Threads and Concurrent Programming

Java Monitors and Mutual Exclusion

An object that contains synchronized methods has a monitor associated with it. A monitor is a widely used synchronization mechanism that ensures that only one thread at a time can execute a synchronized method. When a synchronized method is called, a lock is acquired on that object. For example, if one of the Customer threads calls nextNumber(), a lock will be placed on that TakeANumber object. While an object is locked, no other synchronized method can run in that object. Other threads must wait for the lock to be released before they can execute a synchronized method.

While one Customer is executing nextNumber(), all other Customers will be forced to wait until the first Customer is finished. When the synchronized method is exited, the lock on the object is released, allowing other Customer threads to access their synchronized methods. In effect, a synchronized method can be used to guarantee mutually exclusive access to the TakeANumber object among the competing customers.

One cautionary note here is that although a synchronized method blocks access to other synchronized methods, it does not block access to non-synchronized methods. This could cause problems. We will return to this issue in the next part of our case study when we discuss the testing of our program.