How a static synchronized function works? [duplicate]

Question

When a Java member needs to be thread-safe, we do like the following:

 public synchronized void func() {
     ...
 }

This syntax equivalent to:

 public void func() {
      synchronized(this) {
           ....
      }
 }

That is, it actually uses this for a lock.

My question is, if I use synchronized with a static method, as follows:

class AA {
    private AA() {}

    public static synchronized AA getInstance() {
        static AA obj = new AA();
        return obj;
    }
}

In this case, on what is the lock made for the synchronized method?

Answer 1

In case of static synchronized method, the class object of your class AA will be implicit lock

its equivalent to

class AA {
    private AA() {}

    public static AA getInstance() {
        synchronized(AA.class) {
           AA obj = new AA();
           return obj;
        }
    }
}

Answer 2

From section 8.4.3.6 of the JLS:

A synchronized method acquires a monitor (§17.1) before it executes.

For a class (static) method, the monitor associated with the Class object for the method's class is used.

So your code acquires the monitor for AA.class. As sanbhat says, it's like

synchronized(AA.class) {
    ...
}

... just as with an instance method it would be

synchronized(this) {
    ...
}