Kotlin 和 Jackson - 尝试解析简单类型的子类型时缺少类型 ID

Question

我有一个 Kotlin 密封类 -Pet和两个子类 -Dog和Cat. 我的应用程序需要传输以 JSON 序列化的宠物集合。为了区分子类，我使用 Jackson@JsonTypeInfo和@JsonSubTypes注释。清单如下：

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "type")
@JsonSubTypes(
    JsonSubTypes.Type(value = Dog::class, name = "dog"),
    JsonSubTypes.Type(value = Cat::class, name = "cat")
)
sealed class Pet { abstract val name: String }

data class Dog(override val name: String): Pet()
data class Cat(override val name: String): Pet()

单个实例已正确序列化和反序列化：

    @Test
    fun `serialize dog`() {
        val dog = Dog("Kevin")
        val dogJson = objectMapper.writeValueAsString(dog)

        JsonAssert.assertEquals(dogJson, """{"type":"dog","name":"Kevin"}""")
        val newDog = objectMapper.readValue<Dog>(dogJson)
    }

当对 pets 集合进行序列化和反序列化时，就会出现问题：

    @Test
    fun `serialize dog and cat`() {
        val pets: Set<Pet> = setOf(Dog("Kevin"), Cat("Marta"))
        val petsJson = objectMapper.writeValueAsString(pets)

        JsonAssert.assertEquals(petsJson, """[{"name":"Kevin"},{"name":"Marta"}]""")
        val newPets = objectMapper.readValue<Set<Pet>>(petsJson)
    }

Jackson 在序列化期间吞咽了类型属性，因此objectMapper无法readValue：

com.fasterxml.jackson.databind.exc.InvalidTypeIdException: Missing type id when trying to resolve subtype of [simple type, class s3.moria.workflows.common.model.Pet]: missing type id property 'type'
 at [Source: (String)"[{"name":"Kevin"},{"name":"Marta"}]"; line: 1, column: 17] (through reference chain: java.util.HashSet[0])

有什么想法如何解决这个问题吗？或者解决方法？

杰克逊版本：2.9.0

Answer 1

这实际上不是一个错误，而是一个功能。对于具有泛型的集合，Jackson 将忽略您的子类型注释。这里有一个关于它的讨论：

https://github.com/FasterXML/jackson-databind/issues/1816

以下两个“修复”对我有用，并且比上面的答案需要更少的设置（我认为我们可能使用不同的杰克逊版本，但我无法让杰克逊使用子类的非默认构造函数，所以我用lateinit重写了子类定义）

克服这个问题的一种方法是：

创建您自己的场景作家

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "type")
@JsonSubTypes(
    JsonSubTypes.Type(value = Dog1::class, name = "dog"),
    JsonSubTypes.Type(value = Cat1::class, name = "cat")
)
sealed class Pet1 {
    abstract val name: String
}

class Dog1 : Pet1() {
    override lateinit var name: String
}

class Cat1 : Pet1() {
    override lateinit var name: String
}

这些测试通过（同样 JSONAssert 对我来说似乎是不同的方法签名）

package com.example.demo

import com.fasterxml.jackson.databind.ObjectMapper
import com.fasterxml.jackson.module.kotlin.readValue
import org.junit.jupiter.api.Test
import org.skyscreamer.jsonassert.JSONAssert

internal class PetTest1 {

    private var objectMapper = ObjectMapper()

    @Test
    fun `serialize dog`() {
        val dog = Dog1()
        dog.name = "Kevin"
        val dogJson = objectMapper.writeValueAsString(dog)

        JSONAssert.assertEquals(dogJson, """{"type":"dog","name":"Kevin"}""", true)
        val newDog = objectMapper.readValue<Dog1>(dogJson)
    }

    @Test
    fun `serialize dog and cat with mapper`() {
        val dog = Dog1()
        dog.name = "Kevin"
        val cat = Cat1()
        cat.name = "Marta"
        val pets: Set<Pet1> = setOf(dog, cat)
        val petCollectionType = objectMapper.typeFactory
            .constructCollectionType(Set::class.java, Pet1::class.java)

        val petsJson = objectMapper.writer().forType(petCollectionType).writeValueAsString(pets)

        JSONAssert.assertEquals(
            petsJson, """[{"type":"dog","name":"Kevin"},{"type":"cat","name":"Marta"}]""", true
        )
        val newPets = objectMapper.readValue<Set<Pet1>>(petsJson)
    }
}

您还可以使用此方法： 无需自定义序列化器/反序列化器的解决方法

你的代码看起来像：

@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.EXISTING_PROPERTY)
@JsonSubTypes(
    JsonSubTypes.Type(value = Dog::class, name = "dog"),
    JsonSubTypes.Type(value = Cat::class, name = "cat")
)
sealed class Pet {
    abstract val jacksonMarker: String
        @JsonProperty("@type")
        get
    abstract val name: String
}

class Dog : Pet() {
    override val jacksonMarker: String
        get() = "dog"
    override lateinit var name: String
}

class Cat : Pet() {
    override val jacksonMarker: String
        get() =  "cat"
    override lateinit var name: String
}

以下测试通过

internal class PetTest {

    private var objectMapper = ObjectMapper()

    @Test
    fun `serialize dog`() {
        val dog = Dog()
        dog.name = "Kevin"
        val dogJson = objectMapper.writeValueAsString(dog)

        JSONAssert.assertEquals(dogJson, """{"@type":"dog","name":"Kevin"}""", true)
        val newDog = objectMapper.readValue<Dog>(dogJson)
    }

    @Test
    fun `serialize dog and cat`() {
        val dog = Dog()
        dog.name = "Kevin"
        val cat = Cat()
        cat.name = "Marta"
        val pets: Set<Pet> = setOf(dog, cat)
        val petsJson = objectMapper.writeValueAsString(pets)

        JSONAssert.assertEquals(
            petsJson, """[{"@type":"dog","name":"Kevin"},{"@type":"cat","name":"Marta"}]""", true)
        val newPets = objectMapper.readValue<Set<Pet>>(petsJson)
    }
}