Which real-time (RTSJ) JVM is most preferred?

There seems to be only a handful of JVMs that are implementations of the Real-Time Specification for Java (RTSJ). The Sun/Oracle Java Real-Time System is over $6000 and IBM's real-time WebSphere is over $7000, so many Java developers may never have the opportunity to use either of them. Oracle's JRockit seems to be a free real-time Java implementation with predictable, deterministic behavior, although I am not sure, since their documentation is pretty heavy on marketing language. Other smaller real-time JVMs seem risky by virtue of being developed by small organizations and having smaller user-bases.

Which real-time JVMs are preferred? Which are most used, trusted, and loved by developers?

Solution

Update (March 2012): Oracle appears to have quietly disassembled their JavaRTS development team.

There isn't a good answer to this question. There certainly isn't enough critical mass to say there's a current most-popular RT JVM.

US Defense systems I'm aware of use ~~JavaRTS,~~ IBM's WebSphere RT, and PERC. Aviation platforms use aicas JamaicaVM. Most of those folks haven't been too concerned about licensing costs, to date. They are concerned by much higher program risks. I've worked with a number of these programs, and I find that the products are of high quality, and there is (at times extraordinary) support from the vendors.

AN/FPS-85 radar uses JavaRTS, and though this is a relatively low count, it's a rather high criticality application
US Navy's Aegis Weapon System uses PERC
US Navy's DDG-1000 uses IBM WebSphere RT
aicas' JamaicaVM is in use in avionics platforms, but they are understandably coy about specifics

Many of the vendors have focused on specific clients or domains, and their designs and support infrastructure tend to fall in line with that. If you have a specific application profile in mind, maybe we can get you a better answer.

The high licensing costs you cite reflect the high expectations users of these products have, and the relatively low density of the demand. The developer community (such as it is) is also more fragmentary because systems with real-time or safety-critical constraints are typically of greater sensitivity (defense, aviation, SCADA) or protected proprietary value (automotive) or both. One excellent forum for real-time Java theory and practice is the annual JTRES conference, which is a close-knit group of academics and technically-savvy vendors.

Related on SO: Primitives-only in Java, who uses RTSJ.