Modern Jass is an interesting open source, annotations based Design-by-Contract system for Java 6.
@SpecCasea full method specification (normal and exceptional behaviour)
@Alsoa container for multiple specifications
@Minthe lower bound of a numerical value
My biggest concern with this is that it makes the same mistake assertions did: it uses errors that can be be disabled at runtime where it should be using runtime exceptions. This makes it a debugging tool, but not really part of the running code.