Jackson ObjectMapper activateDefaultTyping 中 JsonTypeInfo 的作用

喜欢ヅ旅行 2024-04-07 14:09 61阅读 0赞

#### 文章目录 ####

*   *  准备工作
     *  JsonTypeInfo作用
     *  测试代码
     *   *  WRAPPER\_ARRAY
         *  WRAPPER\_OBJECT
         *  PROPERTY
         *  EXTERNAL\_PROPERTY
         *  EXISTING\_PROPERTY
     *  结论

### 准备工作 ###

准备一个Person类，用于后续测试。

public class Person {
        
    
        private String name;
        private Integer age;
    
        public Person(){
        }
        public Person(String name, Integer age){
        
            this.name = name;
            this.age = age;
        }
    
        public String getName() {
        
            return name;
        }
    
        public void setName(String name) {
        
            this.name = name;
        }
    
        public Integer getAge() {
        
            return age;
        }
    
        public void setAge(Integer age) {
        
            this.age = age;
        }
    
    
        @Override
        public String toString() {
        
            return "Person{" +
                    "name='" + name + '\'' +
                    ", age=" + age +
                    '}';
        }
    }

### JsonTypeInfo作用 ###

主要用于将Java Class 以指定的格式序列化到Json字符串中，反序列化时就不需要显示的指定转换类型了。

### 测试代码 ###

#### WRAPPER\_ARRAY ####

以数组的方式，Class Type 为第一个元素，Json字符串为第二个元素。

**序列化**

public static void main(String[] args) throws JsonProcessingException {
        
            ObjectMapper mapper = new ObjectMapper();
            mapper.setVisibility(PropertyAccessor.ALL, JsonAutoDetect.Visibility.ANY);
            mapper.activateDefaultTyping(mapper.getPolymorphicTypeValidator(), ObjectMapper.DefaultTyping.NON_FINAL, JsonTypeInfo.As.WRAPPER_ARRAY);
    
            Person person = new Person("kleven", 18);
            System.out.println(mapper.writeValueAsString(person));
        }

结果：

[
        "info.kleven.******.Person", 
        {
        
            "name": "kleven", 
            "age": 18
        }
    ]

**反序列化**

public static void main(String[] args) throws JsonProcessingException {
        
            ObjectMapper mapper = new ObjectMapper();
            mapper.setVisibility(PropertyAccessor.ALL, JsonAutoDetect.Visibility.ANY);
            mapper.activateDefaultTyping(mapper.getPolymorphicTypeValidator(), ObjectMapper.DefaultTyping.NON_FINAL, JsonTypeInfo.As.WRAPPER_ARRAY);
            String jsonString = "[\"info.kleven.******.Person\",{
        \"name\":\"kleven\",\"age\":18}]";
    
            Person person = (Person) mapper.readValue(jsonString, Object.class);
            System.out.println(person);
        }

结果：

Person{
        name='kleven', age=18}

#### WRAPPER\_OBJECT ####

以对象的形式，Class Type 为key， Json字符串为value。

**序列化**

public static void main(String[] args) throws JsonProcessingException {
        
            ObjectMapper mapper = new ObjectMapper();
            mapper.setVisibility(PropertyAccessor.ALL, JsonAutoDetect.Visibility.ANY);
            mapper.activateDefaultTyping(mapper.getPolymorphicTypeValidator(), ObjectMapper.DefaultTyping.NON_FINAL, JsonTypeInfo.As.WRAPPER_OBJECT);
    
            Person person = new Person("kleven", 18);
            System.out.println(mapper.writeValueAsString(person));
        }

结果：

{
        
        "info.kleven.******.Person": {
        
            "name": "kleven", 
            "age": 18
        }
    }

**反序列化**

public static void main(String[] args) throws JsonProcessingException {
        
            ObjectMapper mapper = new ObjectMapper();
            mapper.setVisibility(PropertyAccessor.ALL, JsonAutoDetect.Visibility.ANY);
            mapper.activateDefaultTyping(mapper.getPolymorphicTypeValidator(), ObjectMapper.DefaultTyping.NON_FINAL, JsonTypeInfo.As.WRAPPER_OBJECT);
            String jsonString = "{
        \"info.kleven.******.Person\":{
        \"name\":\"kleven\",\"age\":18}}";
    
            Person person = (Person) mapper.readValue(jsonString, Object.class);
            System.out.println(person);
        }

结果：

Person{
        name='kleven', age=18}

#### PROPERTY ####

以属性的的方式，在Json字符串中体现为："@class"为key，Class Type 为value。

**序列化**

public static void main(String[] args) throws JsonProcessingException {
        
            ObjectMapper mapper = new ObjectMapper();
            mapper.setVisibility(PropertyAccessor.ALL, JsonAutoDetect.Visibility.ANY);
            mapper.activateDefaultTyping(mapper.getPolymorphicTypeValidator(), ObjectMapper.DefaultTyping.NON_FINAL, JsonTypeInfo.As.PROPERTY);
    
            Person person = new Person("kleven", 18);
            System.out.println(mapper.writeValueAsString(person));
        }

结果：

{
        
        "@class": "info.kleven.******.Person", 
        "name": "kleven", 
        "age": 18
    }

**反序列化**

public static void main(String[] args) throws JsonProcessingException {
        
            ObjectMapper mapper = new ObjectMapper();
            mapper.setVisibility(PropertyAccessor.ALL, JsonAutoDetect.Visibility.ANY);
            mapper.activateDefaultTyping(mapper.getPolymorphicTypeValidator(), ObjectMapper.DefaultTyping.NON_FINAL, JsonTypeInfo.As.PROPERTY);
            String jsonString = "{\"@class\":\"info.kleven.******.Person\",\"name\":\"kleven\",\"age\":18}";
    
            Person person = (Person) mapper.readValue(jsonString, Object.class);
            System.out.println(person);
        }

结果：

Person{
        name='kleven', age=18}

#### EXTERNAL\_PROPERTY ####

**序列化**

public static void main(String[] args) throws JsonProcessingException {
        
            ObjectMapper mapper = new ObjectMapper();
            mapper.setVisibility(PropertyAccessor.ALL, JsonAutoDetect.Visibility.ANY);
            mapper.activateDefaultTyping(mapper.getPolymorphicTypeValidator(), ObjectMapper.DefaultTyping.NON_FINAL, JsonTypeInfo.As.EXTERNAL_PROPERTY);
    
            Person person = new Person("kleven", 18);
            System.out.println(mapper.writeValueAsString(person));
        }

结果：  
![Jackson ObjectMapper activateDefaultTyping 代码示例][Jackson ObjectMapper activateDefaultTyping]  
直接报错。 阅读源码可知 2.5版本之后已经不支持***EXTERNAL\_PROPERTY*** 了。  
![ObjectMapper 源码][ObjectMapper]

#### EXISTING\_PROPERTY ####

序列化后的字符串不包含Class Type。

**序列化**

public static void main(String[] args) throws JsonProcessingException {
        
            ObjectMapper mapper = new ObjectMapper();
            mapper.setVisibility(PropertyAccessor.ALL, JsonAutoDetect.Visibility.ANY);
            mapper.activateDefaultTyping(mapper.getPolymorphicTypeValidator(), ObjectMapper.DefaultTyping.NON_FINAL, JsonTypeInfo.As.EXISTING_PROPERTY);
    
            Person person = new Person("kleven", 18);
            System.out.println(mapper.writeValueAsString(person));
        }

结果：

{
        
        "name": "kleven", 
        "age": 18
    }

**反序列化**  
反序列化时需要显示指定读取的Java Class 类型。

public static void main(String[] args) throws JsonProcessingException {
        
            ObjectMapper mapper = new ObjectMapper();
            mapper.setVisibility(PropertyAccessor.ALL, JsonAutoDetect.Visibility.ANY);
            String jsonString = "{\"name\":\"kleven\",\"age\":18}";
    
            Person person =  mapper.readValue(jsonString, Person.class);
            System.out.println(person);
        }

结果：

Person{
        name='kleven', age=18}

### 结论 ###

1.  如果是作为api返回结果，不需要反序列化时：不设置activateDefaultTyping或者使用EXISTING\_PROPERTY。
2.  如果作为缓存等，需要反序列化时：一般使用 **WRAPPER\_ARRAY**、**WRAPPER\_OBJECT**、**PROPERTY**中的一种。如果不指定则默认使用的是**WRAPPER\_ARRAY**。  
    ![Jackson ObjectMapper activateDefaultTyping 中 JsonTypeInfo 的作用][Jackson ObjectMapper activateDefaultTyping _ JsonTypeInfo]

[Jackson ObjectMapper activateDefaultTyping]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/07/e0ecb881e5e143d2838efa1279ca7097.png
[ObjectMapper]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/07/5dcc59a36211404f9b4262fa8b13a130.png
[Jackson ObjectMapper activateDefaultTyping _ JsonTypeInfo]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/07/72e465fc4a68417eb5237efaa60f0509.png