Question

Say we have something like this,

class Class{    
//...
synchronized void m1(Class obj){ obj.m2(); }
synchronized void m2(){ /*...*/ }
}

My question – when is lock of an object obj released? When it returns from method m2, or method m1?

Answer 1

Try to translate the synchronized methods in synchronized blocks:

m1(C obj) {
    synchronized(this) {
        obj.m2();
    }
}

m2() {
    synchronized(this) {
        // some stuff
    }
}

So your obj Object is only locked during the m2() call. This is because that’s the only time when this translates to obj.

The m1() call only locks the current object, which by the way could also be the same obj; if this is the case (this == obj in the first call), then obj is locked until both m1() and m2() complete.