Question

In my java program, when I code

synchronized(a){
    a = 55;
    a.wait();
}

It gives run time exception java.lang.IllegalMonitorStateException on a.wait(); statement. However a=55 runs successfully.

But if I code as

synchronized(a){
    a.wait();
    a = 55;
}

then it runs perfectly without any exception. why so?

Answer 1

Because a is pointing to a different object after reassignment, and is not being synchronized on.

In other words, there are two objects, val1 and val2 that are both assigned to the variable a. You synchronize on val1 but in the first example call wait on val2. The monitor you use is attached to the object, not its variable.

You should thus avoid synchronizing by referencing a non-final variable. This leads to your confusion. If the field is mutable, use another lock, for example:

Object aMonitor = new Object();

synchronized(aMonitor) {
   a = 55;
   aMonitor.wait();
}

Ideally your scenario is just for learning though. wait() and notify() are primitives and shouldn’t be used unless as an exercise or for building your own concurrency library. If it’s for real code, use a higher level mechanism in java.util.concurrent.