What is the meaning of “ReentrantLock” in Java?

Question

Reentrancy means that locks are acquired on a per-thread rather than per-invocation basis.

Since an intrinsic lock is held by a thread, doesn\'t it mean that a threa

Answer 1

Reentrancy means that locks are acquired on a per-thread rather than per-invocation basis.

Let me explain this with an example.

class ReentrantTester {

    public synchronized void methodA() {
      System.out.println("Now I am inside methodA()");
      methodB();
    }

    public synchronized void methodB() {
      System.out.println("Now I am inside methodB()");
    }

    public static void main(String [] args) {
        ReentrantTester rt = new ReentrantTester();
        rt.methodA();  
    }

}

The out put is :

Now I am inside methodA()
Now I am inside methodB()

As in the above code, the ReentrantTester contains two synchronized methods: methodA() & methodB() The first synchronized method methodA() calls the other synchronized method methodB().

When execution enters the methodA(), the current thread acquires the monitor for the ReentrantTester object. Now when methodA() calls methodB(), because methodB() is also synchronized, the thread attempts to acquire the same monitor again. Because Java supports reentrant monitors, this works. The current thread acquire the ReentrantTester's monitor again and continue the execution of both methodA() and methodB().

The Java runtime allows a thread to reacquire a monitor that it already holds, because Java monitors are reentrant. These reentrant monitors are important because they eliminate the possibility of a single thread deadlocking itself on a monitor that it already holds.