OBJ -> String
String str = JSONObject.toJSONString(obj);

JSONObject -> String
String str = JSONObject.toJSONString(jobj);
String str = jobj.toJSONString();

String -> JSONObject
JSONObject jobj = JSONObject.parseObject(str);

String -> OBJ
Teacher teacher = JSONObject.parseObject(str, Teacher.class)
Teacher teacher = JSONObject.parseObject(str, new TypeReference<Teacher>());

*OBJ -> JSONObject
JSONObject jobj = (JSONObject)JSONObject.toJSON(obj);