMonitorStateException

诡异的异常IllegalMonitorStateException

今天的一段代码抛出了java.lang.IllegalMonitorStateException,代码如下:

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private boolean wait = false;

public boolean pleaseWait() {
truesynchronized (this.wait) {
if (this.wait == true) {
return false;
}

this.wait =true;

try {
this.wait.wait();
} catch (InterruptedException e) {

}
return true;
true}
}

JavaDoc上关于IllegalMonitorStateException的解释是:

Thrown to indicate that a thread has attempted to wait on an object's monitor or to notify other threads waiting on an object's monitor without owning the specified monitor.

看起来有些晦涩难懂,比如object’s monitor。其实,在Object.notify()这个函数的JavaDoc中有相关的解释:

A thread becomes the owner of the object’s monitor in one of three ways:

  1. By executing a synchronized instance method of that object.
  2. By executing the body of a synchronized statement that synchronizes on the object.
  3. For objects of type Class, by executing a synchronized static method of that class.

说白了,就是需要在调用wait()或者notify()之前,必须使用synchronized语义绑定住被wait/notify的对象。

可问题是,在上面的代码中,已经对this.wait这个变量使用了synchronzied,然后才调用的this.wait.wait()。按理不应该抛出这个异常。

上网查了很久,终于找到了答案:

真正的问题在于this.wait这个变量是一个Boolean,并且,在调用this.wait.wait()之前,this.wait执行了一次赋值操作:

this.wait = true; 

Boolean型变量在执行赋值语句的时候,其实是创建了一个新的对象。简单的说,在赋值语句的之前和之后,this.wait并不是同一个对象。
synchronzied(this.wait)绑定的是旧的Boolean对象,而this.wait.wait()使用的是新的Boolean对象。由于新的Boolean对象并没有使用synchronzied进行同步,所以系统抛出了IllegalMonitorStateException异常。

相同的悲剧还有可能出现在this.wait是Integer或者String类型的时候。

一个解决方案是采用java.util.concurrent.atomic中对应的类型,比如这里就应该是AtomicBoolean。采用AtomicBoolean类型,可以保证对它的修改不会产生新的对象。

正确的代码:

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private AtomicBoolean wait = new AtomicBoolean(false);  

public boolean pleaseWait() {
synchronized (this.wait) {
if (this.wait.get() == true) {
return false;
}

this.wait.set(true);

try {
this.wait.wait();
} catch (InterruptedException e) {

}

return true;
}
}

备注

该blog内容来自ifeve.com

文章目录
  1. 1. 诡异的异常IllegalMonitorStateException
  2. 2. 诡异的异常IllegalMonitorStateException
  3. 3. 备注
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