Is this ok? Synchronized( thread ), then thread=null in the synch block

Question

I see this:

// thread is a member of this class

synchronized( this.thread )
{
  this.thread.running = false;
  this.thread.notifyAll(); // Wake up anything that was .waiting() on
  // the thread
  this.thread = null;  // kill this thread reference.
  // can you do that in a synchronized block?
}

Is it ok to set the thread=null while still keeping a lock on it?

I found this nugget in a bit of BB code.

Answer 1

Yes, that's fine. The synchronized statement will take a copy of the reference that it's locking on, and use the copy to work out what to unlock at the end.

Section 14.19 of the Java Language Specification isn't actually clear about this, but it does state that the expression is evaluated at the start - and doesn't mention evaluating it again later on.

Answer 2

There's a difference:

synchronized( this.thread )

You are synchronizing on the Object the field this.thread points to

this.thread = null;

You are reassigning the field. You are not doing anything with the object you referenced above, so the lock is still valid.