Summary

In this chapter, we introduced condition synchronization, which in combination with the mutual exclusion provided by synchronized methods, supports the concept of a monitor in Java. Condition synchronization is implemented using the wait(), notify() and notifyAll() primitives which operate on a waiting queue which can be associated with any Java object. Operation wait() suspends the calling thread on the wait queue, notify() unblocks one of the threads on the wait queue and notifyAll() unblocks all the threads on the wait queue. When a thread suspends itself by calling wait(), the monitor mutual exclusion lock is released. Use of these primitives causes an exception if the invoking thread does not currently hold the monitor lock.

Model processes that react to actions rather than instigate them are usually translated into monitors in the program that implements the model. Each guarded action in the model of a monitor is implemented as a synchronized method which uses a while loop and wait() to implement the guard. The while loop condition is the negation of the model guard condition. Changes in the state of the monitor are signaled to waiting threads using notify() or notifyAll().

Nested monitor calls should be used with great care as they can cause a program to deadlock. This can occur since a thread that waits in a monitor releases only its lock, not the lock of any monitor from which it may have been called.