What exactly is RTSJ, the Real-Time Specification for Java?

Question

What exactly is the Java Real Time Specification? That is, how does it differ from the "regular" Java SE or Java EE specifications?

Answer 1

At its core, real-time computing is about predictability -- the knowledge that the system will always perform within the required time frame.

Basically, this is something you won't be able to achieve with the regular VM (and its stop-the-world GC amongst other things), hence the need for another specification:

The Real-Time Specification for Java (RTSJ), or JSR 1, specifies how Java systems should behave in a real-time context and was developed over several years by experts from both the Java and real-time domains.

The RTSJ is designed to seamlessly extend any Java family -- whether the Java Platform, Standard Edition (Java SE); Java Platform, Micro Edition (Java ME); or Java Platform, Enterprise Edition (Java EE) -- and has the requirement that any implementation pass both the JSR 1 technology compatibility kit (TCK) and the TCK of the platform -- Java SE, Java ME, or Java EE -- on which it is based.

The RTSJ introduces several new features to support real-time operations. These features include new thread types, new memory-management models, and other newly introduced frameworks. (...)

I warmly recommend the reading of:

• An Introduction to Real-Time Java Technology: Part 1, The Real-Time Specification for Java (JSR 1)
• An Introduction to Real-Time Java Technology: Part 2, Garbage Collection and the Sun Java Real-Time System (Java RTS)