Jackson中的JsonNode和ObjectMapper类

JsonNode主要是把json字符串转化为JsonNode对象，不用再创建json字符串一样的对象模型，就可以直接通过findvalue()之间获取你想要的字段了。

例如 

 String carJson = "{ \"name\" : \"jack\", \"age\" : 5 }";
 ObjectMapper mapper=new ObjectMapper();
 JsonNode node=mapper.readTree(carJson);
 //获取到brand字段的值 
 String brand=node.findValue("name");

ObjectMapper: Jackson中的ObjectMapper类(com.fasterxml.jackson.databind.ObjectMapper)是使用Jackson解析JSON最简单的方法。

Jackson中的ObjectMapper可以从字符串、流或文件解析JSON，并创建Java对象或对象图来表示已解析的JSON。将JSON解析为Java对象也称为从JSON反序列化Java对象。
Jackson ObjectMapper也可以从Java对象创建JSON. 从Java对象生成JSON的过程也被称为序列化Java对象到JSON。
Jackson对象映射器(Object Mapper)可以把JSON解析为用户自定义类对象, 或者解析为JSON内置的树模型的对象如readTree()方法。

1）ObjectMapper类中的特殊的readTree()方法，该方法返回JsonNode。 使用ObjectMapper readTree()方法将JSON解析为JsonNode。

@Getter
@Setter
@ToString
public class User implements Serializable {
    private static final long  serialVersionUID = -9180229310895087286L;
    private String name;
    private String sex;
    private Integer age;
    private String phoneNo;
    private String address;
    private String hobby;

    public User(String name, String sex, Integer age, String phoneNo, String address, String hobby) {
        this.name = name;
        this.sex = sex;
        this.age = age;
        this.phoneNo = phoneNo;
        this.address = address;
        this.hobby = hobby;
    }
    public User(){}
}

    public static ObjectMapper mapper=new ObjectMapper();
    @Test
    void ObjectMapper1() throws JsonProcessingException {
        String carJson = "{ \"name\" : \"xiao\", \"age\" : 5 }";
        System.out.println("转换为" + mapper.readTree(carJson));
        JsonNode node = mapper.readTree(carJson);
        node.findValue("brand");
        System.out.println("转换为" + node.findValue("name"));
        //将对象转换为jsons字符串
        User user = new User();
        user.setAddress("北京");
        user.setAge(10);
        user.setHobby("上网");
        user.setName("笑笑");
        user.setPhoneNo("11111");
        user.setSex("女");
        String str = mapper.writeValueAsString(user);
        System.out.println("对象转换为字符串 " + str);
        //将json字符串转换为对象
        String str1 = "{ \"name\" : \"xiao\", \"age\" : 5 ,\"address\" : \"上海\",\"hobby\" : \"看书\",\"sex\" : \"男\",\"phoneNo\" :\"88888\"}";
        User users = mapper.readValue(str1, User.class);
        System.out.println("将json字符串转换为对象 " + users);
        //将List转为json字符串
        List<User> userList = new ArrayList<>();
        userList.add(new User("小王", "女", 12, "1234567", "上海", "读书"));
        userList.add(new User("小黑", "男", 22, "7654321", "南京", "读书"));
        userList.add(new User("小丽", "女", 34, "1234567", "上海", "读书"));
        String strList = mapper.writeValueAsString(userList);
        System.out.println("将List转为json字符串 " + strList);
        //将json字符串转为List
        List<User> myUserList = mapper.readValue(strList, List.class);
        System.out.println("将json字符串转为List " + myUserList);
        //将Map转换为json字符串
        HashMap myMap = new HashMap<String, User>();
        myMap.put("第一个数据", new User("小王", "女", 12, "1234567", "上海", "读书"));
        myMap.put("第二个数据", new User("小黑", "男", 22, "7654321", "南京", "读书"));
        myMap.put("第三个数据", new User("小丽", "女", 34, "1234567", "上海", "读书"));
        //将字符串转换Map
        String strMap = mapper.writeValueAsString(myMap);
        System.out.println("将Map转为json字符串 " + strMap);
        try {
            //将字符串转换为Map
            HashMap<String, Object> hash = (HashMap<String, Object>) mapper.readValue(strMap, Map.class);
            System.out.println("将json字符串转Map " + hash);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }