To serialize the response into a class, define the target class
public class Result {
public Long user_id;
}
And map response to it:
Response response = given().body(requestBody).when().post("/admin");
Result result = response.as(Result.class);
You must have Jackson or Gson in the classpath as the documentation states: http://rest-assured.googlecode.com/svn/tags/2.3.1/apidocs/com/jayway/restassured/response/ResponseBodyExtractionOptions.html#as(java.lang.Class)