Question

i have a question, what is difference between

StringBuilder sb = new StringBuilder();
public void sync(){
  synchronized(sb){
  };
}

and

public void sync(){
  synchronized(this){
  };
}

Answer 1

In the first case, you lock on “sb” variable, and in the second case, in “this” object.
This is obvious, but i suppose you want to know which is better.

Well, the first case is better, because you lock on a local variable (consider to make it private) and you are quite sure that no other is going to lock on it than you.
If you lock on “this”, any other thread could use this object to lock, preventing you from running the synchronized code (whereas you safely could).