7 JaRTS: basic concept

For performance reasons it makes sense to compile the code to native machine code at compile time (ahead-of-time compilation). CPU time and memory is saved compared to a JIT-compiler and the costs of dynamic class loading are avoided. The real-time Java parts are used for drivers and embedded control algorithms which mostly do not need dynamic class loading (however a solution for dynamic class loading is planned, see section 14).

JaRTS (Java Real-Time by Siemens) is an implementation of a Core Java compiler and the RTCE libraries. Output of the JaRTS compiler is platform independent ANSI-C code as well as Java code for the Baseline-Core communication. The platform dependent parts are placed in separate operating system dependent include files. In addition the communication between Baseline and Core is implemented in platform dependent C files.

The prototype JaRTS compiler and runtime libraries were implemented for RTLinux. The Core parts of an application (real-time parts) are compiled to native code running directly on the real-time scheduler. For RTLinux the real-time code has to be compiled into kernel modules that can be loaded dynamically. Figure 4 shows the whole build process of a JaRTS real-time Java application. Platform dependent files for RTLinux are encircled.

Figure 4: build process and platform dependent files

We implemented the communication of Baseline and Core Java via JNI (Java Native Interface) that is supported by most JVMs. Since all the real-time processing should take place in the Core part the interface between the Core and the Baseline part of an application is very small and the poor performance of JNI is not an issue.