5 Real-time Java approaches

Currently there are two leading Specifications adding real-time capabilities to Java. Firstly the Real-Time Specification (RTSJ) [3] for Java produced by the Real-Time for Java Expert Group under the auspices of the Java Community Process [18]. In December 2001 the first reference implementation for the Real-Time Specification has been released by TimeSys [13]. Secondly the Real-Time Core Extensions (RTCE) [4] produced by the Real-Time Java Working Group supported by HP, Microsoft and other corporations. Both specifications cover the necessary enhancements to enable Java for real-time tasks:

• Thread scheduling and synchronization: The Java Language Specification [1] does not define the thread scheduling exactly. In addition the ten priorities provided by Java are not enough for most real-time tasks.
• Memory management: Automatic memory management of Java mostly leads to unpredictable timely behavior. There is no definition of worst-case memory allocation times and even concurrent garbage collectors are not preemptible without latencies.
• Asynchrony: Hardware interrupts and software events can occur asynchronously in real-time systems and require an immediate change of the control flow.
• Hardware access: Java does not provide direct access to hardware registers, physical memory and handling of hardware interrupts.

RTSJ implementations require modifications of the Java Virtual Machine (JVM) internals. The application developer can choose the thread scheduling algorithm and there are several strategies for memory management. The approach of RTCE is different. Instead of modifying the whole JVM RTCE can be implemented as a real-time add-on working closely with an arbitrary existing JVM.