如何用Jackson将JSON数组反序列化为Kotlin数据类？

给定如下JSON：

["answer", 42]

以及对应的Kotlin数据类：

data class JsonRow(val s: String, val n: Int)

如何使用Jackson将该数组转换为上述类？

最小可复现示例（Kotlin代码，Java场景下问题一致） #

import com.fasterxml.jackson.module.kotlin.jacksonObjectMapper
import com.fasterxml.jackson.module.kotlin.readValue
import kotlin.test.Test
import kotlin.test.assertEquals


data class JsonRow(val s: String, val n: Int)

object Example {

    @Test
    fun example() {
        // Given
        val jsonArray = """["answer", 42]"""
        val objectMapper = jacksonObjectMapper()

        // When
        val actual = objectMapper.readValue<JsonRow>(jsonArray)

        // Then
        val expected = JsonRow("answer", 42)
        assertEquals(expected, actual)
    }
}

测试执行失败，报错信息如下：

com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize value of type com.example.JsonRowfrom Array value (tokenJsonToken.START_ARRAY) at [Source: (String)"["answer", 42]"; line: 1, column: 1]

解决方案 #

方法1：用@JsonFormat指定数组映射规则 #

直接在数据类上添加注解，告诉Jackson该类需要从JSON数组反序列化，属性会按声明顺序匹配数组元素：

import com.fasterxml.jackson.annotation.JsonFormat
import com.fasterxml.jackson.module.kotlin.jacksonObjectMapper
import com.fasterxml.jackson.module.kotlin.readValue
import kotlin.test.Test
import kotlin.test.assertEquals

@JsonFormat(shape = JsonFormat.Shape.ARRAY)
data class JsonRow(val s: String, val n: Int)

object Example {
    @Test
    fun example() {
        val jsonArray = """["answer", 42]"""
        val objectMapper = jacksonObjectMapper()
        val actual = objectMapper.readValue<JsonRow>(jsonArray)
        val expected = JsonRow("answer", 42)
        assertEquals(expected, actual)
    }
}

方法2：用@JsonProperty自定义元素映射顺序 #

如果数组元素顺序和类属性声明顺序不一致，可通过注解指定每个属性对应数组的索引位置：

import com.fasterxml.jackson.annotation.JsonProperty
import com.fasterxml.jackson.module.kotlin.jacksonObjectMapper
import com.fasterxml.jackson.module.kotlin.readValue
import kotlin.test.Test
import kotlin.test.assertEquals

data class JsonRow(
    @JsonProperty(index = 0) val s: String,
    @JsonProperty(index = 1) val n: Int
)

object Example {
    @Test
    fun example() {
        val jsonArray = """["answer", 42]"""
        val objectMapper = jacksonObjectMapper()
        val actual = objectMapper.readValue<JsonRow>(jsonArray)
        val expected = JsonRow("answer", 42)
        assertEquals(expected, actual)
    }
}

方法3：自定义反序列化器（处理复杂逻辑） #

如果需要对数组元素做额外处理（比如类型转换、参数校验），可以自定义反序列化逻辑：

import com.fasterxml.jackson.core.JsonParser
import com.fasterxml.jackson.databind.DeserializationContext
import com.fasterxml.jackson.databind.JsonDeserializer
import com.fasterxml.jackson.databind.module.SimpleModule
import com.fasterxml.jackson.module.kotlin.jacksonObjectMapper
import com.fasterxml.jackson.module.kotlin.readValue
import kotlin.test.Test
import kotlin.test.assertEquals

data class JsonRow(val s: String, val n: Int)

class JsonRowDeserializer : JsonDeserializer<JsonRow>() {
    override fun deserialize(p: JsonParser, ctxt: DeserializationContext): JsonRow {
        val array = p.readValueAsArray<String>()
        return JsonRow(array[0], array[1].toInt())
    }
}

object Example {
    @Test
    fun example() {
        val jsonArray = """["answer", 42]"""
        val module = SimpleModule().addDeserializer(JsonRow::class.java, JsonRowDeserializer())
        val objectMapper = jacksonObjectMapper().registerModule(module)
        val actual = objectMapper.readValue<JsonRow>(jsonArray)
        val expected = JsonRow("answer", 42)
        assertEquals(expected, actual)
    }
}

内容的提问来源于stack exchange，提问作者Jake