使用@JsonTypeInfo和@JsonSubTypes反序列化枚举至接口时遇异常

尝试使用Jackson将实现同一基础接口的多个枚举类反序列化时，通过@JsonTypeInfo和@JsonSubTypes指定枚举类型，却抛出InvalidTypeIdException异常。

代码示例 #

enum class FooEnum : InterfaceForEnum { BAR_ELEM, BAZ_ELEM }
enum class MooEnum : InterfaceForEnum { CAR_ELEM, CAZ_ELEM }

@JsonTypeInfo(include = JsonTypeInfo.As.EXTERNAL_PROPERTY, property = "enumType",
    use = JsonTypeInfo.Id.NAME)
@JsonSubTypes(
    JsonSubTypes.Type(name = "Foo", value = FooEnum::class),
    JsonSubTypes.Type(name = "Moo", value = MooEnum::class)
)
interface InterfaceForEnum

data class SomeClassWithInterfaceField(val enumInterfaceInstance: InterfaceForEnum)

fun main() {
    val jsonFactory = JsonFactory.builder().enable(StreamReadFeature.INCLUDE_SOURCE_IN_LOCATION).build()
    val mapper = ObjectMapper(jsonFactory).registerKotlinModule()
    val json = """
        {
            "enumType": "Foo",
            "enumInterfaceInstance" : "BAR_ELEM"
        }
    """
    mapper.readValue(json, SomeClassWithInterfaceField::class.java)
}

异常信息 #

Exception in thread "main" 
com.fasterxml.jackson.databind.exc.InvalidTypeIdException: 
Could not resolve subtype of [simple type, class InterfaceForEnum]: 
missing type id property 'enumType' (for POJO property 'enumInterfaceInstance')
 at [Source: (String)"
        {
            "enumType": "Foo",
            "enumInterfaceInstance" : "BAR_ELEM"
        }
    "; line: 4, column: 39] 
(through reference chain: SomeClassWithInterfaceField["enumInterfaceInstance"])

问题原因 #

Jackson对枚举类型有专门的反序列化逻辑，默认会直接将字符串匹配为枚举常量。当在接口上使用EXTERNAL_PROPERTY类型的@JsonTypeInfo时，Jackson无法将外部同级的enumType字段与枚举字符串值关联，导致无法识别要反序列化的具体枚举子类。

解决方案 #

方案1：调整JSON结构与注解配置 #

将枚举序列化为对象格式，把类型信息嵌入枚举对象中，修改注解和JSON结构：

1. 修改接口的注解配置：

@JsonTypeInfo(include = JsonTypeInfo.As.PROPERTY, property = "@type", use = JsonTypeInfo.Id.NAME)
@JsonSubTypes(
    JsonSubTypes.Type(name = "Foo", value = FooEnum::class),
    JsonSubTypes.Type(name = "Moo", value = MooEnum::class)
)
interface InterfaceForEnum

2. 给每个枚举类添加@JsonFormat注解，指定序列化为对象：

@JsonFormat(shape = JsonFormat.Shape.OBJECT)
enum class FooEnum : InterfaceForEnum { BAR_ELEM, BAZ_ELEM }

@JsonFormat(shape = JsonFormat.Shape.OBJECT)
enum class MooEnum : InterfaceForEnum { CAR_ELEM, CAZ_ELEM }

3. 修改JSON结构为：

{
    "enumInterfaceInstance": {
        "@type": "Foo",
        "name": "BAR_ELEM"
    }
}

方案2：保留原JSON结构，手动处理反序列化 #

通过@JsonCreator自定义构造逻辑，根据enumType字段选择对应的枚举类：

data class SomeClassWithInterfaceField(
    val enumType: String,
    val enumInterfaceInstance: InterfaceForEnum
) {
    companion object {
        @JsonCreator
        @JvmStatic
        fun create(
            @JsonProperty("enumType") enumType: String,
            @JsonProperty("enumInterfaceInstance") value: String
        ): SomeClassWithInterfaceField {
            val enumInstance = when (enumType) {
                "Foo" -> FooEnum.valueOf(value)
                "Moo" -> MooEnum.valueOf(value)
                else -> throw IllegalArgumentException("未知枚举类型: $enumType")
            }
            return SomeClassWithInterfaceField(enumType, enumInstance)
        }
    }
}

这样即可直接处理原JSON结构，无需修改接口和枚举类的注解。

内容的提问来源于stack exchange，提问作者Alex78191