GSON中Java对象与JSON互相转换——(一)

json的转换插件是通过java的一些工具，直接将java对象或集合转换成json字符串。

常用的json转换工具有如下几种：

1）jsonlib

2）Gson：google

3）fastjson：阿里巴巴

------GSON操作简单，只需要一个jar包----------

User.java

package bean;

public class User {

    private String id;
    private String name;
    private int age;
    private String address;
    public String getId() {
        return id;
    }
    public void setId(String id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public User(String id, String name, int age, String address) {
        super();
        this.id = id;
        this.name = name;
        this.age = age;
        this.address = address;
    }
    @Override
    public String toString() {
        return "User [id=" + id + ", name=" + name + ", age=" + age + ", address=" + address + "]";
    }
    
    
    
}

1.测试Java对象转JSON(测试结果在后面注释)

操作简单，无论转单个对象还是集合，都是gson.toJson(obj);

package TestJson;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import javax.xml.bind.UnmarshallerHandler;

import org.junit.Test;

import com.google.gson.Gson;

import bean.User;

public class Bean2JSON {

//    测试单个对象转JSON
    @Test
    public void test1(){
        User user = new User("1", "QIAOZHI",25,"山西太原");
        Gson gson = new Gson();
        String json = gson.toJson(user);
        System.out.println(json);   //{"id":"1","name":"QIAOZHI","age":25,"address":"山西太原"}
    }
    
    
    
//    测试list转JSON
    @Test
    public void test2(){
        List<User> list = new ArrayList<User>();
        list.add(new User("1", "QIAOZHI",25,"山西太原"));
        list.add(new User("2", "QIAOZHI",25,"山西太原"));
        list.add(new User("3", "QIAOZHI",25,"山西太原"));
        Gson gson = new Gson();
        String json = gson.toJson(list);
        System.out.println(json);
//        [{"id":"1","name":"QIAOZHI","age":25,"address":"山西太原"},{"id":"2","name":"QIAOZHI","age":25,"address":"山西太原"},{"id":"3","name":"QIAOZHI","age":25,"address":"山西太原"}]
    }
    
//    测试map对象转JSON
    @Test
    public void test3(){
        Map<String,User> map = new HashMap<String,User>();
        map.put("1", new User("1", "QIAOZHI",25,"山西太原"));
        map.put("2", new User("2", "QIAOZHI",25,"山西太原"));
        map.put("3", new User("3", "QIAOZHI",25,"山西太原"));
        Gson gson = new Gson();
        String json = gson.toJson(map);
        System.out.println(json);
//        {"3":{"id":"3","name":"QIAOZHI","age":25,"address":"山西太原"},"2":{"id":"2","name":"QIAOZHI","age":25,"address":"山西太原"},"1":{"id":"1","name":"QIAOZHI","age":25,"address":"山西太原"}}
    }
    
//    测试list<map>对象转JSON
    @Test
    public void test4(){
        List<Map<String,User>> list = new ArrayList<>();
        Map<String,User> map1 = new HashMap<String,User>();
        Map<String,User> map2 = new HashMap<String,User>();
        map1.put("map11", new User("1", "QIAOZHI",25,"山西太原"));
        map1.put("map12", new User("2", "QIAOZHI",25,"山西太原"));
        map2.put("21", new User("11", "QIAOZHI",25,"山西太原"));
        map2.put("22", new User("21", "QIAOZHI",25,"山西太原"));
        list.add(map1);
        list.add(map2);
        Gson gson = new Gson();
        String json = gson.toJson(list);
        System.out.println(json);
//        [{"map12":{"id":"2","name":"QIAOZHI","age":25,"address":"山西太原"},"map11":{"id":"1","name":"QIAOZHI","age":25,"address":"山西太原"}},{"21":{"id":"11","name":"QIAOZHI","age":25,"address":"山西太原"},"22":{"id":"21","name":"QIAOZHI","age":25,"address":"山西太原"}}]
    }
    
}

 2.测试Json转Java对象与集合(测试结果在后面注释)

package TestJson_GSON;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import org.junit.Test;

import com.google.gson.Gson;
import com.google.gson.reflect.TypeToken;

public class JSON2Bean {
//    测试json转单个对象
    @Test
    public void test1(){
        String json = "{'id':'1','name':'QIAOZHI','age':25,'address':'山西太原'}";
        Gson gson = new Gson();
        User user = gson.fromJson(json, User.class);
        System.out.println(user);  
//        User [id=1, name=QIAOZHI, age=25, address=山西太原]
    }
    
    
    
//    测试JSON转list
    @SuppressWarnings("unchecked")
    @Test
    public void test2(){
        String json = "[{'id':'1','name':'QIAOZHI','age':25,'address':'山西太原'},{'id':'2','name':'QIAOZHI','age':25,'address':'山西太原'},{'id':'3','name':'QIAOZHI','age':25,'address':'山西太原'}]";
        List<User> list = new ArrayList<User>();
        Gson gson = new Gson();
        list = (List<User>) gson.fromJson(json, new TypeToken<List<User>>(){}.getType());
        System.out.println(list);
        for(User user:list){
            System.out.println(user);
        }
//        [{id=1, name=QIAOZHI, age=25.0, address=山西太原}, {id=2, name=QIAOZHI, age=25.0, address=山西太原}, {id=3, name=QIAOZHI, age=25.0, address=山西太原}]
        System.out.println(list.get(1));
//        {id=2, name=QIAOZHI, age=25.0, address=山西太原}
    }
    
    
//    测试JSON转Map
    @Test
    public void test3(){
        String json = "{'3':{'id':'3','name':'QIAOZHI','age':25,'address':'山西太原'},'2':{'id':'2','name':'QIAOZHI','age':25,'address':'山西太原'},'1':{'id':'1','name':'QIAOZHI','age':25,'address':'山西太原'}}";
        Map<String,User> map = new HashMap<String,User>();
        Gson gson = new Gson();
        map = (Map<String,User>) gson.fromJson(json, new TypeToken<Map<String,User>>(){}.getType());
        System.out.println(map);
//        {3={id=3, name=QIAOZHI, age=25.0, address=山西太原}, 2={id=2, name=QIAOZHI, age=25.0, address=山西太原}, 1={id=1, name=QIAOZHI, age=25.0, address=山西太原}}
        User user = map.get("1");
        System.out.println(user);
//        {id=1, name=QIAOZHI, age=25.0, address=山西太原}
    }
    
    
//    测试JSON转list<map>对象
    @SuppressWarnings("unchecked")
    @Test
    public void test4(){
        String json = "[{'map12':{'id':'2','name':'QIAOZHI','age':25,'address':'山西太原'},'map11':{'id':'1','name':'QIAOZHI','age':25,'address':'山西太原'}},{'21':{'id':'11','name':'QIAOZHI','age':25,'address':'山西太原'},'22':{'id':'21','name':'QIAOZHI','age':25,'address':'山西太原'}}]";
        List<Map<String,User>> list = new ArrayList<>();
        Gson gson = new Gson();
        list = gson.fromJson(json, new TypeToken<List<Map<String,User>>>(){}.getType());
        System.out.println(list);
//        [{map12={id=2, name=QIAOZHI, age=25.0, address=山西太原}, map11={id=1, name=QIAOZHI, age=25.0, address=山西太原}}, {21={id=11, name=QIAOZHI, age=25.0, address=山西太原}, 22={id=21, name=QIAOZHI, age=25.0, address=山西太原}}]
        System.out.println(list.get(1));
//        {21={id=11, name=QIAOZHI, age=25.0, address=山西太原}, 22={id=21, name=QIAOZHI, age=25.0, address=山西太原}}
    }
}

注意:

json转普通的Javabean只需要两个简单的参数，第一个是json字符串，第二个是需要转换对象的类型。  

User user = gson.fromJson(json, User.class);

json转集合与map类型需要使用了TypeToken，它是gson提供的数据类型转换器，可以支持各种数据集合类型转换。也就是通知GSON将json转为那种集合

list = (List<User>) gson.fromJson(json, new TypeToken<List<User>>(){}.getType());

map = (Map<String,User>) gson.fromJson(json, new TypeToken<Map<String,User>>(){}.getType());

list = gson.fromJson(json, new TypeToken<List<Map<String,User>>>(){}.getType());