java - What is the difference between synchronized(this) and synchronized method

Question

Lets say we have these 2 sample code :

public synchronized void getSomething(){
     this.hello = "hello World";
}

and this one

public void getSomething(){
   synchronized(this){
     this.hello = "hello World";
   }
}

So some one can tell me what's the difference now?

Answer 1

The two different methods are functionally equivalent. There may be a very small performance difference:

At the bytecode level, the synchronized method advertises its need for synchronization as a bit set in the method's access flag. The JVM looks for this bit flag and synchronizes appropriately.

The synchronized block implements its synchronization through a sequence of bytecode operations stored in the class file's definition of the method.

So the synchronized method might potentially execute slightly faster and take up less space in terms of bytecode.

Again, the two are, by specification, functionally identical.

I'm guessing that the performance difference is negligible and code style guidelines should win out. Some compilers might even optimize away the block into an access flag. And JIT may take the performance difference away.