Question

Before a thread can wait on an object, it has to acquire a monitor on that object. The monitor is then released, and the thread attempts to re-acquired it once it awakes.

But what happens to other monitors the thread holds when it calls wait?

Consider this example:

   Object a = // ...
   Object b = // ...

   synchronized(a)
   {
       synchronized(b)
       {
           b.wait();
           // continue
       }
   }

When the thread calls b.wait(), will it release the locks on both a and b, or only b?

Answer 1

Only b.

The authoritarian source for these type of questions is the Java Language Specification. The relevant section in this case is 17.8 Wait Sets and Notification:

Let thread t be the thread executing the wait method on object m, and let n be the number of lock actions by t on m that have not been matched by unlock actions. One of the following actions occurs.

• […]

• Otherwise, the following sequence occurs:

  1. Thread t is added to the wait set of object m, and performs n unlock actions on m.
  2. […]