IllegalMonitorStateException

Question

What can cause that i get IllegalMonitorStateException in this code

synchronized(syncCount){
    syncCount--;
    syncCount.notify();
}

I'm little confused, since, as far as I know running thread must have monitor on object who's notify is called. It looks to me that my code can not be wrong, but somehow it is.

Answer 1

Of type Integer or similar? -- replaces the immutable Integer object with another one. Therefore you are calling notify on a different object than the synchronized.

Your code is the equivalent of:

Integer syncConunt = Integer.valueOf(5);
[...]
synchronized (syncCount) {
    syncCount = Integer.valueOf(syncCount.intValue() + 1);
    syncCount.notify();
}

You are not alone. Even before J2SE 5.0, I have seen example code published in a book that assigned a reference within a synchronized block. In general it is a good idea to mark lock fields final.

Another significant point is that the code synchronising on an object that it does not "own". Integer objects are shared (Integer.valueOf(int) will return exactly the same instance if called with values between -128 and 127, and perhaps further). If this were done by two pieces of unrelated code, then there would hidden interactions. This applies to any type where instances are shared between unrelated code. Common examples are Integer, String, Class (used by static synchronised methods) and Thread (in Sun's implementation, Thread happens to be used as a lock for join).