Threads of Control

Java Monitors Are Reentrant

The Java runtime system allows a thread to re-acquire a monitor that it already holds because Java monitors are reentrant. Reentrant monitors are important because they eliminate the possibility of a single thread deadlocking itself on a monitor that it already holds.

Consider this class:

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 synchronized method, a, calls the other synchronized method, b.

When control enters method a, the current thread acquires the monitor for the Reentrant object. Now, a calls b and because b is also synchronized the thread attempts to acquire the same monitor again. Because Java supports reentrant monitors, this works. The current thread can acquire the Reentrant object's monitor 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()

In systems that don't support reentrant monitors, this sequence of method calls would cause deadlock.

Threads of Control