Jainja is a JVM (Java Virtual Machine) written in Java.

Focus is set on portability, not on performance. Currently the supported platforms are Linux, Windows, DOS, xBSD, Minix, Haiku, HelenOS, RTEMS, Genode, Fuchsia OS, RISC OS, Java SE, Java ME, Android, Javascript (GWT, TeaVM or Emscripten), Dart, .Net  (Mono / .Net Core)

You can see Jainja in action in your HTML5 browser (demo) 

The project is licensed under the GPL with a linking exception