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The object upon which the method is being executed.

The Object that is actually synchronized while invoking a synchronised instance method (non-static) is the 'this' of scope of this method- which means - the class that contains this method as its instances' methods.

when thread t1 is executing on synch method1() and nonsynch method2. it has to acquire a lock on calling object of method1() but not for method2().

say for example: there are two threads t1 and t2. and 3 mothods A(), B() and C();

Method A() and B() are sycnhronized but method C() is not.
So, t1 and t2 both can execute C() without acquiring a lock but they (t1 and t2) need to acquire a lock for A() and B(). Means only one thread can execute A() or B() at a time.

If synchronized method is an instance method, then the called object will be locked. but if synchronized method is an static method, then the Classname.class object will be locked.