The RTSJ is an essential refinement of Java semantics and libraries for safety-critical applications plus additional APIs to support realtime and embedded programming. It provides APIs for defining realtime threads and event handlers with realtime priorities above the standard 10 Java priorities, interacting with devices, handling POSIX signals, asynchronous transfer of control, enforcing resource limits, and scope-based memory management. The RTSJ provides a means of writing and testing memory-safe time-critical applications efficiently.