Google released Contract for Java - http://code.google.com/p/cofoja/ - which is a Java implementations for Design by Contract. Googles version uses a set of annotations, an annotation processor and a runtime java agent (see their documentation).
We have all the pieces for doing this as well.
- We have the constraint annotations
- We have an annotation processor (obviously it does something else now, but we know how to write one)
- And we have Byteman
I wonder whether we couldn't put these things together and create our own Contract for Java.