Why do we write Synchronized(ClassName.class)

I have a question in singleton pattern. In singleton pattern we write

synchronized(ClassName.class){

     // other code goes here

}

What is the purpose of writing ClassName.class?

In a member method (non-static) you have two choices of which monitor (lock) to use: "this" and "my class's single static lock".

If your purpose is to coordinate a lock on the object instance, use "this":

...
synchronized (this) {
  // do critical code
}

or

public synchronized void doSomething() {
 ...
}

However, if you are trying to have safe operations including either:

• static methods

• static members of your class

Then it is critical to grab a class-wide-lock. There are 2 ways to synchronize on the static lock:

...
synchornized(ClassName.class) {
   // do class wide critical code
}

or

public static synchronized void doSomeStaticThing() {
   ...
}

VERY IMPORTANTLY, the following 2 methods DO NOT coordinate on the same lock:

public synchronized void doMemberSomething() {
   ...
}

and

public static synchronized void doStaticSomething() {
   ...
}