Chapter 12: Timed Systems

Overview

The programs we have developed in the previous chapters of this book are not explicitly concerned with the passage of time. We have used delays to make the execution of example programs viewable in a convenient way, however the correct operation of these programs is independent of time. Correctness depends on performing actions in a certain order and not on performing an action by a certain time. This has the advantage that the programs work safely, independently of processor speed and thread scheduling.

In this chapter, we develop programs that are concerned with the passage of time and in which correctness does depend on performing actions by specific times. In general, we deal with the relationship between the time at which program actions are executed and the time that events occur in the real world. We make the simplifying assumption that program execution proceeds sufficiently quickly such that, when related to the time between external events, it can be ignored. For example, consider a program that detects double mouse clicks. The program notes the time that passes between successive mouse clicks and causes a double-click action to be executed when this is less than some specified period. The inter-click time for such a program would be specified in tenths of a second, while the instructions necessary to recognize clicks and measure the inter-click time would require tenths of a millisecond. In modeling and developing such a program, it is reasonable to ignore the execution time of actions. We can assume that the execution of an action takes zero time, although we do not assume that executing an infinite number of actions takes zero time. A much more difficult approach to action execution time is to specify and subsequently guarantee an upper bound to execution time. This falls into a specialized area of concurrent software design termed real-time and it is beyond the scope of this book.

In the following, we first examine how we can incorporate time into models and then how the behavior exhibited by these models can be implemented in Java.