The homepage is here:
From the web page (update when appropriate):
JEmacs is a re-implementation of Emacs, written in a mix of Java, Scheme, and EmacsLisp.
JEmacs uses Kawa to compile Scheme and ELisp into Java bytecodes. Kawa is a widely-praised Scheme implementation that includes many Scheme extensions, powerful Java/Scheme interoperability, and high performance though a sophisticated compiler.
Kawa is being extended to support ELisp. This is so JEmacs can run the many existing Emacs packages written in ELisp. These will be compiled into Java bytecodes, which means ELisp will run on any Java engine, and benefit from the engineering being put into JVMs, such as “Just-In-Time” compilers. This could be substantially faster than the existing Emacs bytecode interpreter.
It will also be possible to write Emacs packages using Scheme. This should provide higher performance and cleaner design through the use of static scoping. You also get all the extensions and optimizations of Kawa. (The FSF also plans to base a future Emacs on Scheme, using Guile; however, I believe Kawa will provide better performance and a better design.)
Currently, JEmacs is a functional but very minimal editor. For example, searching commands are not implemented. And JEmacs is not robust or reliable. Many of the core Emacs concepts (keymaps, windows, frames, buffers, markers) are implemented, and generally in such a way as to provide the standard Emacs semantics. Enough of ELisp is implemented to compile non-trivial ELisp file, though many of the necessary functions and macros are missing.