Welch, P.H.
(1998)
Java Threads in the Light of occam/CSP.
In: Welch, P.H. and Bakkers, A.W.P., eds.
Architectures, Languages and Patterns for Parallel and Distributed Applications.
Concurrent Systems Engineering Series, 52.
IOS Press, Amsterdam
pp. 259-284.
ISBN 90-5199-391-9.
Abstract
Java provides support for parallel computing through a model that is built
into the language itself. However, the designers of Java chose to be fairly conservative
and settled for the concepts of threads and monitors. Monitors were developed by
Tony Hoare (and others) in the early 1970s as a structured way of using semaphores
to control access to shared resources. Hoare moved away from this, in the late 1970s,
to develop the theory of Communicating Sequential Processes (CSP). One reason was
that the semantics of monitors are not WYSIWYG, so that designing robust parallel
algorithms at this level is seriously hard. This tutorial will look at how this impacts on
threaded applications written in Java.
Fortunately, it is possible to introduce the CSP model into Java through sets of
classes implemented on top of its monitor support. By restricting interaction between
active Java objects to CSP synchronisation primitives, Java thread semantics
become compositional and systems with arbitrary levels of complexity become possible.
Multi-threadedWeb applets and distributed applications become simpler to design
and implement, race hazards never occur, difficulties such as starvation, deadlock
and livelock are easier to confront and overcome; and performance is no worse than
that obtained from directly using the raw monitor primitives.
The advantages of teaching parallelism in Java purely through the CSP class libraries
will be discussed. These libraries were developed jointly at Kent and Oxford
Universities in the U.K. and the University of Twente in the Netherlands.
This paper was developed from material first presented at the Java Threads Workshop
[1]. It presents the basic threads model of Java, discusses why they may be
a good thing but why they need special care in their management, runs through the
monitor mechanisms provided in Java for their control and points out weaknesses in
that control. Finally, the CSP primitives are introduced and the case for ignoring the
monitor methods presented.
Thiswork is one of the foundation stones of the JavaPP project [2], which spun out
from the above workshop. The other founding stones [3, 4, 5, 6] were first presented
at the WoTUG-20 conference last year.
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