Need to assert on json content in Response

hi
Kindly let me know how to examine json content returned in Response to a REST request.
If the response contained XML then i understand that following works

def holder = new XmlHolder( messageExchange.responseContentAsXml )
assert holder["//ns1:RequestId"] != null

But if the content is in json format is there a "JsonHolder" that i can use??

Thank You
Mahesh