You could try the REST Assured library which makes it very easy to test REST services from Java. For example given that your resource is called "greeting" and returns the following JSON:
{ "greeting" : { "firstName" : <first_name>, "lastName" : <last_name> } }
you can test it like this in REST Assured:
given().
param("first_name", "John").
param("last_name", "Doe").
when().
get("/greeting").
then().
statusCode(200).
body("greeting.firstName", equalTo("John").
body("greeting.lastName", equalTo("Doe");