Threads of Control

Java Monitors Are Re-entrant

The Java runtime system allows a thread to re-acquire a monitor that it already holds because Java monitors are re-entrant. Re-entrant 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()");
    }
}

Reentrantcontains 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 re-entrant 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 re-entrant monitors, this sequence of method calls would cause deadlock.

Threads of Control