// 引入相应的包
//json-lib-2.2-jdk15.jar
 import net.sf.json.JSONArray;
 import net.sf.json.JSONObject;

1. //把java 对象列表转换为json对象数组,并转为字符串

JSONArray array = JSONArray.fromObject(userlist);
     String jsonstr = array.toString();

2.//把java对象转换成json对象,并转化为字符串

JSONObject object = JSONObject.fromObject(invite);
   Log4jInit.ysulogger.debug(object.toString());

3.//把JSON字符串转换为JAVA 对象数组

String personstr = getRequest().getParameter("persons");
   JSONArray json = JSONArray.fromObject(personstr);
   List persons = (List)JSONArray.toCollection(json, nvoidPerson.class);