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The Real-Time Specification for Java (RTSJ) provides facilities for deterministic, real-time execution in a language that is otherwise subject to variable latencies in memory allocation and garbage collection. A major consequence of these facilities is that the normal Java practice of passing around references to objects in heap memory cannot be used in hard real-time activities. Instead, designers must think carefully about what type of non-heap memory to use and how to transfer data between components without violating RTSJ's memory-area assignment rules. This report explores the issues of programming with non-heap memory from a practitioner's view in designing and programming realtime control loops using a commercially available implementation of the RTSJ.

Missions involving robotic space flight typically have a way to change the software that controls the flight system, or some part of it, such as an instrument, after launch. Usually this is accomplished by uplinking small sets of binary machine instructions and writing them to known locations in memory. We present an approach, used on the Aquarius mission, that involves replacing running components of, or adding components to, the running software at a higher logical level, specifically at the software architecture level, and on the C++ rather than machine-language level. This approach provides significant advantages in flexibility, robustness, reliability, and testability. We present the component-based flight software (FSW) design features that enable these capabilities. We then discuss the approach used to verify the robustness and reliability of these techniques, and finally describe usages to date.