java中对象转json,json转list，json转map

在IDEA中的springboot项目里写的一个测试例子，新建User类，四个字段，Long id ， String password，String userName，int age; 以及带参构造，toString方法

在pom.xml中添加 net.sf.json-lib  依赖：

 <!-- 对象转json依赖包 -->
         <!-- https://mvnrepository.com/artifact/net.sf.json-lib/json-lib -->
         <dependency>
             <groupId>net.sf.json-lib</groupId>
             <artifactId>json-lib</artifactId>
             <version>2.4</version>
             <!--不加下面这一项会报错，是因为json-lib支持版本是jdk1.5，所以要指定下版本-->
             <classifier>jdk15</classifier>
         </dependency>

json转换测试类：

 package com.lch.springboot04.jsonparse;

 import com.lch.springboot04.domain.User;
 import net.sf.json.JSONArray;
 import net.sf.json.JSONObject;

 import java.util.*;

 /**
  * 对象转换为json数据测试，通过net.sf.json-lib.jar
  */
 public class JsonParse {

     public static void main(String[] args) {
         /**
          *  object 转为json数组 :数组，对象，map,list,list<Map<String,Object>> 都可以；
          *  object 转为json字符串：只有 自定义对象  和 map可以转换，其他都不能这样转；
          */

         String[] arr = {"a", "b", "c"};
         System.out.println("string数组转为json数组" + JSONArray.fromObject(arr)); //["a","b","c"]
 //        System.out.println("string数组转为json字符串"+ JSONObject.fromObject(arr).toString()); //无法转换：'object' is an array. Use JSONArray instead

         //java自定义对象转json
         User jack = new User("123", "jack", 18);
         System.out.println("Java对象转为json数组" + JSONArray.fromObject(jack));
         //结果：[{"age":18,"id":0,"password":"123","userName":"jack"}]
         System.out.println("Java对象转为json字符串" + JSONObject.fromObject(jack).toString());
         //结果：{"age":18,"id":0,"password":"123","userName":"jack"}

         //list转json
         User tom = new User("456", "tom", 20);
         List<User> userList = new ArrayList<>();
         userList.add(jack);
         userList.add(tom);
         System.out.println("list转为json数组" + JSONArray.fromObject(userList));
 //        结果：list转为json数组[{"age":18,"id":0,"password":"123","userName":"jack"},{"age":20,"id":0,"password":"456","userName":"tom"}]
 //        System.out.println("list转为json字符串"+ JSONObject.fromObject(userList).toString()); //不能转为json字符串

 //        list<Map<String,Object>> 转json
         List<Map<String, Object>> mapList = new ArrayList();
         Map<String, Object> m1 = new HashMap<String, Object>();
         m1.put("jack", jack);
         m1.put("tom", tom);
         Map<String, Object> m2 = new HashMap<String, Object>();
         m2.put("alex", new User("789", "alex", 20));
         mapList.add(m1);
         mapList.add(m2);
         System.out.println("map转json数组" + JSONArray.fromObject(m2));
 //        转换结果：[{"alex":{"age":20,"id":0,"password":"789","userName":"alex"}}]
         System.out.println("map转json字符串" + JSONObject.fromObject(m2));
         //结果：{"alex":{"age":20,"id":0,"password":"789","userName":"alex"}}
         System.out.println(" list<Map<String,Object>> 转json数组" + JSONArray.fromObject(mapList));
 // 结果：[{"tom":{"age":20,"id":0,"password":"456","userName":"tom"},"jack":{"age":18,"id":0,"password":"123","userName":"jack"}},{"alex":{"age":20,"id":0,"password":"789","userName":"alex"}}]
 //        System.out.println(" list<Map<String,Object>> 转json字符串"+JSONObject.fromObject(mapList));//不能转，'object' is an array. Use JSONArray instead

         /**
          * json字符串转map
          */
      /*   String jsonStr = "{\"alex\":{\"age\":20,\"id\":0,\"password\":\"789\",\"userName\":\"alex\"}}";
         Map<String,Object> map = jsonToMap(JSONObject.fromObject(jsonStr).get("alex").toString());
         System.out.println("json字符串转map:"+map.get("age"));*/
 //        {"total":2,"items":[{"age":15,"id":1,"name":"jack"},{"age":16,"id":2,"name":"tom"}]}
         String jsonStr2 = "{\"total\":2,\"items\":[{\"age\":15,\"id\":1,\"userName\":\"jack\"},{\"age\":16,\"id\":2,\"userName\":\"tom\"}]}";
         Map<String, Object> stringObjectMap = jsonToMap(jsonStr2);
         for (Map.Entry<String, Object> entry : stringObjectMap.entrySet()) {
             System.out.println(entry.getKey() + "--" + entry.getValue());
         }
         /**输出结果：
          * total--2
          items--[{"age":15,"id":1,"userName":"jack"},{"age":16,"id":2,"userName":"tom"}]
          */

 //        jsonstr转list,list中存放Java Bean
         String jsonStr3 = JSONObject.fromObject(jsonStr2).get("items").toString();
         System.out.println("jsonStr3=" + jsonStr3);
         List list = jsonStr2ObjectList(jsonStr3, User.class);
         for (Object u : list) {
             System.out.println(u);
         }
         /**
          * 输出User{id=1, password='null', userName='jack', age=15}
          User{id=2, password='null', userName='tom', age=16}
          */

     }

     /**
      * json字符串转map
      *
      * @param jsonStr
      * @return
      */
     public static Map<String, Object> jsonToMap(String jsonStr) {
         JSONObject jsonObject = JSONObject.fromObject(jsonStr); //首先转换为 JSONObject
         Iterator<String> nameItr = jsonObject.keys();//获取jsonObject的keySet()的迭代器
         String key;//接收key值
         Map<String, Object> outMap = new HashMap<>(); //存放转换结果
         while (nameItr.hasNext()) {
             key = nameItr.next();
             outMap.put(key, jsonObject.get(key));
         }
         return outMap;
     }

     /**
      * JSONObject类中keys()方法默认返回jsonObject的keyset的迭代器
      * public Iterator keys() {
      * this.verifyIsNull();
      * return this.keySet().iterator();
      * }
      */

     /** json转list,list中存放JavaBean
      * @param jsonStr 要转换的json字符串
      * @param clazz   javaBean的Class对象
      * @return
      */
     public static List jsonStr2ObjectList(String jsonStr, Class clazz) {
         JSONArray array = JSONArray.fromObject(jsonStr);
         List list = new ArrayList();
         for (Iterator itr = array.iterator(); itr.hasNext(); ) {
             JSONObject jsonObject = (JSONObject) itr.next();
             //取出json字符串中的一个子字符串，转换为指定类型的JavaBean
             Object bean = JSONObject.toBean(jsonObject, clazz);
             list.add(bean);
         }
         return list;
     }
 }

结果：