java - Can anyone explain thread monitors and wait?

Question

Someone at work just asked for the reasoning behind having to wrap a wait inside a synchronized.

Honestly I can't see the reasoning. I understand what the javadocs say--that the thread needs to be the owner of the object's monitor, but why? What problems does it prevent? (And if it's actually necessary, why can't the wait method get the monitor itself?)

I'm looking for a fairly in-depth why or maybe a reference to an article. I couldn't find one in a quick google.

Oh, also, how does thread.sleep compare?

edit: Great set of answers--I really wish I could select more than one because they all helped me understand what was going on.

Answer 1

Lots of good answers here already. But just want to mention here that the other MUST DO when using wait() is to do it in a loop dependent on the condition you are waiting for in case you are seeing spurious wakeups, which in my experience do happen.

To wait for some other thread to change a condition to true and notify:

synchronized(o) {
  while(! checkCondition()) {
    o.wait();
  }
}

Of course, these days, I'd recommend just using the new Condition object as it is clearer and has more features (like allowing multiple conditions per lock, being able to check wait queue length, more flexible schedule/interrupt, etc).

 Lock lock = new ReentrantLock();
 Condition condition = lock.newCondition();
 lock.lock();
 try {
   while (! checkCondition()) {
     condition.await();
   }
 } finally {
   lock.unlock();
 }

}