Concurrency: State Models and Java Programs

Chapter 4. Shared Objects and Mutual Exclusion

In the last chapter, we discussed the execution of multiple processes on one or more processors, modeling concurrent execution by interleaving and executing multiple concurrent threads in a Java program. We explained how process interaction is modeled using shared atomic actions, but not how real processes or threads interact. In this chapter, we turn to the issues involved in constructing concurrent programs in which threads interact to communicate and cooperate.

The simplest way for two or more threads in a Java program to interact is via an object whose methods can be invoked by the set of threads. This shared object's state can of course be observed and modified by its methods. Consequently, two threads can communicate by one thread writing the state of the shared object and the other thread reading that state. Similarly, a set of threads may cooperate to update some information encapsulated in a shared object. Unfortunately, as we will explain, this simple scheme of interaction does not work.

Interference

We have seen that the execution of the instructions from a set of threads can be interleaved in an arbitrary fashion. This interleaving can result in incorrect updates to the state of a shared object. The phenomenon is known as interference. The problem of interference, and how to deal with it, is the main topic of this chapter.

Ornamental Garden Problem

To focus on the issues of thread interaction, we use an example known as the problem ...