The segv was relatively easy to find and fix. A regression introduced in JamVM 1.5.1, when I added unloader objects to unload JNI libraries after the classloader which loaded them is garbage-collected. This was itself a fix for library unloading, which used to be done within GC, but broke if the JNI_OnUnload function called back into Java.
Basically, I didn't take into account libraries which have a JNI_OnUnload function being loaded by the boot classloader (the NULL loader). This is pointless, as the boot classloader is never collected, and therefore no library loaded by it can be unloaded. However, the fix was simple - just ignore them.
The next problem was with MemoryManagerMXBean. I spent some time implementing native support for ThreadMXBean in JamVM 1.4.4 as part of the thread re-work, but never got round to implementing the full set, as nothing much seemed to use them. For now, a simple implementation which just returns "no memory managers" appears to be sufficient.
After that there was a problem with annotations, where an AnnotationTypeMismatchException was being thrown. This took some time to track down because I had to remember how annotations worked! It ended up being a mismatch between an annotation array value and the method return type. When parsing the annotation, the array values can be any one of a number of types, so in JamVM an Object array is created (when the array is created, the elements haven't yet been parsed so the type isn't known). But the method return value is the specific type, in this case String. Luckily, a similar problem has been found with the implementation in gcj, so I was able to adapt the fix into JamVMs version of sun.reflect.annotation.AnnotationInvocationHandler.
Problem number 4 was with VMClassLoader.getPackage(). The default GNU Classpath implementation relies on a META-INF/INDEX.LIST file existing in the first Jar in the bootclasspath. JRuby uses Constantine, which uses the package name to load an appropriate constant class. As the Constantine Jar is added to the bootclasspath, even if the INDEX.LIST existed, it wouldn't have any package information for it. A quick and dirty implementation of VMClassLoader.getBootPackages() which doesn't need INDEX.LIST fixed this.
Finally, there was a problem with Class.getSimpleName(). The simple name is appended to the package name to locate the constant class. However, the GNU Classpath implementation of getSimpleName (which delegates to VMClass) is broken. Again, I took the fix from gcj.
After all this, jRuby runs!
rob@dougal:~$ jruby hello.rb
The next thing to try was jirb (interactive shell). For some reason, the default prompt doesn't work (nothing is shown, it may be related to sun.misc.Signal, as jirb complains about an unsupported trap). However, the simple prompt does.
rob@dougal:~$ jirb --prompt simple
trap not supported or not allowed by this VM
>> include_class java.lang.System
=> "GNU Classpath"