<p>I'm working on a cross platform application in Java which currently works nicely on Windows, Linux and MacOS X. I'm trying to work out a nice way to do detection (and handling) of 'crashes'. Is there an easy, cross-platform way to detect 'crashes' in Java and to do something in response?</p>

<p>I guess by 'crashes' I mean uncaught exceptions. However the code does use some JNI so it'd be nice to be able to catch crashes from bad JNI code, but I have a feeling that's JVM specific.</p>

For handling uncaught exceptions you can provide a new ThreadGroup which provides an implementation of ThreadGroup.uncaughtException(...). You can then catch any uncaught exceptions and handle them appropriately (e.g. send a crash log home).
I can't help you on the JNI front, there's probably a way using a native wrapper executable before calling the JVM, but that executable is going to need to know about all the possible JVMs it could be calling and how the indicate crashes and where crash logs are placed etc.