多线程环境中的布尔值

Question

I have boolean field:

private boolean isReady = false;
private boolean isReady() {
  return isReady;
}

and I am using it inside two methods:

synchronized (topologyLock)
{
   try 
    {
      while(!instance.isReady()) 
      {
        topologyLock.wait();
      }
    } 
    catch (InterruptedException e)
    {               
        Thread.currentThread().interrupt();
    } 

private synchronized boolean topologyChanged()
{           
    synchronized(topologyLock)
    {
        isReady = true;
        topologyLock.notifyAll();
    }
}

I think that above code should work perfectly - or do I need to make this boolean variable volatile?

Answer 1

Quoting from stone ages :

So, when to make a variable volatile?

When you have a variable which can be accessed by many threads and you want every thread to get the latest updated value of that variable even if the value is updated by any other thread/process/outside of the program.

So one could think that volatile is necessary here. But (as Kayaman pointed out correctly): it is all about the Java memory model. And the fact that synchronized not only prevents race conditions (as only one thread can update data at any point in time) but also establishes a happens before relation.

Therefore: the above code doesn't have race conditions; and it also makes sure that each thread sees the "correct" value of isReady .

Of course, if you ever happen to manipulate isReady out of a synchronized block, then all bets are off.

Answer 2

不，因为您仅在synchronized块中与之交互，按照定义， synchronized块仅在一个线程上运行。

Answer 3

If the code that you exported is the only code accessing isReady , it is correct.

Instead if this is the only part of code writing isReady , you need to define isReady as volatile to read it correctly from another part of code.

In any case if the relevant synchronized code is the code that you posted, is incorrect defining the method topologyChanged as synchronized because internally you are using the monitor topologyLock . It is enough.