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：