Java对象与JSON字符串互相转换

1.准备工作

在与前端页面交互时，经常需要传递json字符串数据，我们可以使用JSONObject来处理json字符串。

需要导入2个jar包：

fastjson-1.2.2.jar，fastjson-1.2.2-sources.jar

import com.alibaba.fastjson.JSONObject;
import org.junit.Test;

import java.util.*;

作为测试，先准备几个字符串数据：

 private String s1 = " {\"AreaName\": \"北京\",\n" +
      "   \"CityId\": 110100,\n" +
      "   \"NoMarket\": false,\n" +
      "   \"OldCityId\": 646,\n" +
      "   \"Pinyin\": \"beijing\",\n" +
      "   \"ProvinceId\": 110000}";
  private String s2 = "[\n"+
      "       {\"name\":\"tom\",\"age\":18,\"gender\":\"男\"},\n"+
      "       {\"name\":\"lily\",\"age\":19,\"gender\":\"女\"},\n"+
      "       {\"name\":\"hanmeimei\",\"age\":17,\"gender\":\"女\"}]";

  private String s3 = "{\"id\":1, \"from\":\"xiaoxin\", \n" +
    "       \"result\":[\n" +
    "           {\"age\":18,\"gender\":\"男\",\"name\":\"tom\"},\n" +
    "           {\"age\":19,\"gender\":\"女\",\"name\":\"lily\"},\n" +
    "           {\"age\":17,\"gender\":\"女\",\"name\":\"hanmeimei\"}]}";

  private String s4 = "{\"age\":18,\"gender\":\"男\",\"name\":\"tom\"}";
  private String s5 ="[1,2,3,4,5]";

2.字符串转对象

我们需要使用JSONObject.parse(),JSONObject.parseArray(),JSONObject.parseObject()三个方法来转换json数据，JSONObject.parse()获得后的数据需要强转为JSONObject才能进行下一步操作。

json字符串转为JSONObject对象后,操作数据与map类似，使用get()方法来获取数据，也可以使用getInteger(),getString()…等方法来获取数据。

若要转换为java对象，则需要创建一个有相应字段的类，下例中为User类

 @Test
  public void test1(){
    //json字符串转json对象,与map类似
    //只写parse,需要强转，写parseObject，则直接得到JSONObject
    JSONObject jsonObject = (JSONObject) JSONObject.parse(s1);
    System.out.println(jsonObject);//{"AreaName":"北京","CityId":110100,"NoMarket":false,"OldCityId":646,"Pinyin":"beijing","ProvinceId":110000}
    //从json对象中获取值
    String areaName = jsonObject.getString("AreaName");
    System.out.println("----------------");
    System.out.println(areaName);
    System.out.println(jsonObject.getInteger("OldCityId"));
    System.out.println(jsonObject.get("CityId"));
    System.out.println("----------------");
    //遍历jsonObject
    jsonObject.forEach((key,value)-> System.out.println(key+"->"+value));

    //将字符串转换为java对象
    User user = JSON.parseObject(s4, User.class);
    System.out.println(user);
   }

User类：

public class User {
  private String name;
  private int age;
  private String gender;

  @Override
  public String toString() {
    return "User{" +
        "name='" + name + '\'' +
        ", age=" + age +
        ", gender='" + gender + '\'' +
        '}';
   }

  public String getName() {
    return name;
   }

  public void setName(String name) {
    this.name = name;
   }

  public int getAge() {
    return age;
   }

  public void setAge(int age) {
    this.age = age;
   }

  public String getGender() {
    return gender;
   }

  public void setGender(String gender) {
    this.gender = gender;
   }



  public User() {
   }

  public User(String name, int age, String gender) {
    this.name = name;
    this.age = age;
    this.gender = gender;
   }
}
            

转换json数组，需要使用 JSONObject.parseArray()方法，转换为JSONArray类，之后的操作方法与操作ArrayList类似。

 @Test
  public void test2(){
    //json对象数组字符串转json对象数组，相当于ArrayList
    JSONArray jsonS2 = JSONObject.parseArray(s2);
    System.out.println(jsonS2);//[{"age":18,"gender":"男","name":"tom"},{"age":19,"gender":"女","name":"lily"},{"age":17,"gender":"女","name":"hanmeimei"}]
    //获取索引为1的值,需要强转为JSONObject
    JSONObject o1 = (JSONObject)jsonS2.get(0);
    System.out.println(o1);
    //遍历
//     jsonS2.forEach(item-> System.out.println(item));
    jsonS2.forEach(System.out::println);

    //转化为java ArrayList
    List<User> users = JSON.parseArray(s2, User.class);
    System.out.println(users);
   }

转化复杂的json数据，需要一步一步转换，获取相应的数据可以使用getJSONArray(),getJSONObject()方法

 @Test
  public void test3(){
    //复杂数据转json对象，与上面的一样
    JSONObject objectS3 = JSONObject.parseObject(s3);
    //拿到result  JSONArray
    JSONArray result = objectS3.getJSONArray("result");
    //遍历result
    result.forEach(System.out::println);
    //获取指定索引的值
    JSONObject o = result.getJSONObject(1);
    System.out.println(o.getString("name")); //lily

    //将json数组转换为List
    List<Integer> numArr = JSON.parseArray(s5, Integer.class);
    System.out.println(numArr);

    //将result转换为List对象
    List<User> users = JSON.parseArray(objectS3.getJSONArray("result").toJSONString(), User.class);
    System.out.println(users);//[User{name='tom', age=18, gender='男'}, User{name='lily', age=19, gender='女'}, User{name='hanmeimei', age=17, gender='女'}]

   }

3.对象转字符串

先准备一个Map和一个List

private HashMap<String, String> map1 = new HashMap<String, String>();
private List<String> list1 = new ArrayList<>();
private List<User> list2 = new ArrayList<>();

使用JSONObject.toJSONString()方法来将对象转换为json字符串

@Test
  public void test1(){
    map1.put("apple","新鲜的苹果"); //向集合中添加对象
    map1.put("computer","配置优良的计算机");
    map1.put("book","堆积成山的图书");
    System.out.println(map1);
    //map对象转json字符串
    String s = JSONObject.toJSONString(map1);
    System.out.println(s);

    //java对象转json字符串
    String s1 = JSONObject.toJSONString(new User("tom", 18, "男"));
    System.out.println(s1);
   }

转换数组也是一样：

 @Test
  public void test2(){
    Collections.addAll(list1,"tom","18","男");
    System.out.println(list1);
    //ArrayList转json字符串
    String s = JSONObject.toJSONString(list1);
    System.out.println(s);

    //java对象的ArrayList转json字符串
    Collections.addAll(list2,new User("tom",18,"男"),new User("lily",16,"女"));
    System.out.println(JSONObject.toJSONString(list2));
   }