Java抽象类Map的Jackson序列化问题：无法生成目标JSON格式

目标API要求的请求JSON格式：

{
  "firstName" : "XYZ",
  "family" : {
     "commonDetails" : {
         "secondName" : "ABC"
     }, 
     "1" : "Mother name",
     "2" : "Father name",
     "3" : "Spouse name"
  }
}

我定义的Java请求类如下：

UserDetails类 #

public class UserDetails {
   private String firstName;
   private Map<String, AbstractFamilyDetails> details;
}

AbstractFamilyDetails抽象类 #

@Data
@NoArgsConstructor
@AllArgsConstructor
@JsonIgnoreProperties("type")
@JsonTypeInfo(
        use = JsonTypeInfo.Id.NAME,
        include = JsonTypeInfo.As.EXISTING_PROPERTY,
        property = "type"
)
@JsonSubTypes({
        @JsonSubTypes.Type(value = FamilyCommonDetails.class, name = "COMMON_DETAILS"),
        @JsonSubTypes.Type(value = FamilyIndividual.class, name = "FAMILY")
})
public abstract class AbstractFamilyDetails {
    private String type;
}

FamilyCommonDetails子类 #

@Data
@EqualsAndHashCode(callSuper = true)
public class FamilyCommonDetails extends AbstractFamilyDetails {
    private String secondName;

    @Builder
    public FamilyCommonDetails(String secondName) {
        super("COMMON_DETAILS");
        this.secondName = secondName;
    }

    public FamilyCommonDetails() {
        super("COMMON_DETAILS");
    }

    @Override
    public String toString() {
        return this.secondName;
    }
}

FamilyIndividual子类 #

@Data
@EqualsAndHashCode(callSuper = true)
public class FamilyIndividual extends AbstractFamilyDetails {
    private String individual;

    @Builder
    public FamilyIndividual(String individual) {
        super("FAMILY");
        this.individual = individual;
    }

    public FamilyIndividual() {
        super("FAMILY");
    }

    @Override
    public String toString() {
        return this.individual;
    }
}

但实际发送给服务器的JSON是：

{
  "firstName" : "XYZ",
  "family" : {
     "commonDetails" : {
         "secondName" : "ABC",
     }, 
     "1" : {"individual": "Mother name"},
     "2" : {"individual": "Father name"},
     "3" : {"individual": "Spouse name"}
  }
}

请问哪里出错了？是否需要自定义Jackson序列化器实现需求？

问题根源 #

1. FamilyIndividual序列化方式不符合要求：Jackson默认会把对象序列化为键值对结构，所以你的FamilyIndividual实例被转成了{"individual": "xxx"}的嵌套对象，但API需要的是直接的字符串值。
2. 字段名映射问题：你的UserDetails类里字段叫details，但目标JSON里对应的键是family（看你实际输出已经是family，应该是已经补了@JsonProperty，但这里还是提一下避免遗漏）。

解决步骤 #

不需要写复杂的自定义序列化器，用Jackson自带的@JsonValue注解就能解决核心问题：

1. 修改FamilyIndividual类

给individual字段加上@JsonValue注解，告诉Jackson直接序列化这个字段的字符串值，而不是整个对象：

@Data
@EqualsAndHashCode(callSuper = true)
public class FamilyIndividual extends AbstractFamilyDetails {
    @JsonValue
    private String individual;

    @Builder
    public FamilyIndividual(String individual) {
        super("FAMILY");
        this.individual = individual;
    }

    public FamilyIndividual() {
        super("FAMILY");
    }

    @Override
    public String toString() {
        return this.individual;
    }
}

2. 修正字段名映射（如果没做的话）

给UserDetails的details字段加上@JsonProperty("family")，确保序列化后的键名符合API要求：

public class UserDetails {
   private String firstName;
   @JsonProperty("family")
   private Map<String, AbstractFamilyDetails> details;
}

验证效果 #

修改后生成的JSON会完全匹配API要求的格式：

{
  "firstName" : "XYZ",
  "family" : {
     "commonDetails" : {
         "secondName" : "ABC"
     }, 
     "1" : "Mother name",
     "2" : "Father name",
     "3" : "Spouse name"
  }
}

内容的提问来源于stack exchange，提问作者kgsd