It's Time for a Change: The Sun Java Real-Time System for Automation Systems

Abstract

The Sun Java Real-Time System (Java RTS) 2.0 was released as a produce in June of 2007. This release includes several innovative features new to Sun and Java; real-time garbage collection and initialization-time compilation. Additionally, this release offers customers remarkable precision in the control of the execution of logic in the temporal dimension.The Java Platform requires, for any usable implementation, internal processes which automatically collect and return to the free pool unused blocks of memory. Typically called garbage collection this internal process executes in, what is essentially, opposition to the application in its access to the memory referencing data word topology (reference topology from now on) and in its movement and access to application defined blocks of memory (objects from now on). Garbage collectors need, at some level, to synchronize with the application to avoid mutually destructive simultaneous access to the reference topology and objects. Two points we can now assert, garbage collectors require processor cycles, i.e., overhead, to complete their work and garbage collectors contend with the application for access to application data structures; hence garbage collectors introduce inconsistencies in the execution of logic when viewed in the temporal dimension. For general-purpose applications these inconsistencies, also known as pause times, are currently small enough to be unnoticeable in all but the most demanding applications. For applications which control and monitor physical machines or processes, e.g., robots, cars, trains, oil refineries, manufacturing systems, financial markets, etc., these pause times are too large by three orders of magnitude and in the past have completely eliminated the possibility of using the Java Platform to develop and execute such applications.Java Specification Request 01, finalized in 2001, known as The Real-Time Specification for Java (RTSJ), defines libraries and semantics which when implemented in a Java Virtual Machine (JVM) give developers and, consequently, applications precise control over temporal behavior. The semantics are a strict subset of the semantics of the general-purpose Java Platform which means that implementation of JSR-01 or the RTSJ are, in fact, also conforming implementations of the general-purpose Java Platform. The RTSJ offers developers a rich and deep set of abstractions in which logic can be executed but the one we are interested in is instances of RealtimeThread (RTT), (a class in the RTSJ which is a subclass of Thread). In all observable dimensions and function except the temporal domain logic executing in the context of an RTT is exactly equivalent to that logic executing in the context of a Thread.