http://www.mistybeach.com/products/Forth/ -- Embedding Forth in the JavaVirtualMachine, with the ability to call into the Java class libraries. Not fully baked, but quite promising. A pretty good environment for learning Forth and the class libraries, due to the stack and dictionary browsers. (Freeware, but not open source.)
http://leonardoce.interfree.it/leoforth/ -- LeoForth?, which has some Java-related words (jnewn and jinvoken, for creating instances of java classes and invoking methods on java objects, respectively), and in which the stack is composed of java objects.