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Reaquiring a Lock

The same thread can call a synchronized method on an object for which it already holds the lock, thereby reacquiring the lock. The Java runtime environment allows a thread to reacquire a lock because the locks are reentrant. Reentrant locks are important because they eliminate the possibility of a single thread�s waiting for a lock that it already holds.

Consider this class:

public class Reentrant {
    public synchronized void a() {
	b();
	System.out.println("here I am, in a()");
    }
    public synchronized void b() {
	System.out.println("here I am, in b()");
    }
}

Reentrant contains two synchronized methods: a and b. The first, a, calls the other, b. When control enters method a, the current thread acquires the lock for the Reentrant object. Now, a calls b; because b is also synchronized, the thread attempts to acquire the same lock again. Because the Java platform supports reentrant locks, this works. In platforms that don�t support reentrant locks, this sequence of method calls causes deadlock. The current thread can acquire the Reentrant object's lock again, and both a and b execute to conclusion, as is evidenced by the output:

here I am, in b()
here I am, in a()

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