1.字符串String转Json

JSONObject json=new JSONObject(str);

2.Json内数据转List

如json内数据list内是map数组,
{"data":"la","list":[{"name":"lisi","age":12}]}

JSONArray jsonArray = null;
try {
    jsonArray= (JSONArray) listData.get("list");
} catch (JSONException e) {
    throw new RuntimeException(e);
}
List<JSONObject> mlist=new ArrayList<>();
for (int i = 0; i < jsonArray.length(); i++) {
    try {
        mlist.add(jsonArray.getJSONObject(i));
    } catch (JSONException e) {
        throw new RuntimeException(e);
    }
}

3.Json转map

String json=null;
Map<String,Object> parse = (Map<String, Object>) JSON.parse(json);

4.Map转化为Json

HashMap<String, Object> map = new HashMap<>();
map.put("name","lisi");
String str = JSONObject.toJSONString(map);

4.1Java连接数据库返回的数据转为JSON

JSONObject json=new JSONObject();
List jsonList = new ArrayList();
ResultSet rs = queryDb.executeQuery(sql);
ResultSetMetaData rsmd = rs.getMetaData();
int fieldCount = rsmd.getColumnCount();
while (rs.next()) {
    Map map = new HashMap();
    for (int i = 0; i < fieldCount; i++) {
        map.put(rsmd.getColumnName(i + 1), rs.getString(rsmd.getColumnName(i + 1)));
    }
    jsonList.add(map);
}
json.put("aaData",jsonList);

5.实体类转为JSON

User user = new User(); #User实体类
user.setName("lisi");
user.setAge("18");
String userJson = JSONObject.toJSONString(user);