Concurrency: State Models and Java Programs

Chapter 2. Processes and Threads

In Chapter 1, we noted that in concurrent programs, computational activities are permitted to overlap in time and that the subprogram executions describing these activities proceed concurrently. The execution of a program (or subprogram) is termed a process and the execution of a concurrent program thus consists of multiple processes. In this chapter, we define how processes can be modeled as finite state machines. We then describe how processes can be programmed as threads, the form of process supported by Java.

Modeling Processes

A process is the execution of a sequential program. The state of a process at any point in time consists of the values of explicit variables, declared by the programmer, and implicit variables such as the program counter and contents of data/address registers. As a process executes, it transforms its state by executing statements. Each statement consists of a sequence of one or more atomic actions that make indivisible state changes. Examples of atomic actions are uninterruptible machine instructions that load and store registers. A more abstract model of a process, which ignores the details of state representation and machine instructions, is simply to consider a process as having a state modified by indivisible or atomic actions. Each action causes a transition from the current state to the next state. The order in which actions are allowed to occur is determined by a transition graph that is an abstract representation ...