Precondition (@Requires) should be checked before the method, closure is invoked

The converse is true for the post conditions (@Ensures).
The check should be after the the method, closure invocation.

class G {

String g

}

class H {

@Requires({ g != null })
static void h(G g) {
    g.g
}

}

if you call H.h(null) it will fail with a null pointer exception instead of AssertionViolation (more specifically PreconditionViolation)