var json = {
"String": string,
"Array": [array]
};

var params = 'array=' + encodeURIComponent(json);
xmlhttp.open("POST",callservlet, true);
xmlhttp.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
...
xmlhttp.send(params);

String json = "{\"paramsArray\": [\"first\", 100], "
+ "\"paramsObj\": {\"one\": \"two\", \"three\": \"four\"}, "
+ "\"paramsStr\": \"some string\"}";

JSONParser parser = new JSONParser();

Object obj = parser.parse(json);

JSONObject jsonObj = (JSONObject) obj;
System.out.println(jsonObj.get("paramsStr"));
// some string

JSONObject jo = (JSONObject) jsonObj.get("paramsObj");
System.out.println(jo.get("three"));
// four

JSONArray ja = (JSONArray) jsonObj.get("paramsArray");
System.out.println(ja.get(0));