Jackson反序列化时如何映射org.apache.commons.lang3.tuple.Pair至具体类型？

问题场景 #

待反序列化的JSON #

{
  "page" : 0,
  "size" : 4,
  "leafDistribution" : {
    "D_a" : {
      "1" : 4
    },
    "D_b" : {
      "3" : 4
    }
  }
}

Npd类定义 #

@Getter
public class Npd {
   private Integer page;
   private Integer size;
   protected Map<String, Pair<Integer, Integer>> leafDistribution;
}

报错信息 #

com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of
org.apache.commons.lang3.tuple.Pair (no Creators, like default constructor, exist): abstract
types either need to be mapped to concrete types, have custom deserializer, or contain additional
type information
at [Source: (String)"{
"page" : 0,
"size" : 4,
"leafDistribution" : {
"D_a" : {
"1" : 4
},
"D_b" : {
"3" : 4
}
}
}"; line: 5, column: 15] (through reference chain:
prac.filter.dto.NestedFilterRequestDto["leafDistribution"]->java.util.LinkedHashMap["D_a"])

无效的Mixin尝试代码 #

public abstract class NpdMixin {
   @JsonDeserialize(as= org.apache.commons.lang3.tuple.MutablePair.class)
    public abstract class Pair<L, R> implements Map.Entry<L, R>, Comparable<org.apache.commons.lang3.tuple.Pair<L, R>>, Serializable  { }
}

有效解决方法 #

方法一：直接在字段上指定反序列化实现类 #

无需编写Mixin，直接在leafDistribution字段上添加@JsonDeserialize注解，用contentAs指定Map值的具体Pair实现类（如MutablePair或ImmutablePair）：

@Getter
public class Npd {
   private Integer page;
   private Integer size;
   @JsonDeserialize(contentAs = org.apache.commons.lang3.tuple.MutablePair.class)
   protected Map<String, Pair<Integer, Integer>> leafDistribution;
}

方法二：正确使用Mixin注解 #

之前的Mixin写法错误，需直接为原Pair类定义Mixin，而非在NpdMixin内部嵌套新类：

1. 定义Pair的Mixin类：

@JsonDeserialize(as = org.apache.commons.lang3.tuple.MutablePair.class)
public abstract class PairMixin {}

2. 在ObjectMapper中注册该Mixin：

ObjectMapper mapper = new ObjectMapper();
mapper.addMixIn(org.apache.commons.lang3.tuple.Pair.class, PairMixin.class);

方法三：自定义反序列化器（灵活适配场景） #

如果JSON结构与Pair的映射需要特殊处理，可自定义反序列化器：

1. 编写反序列化器类：

public class PairDeserializer extends StdDeserializer<Pair<Integer, Integer>> {
    protected PairDeserializer() {
        super(Pair.class);
    }

    @Override
    public Pair<Integer, Integer> deserialize(JsonParser p, DeserializationContext ctxt) throws IOException {
        JsonNode node = p.getCodec().readTree(p);
        // 提取JSON单键值对转为Pair
        Map.Entry<String, JsonNode> entry = node.fields().next();
        Integer key = Integer.parseInt(entry.getKey());
        Integer value = entry.getValue().asInt();
        return MutablePair.of(key, value);
    }
}

2. 在Npd类字段上注册该反序列化器：

@Getter
public class Npd {
   private Integer page;
   private Integer size;
   @JsonDeserialize(contentUsing = PairDeserializer.class)
   protected Map<String, Pair<Integer, Integer>> leafDistribution;
}

内容的提问来源于stack exchange，提问作者Saptarshi