使用 Jackson 将 JSON 反序列化为多态类型 - 一个完整的示例给了我一个编译错误-Java问题

Deserialize JSON with Jackson into Polymorphic Types - A Complete Example is giving me a compile error(使用 Jackson 将 JSON 反序列化为多态类型 - 一个完整的示例给了我一个编译错误)

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问题描述

我正在尝试完成程序员 Bruce 的教程，该教程应该允许对多态 JSON 进行反序列化.

I am attempting to work through a tutorial from Programmer Bruce that is supposed to allow the deserialization of polymorphic JSON.

完整的列表可以在这里找到程序员布鲁斯教程(顺便说一句很棒)

The complete list can be found here Programmer Bruce tutorials (Great stuff btw)

我已经完成了前五个，没有任何问题，但我在最后一个(示例 6)上遇到了障碍，这当然是我真正需要开始工作的那个.

I have worked through the first five with no problems but I have hit a snag on the last one (Example 6), which of course is the one I really need to get working.

我在编译时收到以下错误

I am getting the following error at compile time

ObjectMapper 类型中的方法 readValue(JsonParser, Class) 不适用于参数 (ObjectNode, Class)

The method readValue(JsonParser, Class) in the type ObjectMapper is not applicable for the arguments (ObjectNode, Class)

这是由代码块引起的

  public Animal deserialize(  
      JsonParser jp, DeserializationContext ctxt)   
      throws IOException, JsonProcessingException  
  {  
    ObjectMapper mapper = (ObjectMapper) jp.getCodec();  
    ObjectNode root = (ObjectNode) mapper.readTree(jp);  
    Class<? extends Animal> animalClass = null;  
    Iterator<Entry<String, JsonNode>> elementsIterator =   
        root.getFields();  
    while (elementsIterator.hasNext())  
    {  
      Entry<String, JsonNode> element=elementsIterator.next();  
      String name = element.getKey();  
      if (registry.containsKey(name))  
      {  
        animalClass = registry.get(name);  
        break;  
      }  
    }  
    if (animalClass == null) return null;  
    return mapper.readValue(root, animalClass);
  }  
}

具体按行

return mapper.readValue(root, animalClass);

return mapper.readValue(root, animalClass);

以前有没有人遇到过这种情况，如果有，有解决办法吗?

Has anyone run into this before and if so, was there a solution?

任何人都可以提供任何帮助，我将不胜感激提前致谢乔恩 D.

I'd appreciate any help anyone can give Thanks in advance Jon D.

推荐答案

正如我所承诺的，我将举一个例子来说明如何使用注解来序列化/反序列化多态对象，这个例子基于 Animal 类.


As promised, I'm putting an example for how to use annotations to serialize/deserialize polymorphic objects, I based this example in the Animal class from the tutorial you were reading.
首先你的 Animal 类带有子类的 Json 注释.
First of all your Animal class with the Json Annotations for the subclasses.
import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
import com.fasterxml.jackson.annotation.JsonSubTypes;
import com.fasterxml.jackson.annotation.JsonTypeInfo;

@JsonIgnoreProperties(ignoreUnknown = true)
@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY)
@JsonSubTypes({
    @JsonSubTypes.Type(value = Dog.class, name = "Dog"),

    @JsonSubTypes.Type(value = Cat.class, name = "Cat") }
)
public abstract class Animal {

    private String name;

    public String getName() {
        return name;
    }

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

}

然后是你的子类，Dog 和 Cat.
Then your subclasses, Dog and Cat.
public class Dog extends Animal {

    private String breed;

    public Dog() {

    }

    public Dog(String name, String breed) {
        setName(name);
        setBreed(breed);
    }

    public String getBreed() {
        return breed;
    }

    public void setBreed(String breed) {
        this.breed = breed;
    }
}

public class Cat extends Animal {

    public String getFavoriteToy() {
        return favoriteToy;
    }

    public Cat() {}

    public Cat(String name, String favoriteToy) {
        setName(name);
        setFavoriteToy(favoriteToy);
    }

    public void setFavoriteToy(String favoriteToy) {
        this.favoriteToy = favoriteToy;
    }

    private String favoriteToy;

}

如您所见，Cat 和 Dog 并没有什么特别之处，唯一了解它们的是 abstract 类 Animal，因此在反序列化时，您将定位到 Animal 并且 ObjectMapper 将返回实际实例，如下面的测试所示:
As you can see, there is nothing special for Cat and Dog, the only one that know about them is the abstract class Animal, so when deserializing, you'll target to Animal and the ObjectMapper will return the actual instance as you can see in the following test:
public class Test {

    public static void main(String[] args) {

        ObjectMapper objectMapper = new ObjectMapper();

        Animal myDog = new Dog("ruffus","english shepherd");

        Animal myCat = new Cat("goya", "mice");

        try {
            String dogJson = objectMapper.writeValueAsString(myDog);

            System.out.println(dogJson);

            Animal deserializedDog = objectMapper.readValue(dogJson, Animal.class);

            System.out.println("Deserialized dogJson Class: " + deserializedDog.getClass().getSimpleName());

            String catJson = objectMapper.writeValueAsString(myCat);

            Animal deseriliazedCat = objectMapper.readValue(catJson, Animal.class);

            System.out.println("Deserialized catJson Class: " + deseriliazedCat.getClass().getSimpleName());



        } catch (Exception e) {
            e.printStackTrace();
        }

    }
}

运行Test类后的输出:
{"@type":"Dog","name":"ruffus","breed":"english shepherd"}
反序列化的 dogJson 类:Dog
{"@type":"Cat","name":"goya","favoriteToy":"mice"}
反序列化的 catJson 类:猫
希望这会有所帮助，
何塞·路易斯

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