Java locks with synchronized keyword

Question

I saw this in a tutorial, they asked if there is a problem with the following code. To me it looked like b() cannot be accessed as a() is already having the control over the monitor. Am I right in thinking so?

public class Test {
    public synchronized void a() {
        b();
        System.out.println("I am at a");
    }
    public synchronized void b() {
        System.out.println("I am at b");
    }
}

Answer 1

No, there is no problem with that code. Note two things:

1. synchronized SomeType foo() { ... } is equivalent to

   SomeType foo() {
       synchronized (this) { ... } 
   }
   

   It locks the this instance of the enclosing class. So, in your case a() and b() are locking the same thing

2. If a thread is already holding a lock on some object's monitor, it prevents another thread from acquiring a lock on the same object, but the same thread can acquire more locks if it needs too, that is not affected. So

   public synchronized void a() {  // acquires lock on this
      b();                         // also aquires lock on this, but it's ok because it is the same thread
      System.out.println("I am at a");
   }      
   

   While a thread is inside a(), no other thread will be able to call either a() or b() on the same instance. If they try to, they will have to wait until the current thread exits a(). But the current thread itself is not affected, it can call any synchronized method on this object, because it is already holding the lock.

Answer 2

No, you're incorrect. The thread has control of the monitor, not the method, and so it's capable of following execution into as many methods synchronized on the same object as needed.