Reaquiring a Lock

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Trail:
Essential Java Classes

Lesson:
Doing Two or More Tasks At Once: Threads

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Reaquiring a Lock
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:
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 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.

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