Marker class used with annotations to indicate "no class". This is a silly but necessary work-around -- annotations cannot take nulls as either default or explicit values. Hence for class values we must explicitly use a bogus placeholder to denote equivalent of "no class" (for which 'null' is usually the natural choice).

Note that since 2.4, most (but not all! com.fasterxml.jackson.annotation.JsonTypeInfo#defaultImpl is a notable exception}) usage should start using java.lang.Void instead as the "not defined" marker.