> Cross platform in theory, not so much in practice.
Optimistic that the OpenJDK folks would rise to the challenge if there was anything to play for. Writing a serious optimising JIT for modern ARM CPUs would doubtless be no small task, but wouldn't be breaking the mould. I believe it's a similar situation for RISC-V, currently.
Googles But wait, there's more! 'Graal'! Shiny new JIT engines are on the way, and ARM support is in there. Hopefully they'll perform well.  
Java is intended to be used by optimising JVMs. Java bytecode is rarely optimised -- that's left to the JIT. Using the Jazelle approach, where is the compiler optimisation step supposed to occur? Nowhere, of course! You'd be far better off with a decent conventional ARM JVM.
If you're on a system so lightweight that this isn't an option, well, you probably shouldn't have gone with Java. (Remember Java ME?)
[Not that I've ever actually worked with this stuff, mind.]