Jackson高级多态反序列化：处理键值对中类型信息的复杂JSON结构

{"bulli":"dog","barkVolume":10}
{"kitty":"cat", "likesCream":true, "lives":3}

public class Zoo {
    public Animal animal;

    @JsonTypeInfo(
      use = JsonTypeInfo.Id.NAME,
      include = As.PROPERTY,
      property = "type") // 期望JSON中有一个名为"type"的属性
    @JsonSubTypes({
        @JsonSubTypes.Type(value = Dog.class, name = "dog"),
        @JsonSubTypes.Type(value = Cat.class, name = "cat")
    })
    public static class Animal {
        public String name;
    }
    // ... Dog, Cat subclasses
}

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
import com.fasterxml.jackson.databind.annotation.JsonDeserialize;

@JsonDeserialize(using = AnimalDeserializer.class) // 指定使用自定义反序列化器
@JsonIgnoreProperties(ignoreUnknown = true) // 忽略JSON中Java类未定义的属性
public abstract class Animal {
    public String name; // 用于存储实例的名称（JSON的键）
}

import com.fasterxml.jackson.annotation.JsonProperty;

public class Dog extends Animal {
    @JsonProperty // 可选，但明确标识属性
    public int barkVolume;
}

import com.fasterxml.jackson.annotation.JsonProperty;

public class Cat extends Animal {
    @JsonProperty
    public boolean likesCream;
    @JsonProperty
    public int lives;
}

import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonDeserializer;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.node.ObjectNode;

import java.io.IOException;
import java.util.Iterator;

public class AnimalDeserializer extends JsonDeserializer<Animal> {

    @Override
    public Animal deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException {
        // 获取ObjectMapper实例，用于将JSON树转换为Java对象
        ObjectMapper mapper = (ObjectMapper) jsonParser.getCodec();
        // 将当前JSON对象解析为ObjectNode（树模型），以便遍历其字段
        ObjectNode node = mapper.readTree(jsonParser);

        // 遍历JSON对象的所有字段名
        Iterator<String> fieldIterator = node.fieldNames();
        while (fieldIterator.hasNext()) {
            String fieldName = fieldIterator.next(); // 获取当前字段的键
            // 检查当前字段的值是否指示了动物类型
            if (node.get(fieldName).asText().equalsIgnoreCase("dog")) {
                // 如果是"dog"，则将整个JSON节点转换为Dog对象
                Dog dog = mapper.treeToValue(node, Dog.class);
                dog.name = fieldName; // 手动设置Dog对象的名称为JSON的键
                return dog;
            } else if (node.get(fieldName).asText().equalsIgnoreCase("cat")) {
                // 如果是"cat"，则将整个JSON节点转换为Cat对象
                Cat cat = mapper.treeToValue(node, Cat.class);
                cat.name = fieldName; // 手动设置Cat对象的名称为JSON的键
                return cat;
            }
        }
        // 如果遍历完所有字段仍未找到匹配的类型，则抛出异常
        throw new IllegalArgumentException("无法识别的动物类型: " + node.toString());
    }
}

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;

import java.util.List;

public class Farm {

    private static final String json = "[" +
        "{\"bulli\":\"dog\",\"barkVolume\":10},\n" +
        "{\"dogi\":\"dog\", \"barkVolume\":7},\n" +
        "{\"kitty\":\"cat\", \"likesCream\":true, \"lives\":3},\n" +
        "{\"milkey\":\"cat\", \"likesCream\":false, \"lives\":9}" +
        "]";

    public static void main(String[] args) throws JsonProcessingException {
        ObjectMapper mapper = new ObjectMapper();
        // 使用TypeReference处理泛型集合的反序列化
        List<Animal> animals = mapper.readValue(json, new TypeReference<List<Animal>>() {});

        animals.forEach(a -> {
            System.out.println("动物名称: " + a.name);
            if (a instanceof Dog) {
                Dog dog = (Dog) a;
                System.out.println("  种类: 狗, 吠叫音量: " + dog.barkVolume);
            } else if (a instanceof Cat) {
                Cat cat = (Cat) a;
                System.out.println("  种类: 猫, 喜欢奶油: " + cat.likesCream + ", 生命数: " + cat.lives);
            }
            System.out.println("---");
        });
    }
}

动物名称: bulli
  种类: 狗, 吠叫音量: 10
---
动物名称: dogi
  种类: 狗, 吠叫音量: 7
---
动物名称: kitty
  种类: 猫, 喜欢奶油: true, 生命数: 3
---
动物名称: milkey
  种类: 猫, 喜欢奶油: false, 生命数: 9
---