Question

What happens when a static synchronized method is called by two threads using different instances at the same time? Is it possible? The object lock is used for non static synchronized method but what type lock is used for static synchronized method?

Answer 1

It is the same as synchronizing on the Class object implementing the method, so yes, it is possible, and yes, the mechanism effectively ignores the instance fro which the method is called:

class Foo {
    private static synchronized doSomething() {
        // Synchronized code
    }
}

is a shortcut for writing this:

class Foo {
    private static doSomething() {
        synchronized(Foo.class) {
            // Synchronized code
        }
    }
}