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.