JSON 如何在 Kotlinx 序列化中对 BigDecimal 和 BigInteger 进行编码而不丢失精度？

Question

我正在使用 Kotlin/JVM 1.8.0 和 Kotlinx Serialization 1.4.1。

我需要将java.math.BigDecimal和java.math.BigInteger编码为 JSON。

我正在使用BigDecimal和BigInteger ，因为我想要编码的值可能大于Double可以容纳的值，而且我还想避免出现浮点精度错误。 我不想将数字编码为字符串，因为其他程序会读取 JSON，因此它需要正确。

JSON 规范对 numbers 的长度没有限制，所以应该可以。

当我尝试直接使用BigDecimal和BigInteger时，出现错误

import java.math.*
import kotlinx.serialization.*
import kotlinx.serialization.json.*

@Serializable
data class FooNumbers(
  val decimal: BigDecimal,
  val integer: BigInteger,
)

Serializer has not been found for type 'BigDecimal'. To use context serializer as fallback, explicitly annotate type or property with @Contextual
Serializer has not been found for type 'BigInteger'. To use context serializer as fallback, explicitly annotate type or property with @Contextual

我尝试为BigDecimal和BigInteger （ 和typealias es 方便）创建自定义序列化器，但是因为它们使用toDouble()和toLong()它们失去了精度！

typealias BigDecimalJson = @Serializable(with = BigDecimalSerializer::class) BigDecimal

private object BigDecimalSerializer : KSerializer<BigDecimal> {

  override val descriptor = PrimitiveSerialDescriptor("java.math.BigDecimal", PrimitiveKind.DOUBLE)

  override fun deserialize(decoder: Decoder): BigDecimal =
    decoder.decodeDouble().toBigDecimal()

  override fun serialize(encoder: Encoder, value: BigDecimal) =
    encoder.encodeDouble(value.toDouble())
}

typealias BigIntegerJson = @Serializable(with = BigIntegerSerializer::class) BigInteger

private object BigIntegerSerializer : KSerializer<BigInteger> {

  override val descriptor = PrimitiveSerialDescriptor("java.math.BigInteger", PrimitiveKind.LONG)

  override fun deserialize(decoder: Decoder): BigInteger =
    decoder.decodeLong().toBigInteger()

  override fun serialize(encoder: Encoder, value: BigInteger) =
    encoder.encodeLong(value.toLong())
}

当我对示例实例进行编码和解码时，返回了不同的结果。

import java.math.*
import kotlinx.serialization.*
import kotlinx.serialization.json.Json

@Serializable
data class FooNumbers(
  val decimal: BigDecimalJson,
  val integer: BigIntegerJson,
)

fun main() {
  val fooDecimal = BigDecimal("0.1234567890123456789012345678901234567890")
  val fooInteger = BigInteger("9876543210987654321098765432109876543210")

  val fooNumbers = FooNumbers(fooDecimal, fooInteger)
  println("$fooNumbers")

  val encodedNumbers = Json.encodeToString(fooNumbers)
  println(encodedNumbers)

  val decodedFooNumbers = Json.decodeFromString<FooNumbers>(encodedNumbers)
  println("$decodedFooNumbers")

  require(decodedFooNumbers == fooNumbers)
}

require(...)失败：

FooNumbers(decimal=0.1234567890123456789012345678901234567890, integer=9876543210987654321098765432109876543210)
{"decimal":0.12345678901234568,"integer":1086983617567424234}
FooNumbers(decimal=0.12345678901234568, integer=1086983617567424234)
Exception in thread "main" java.lang.IllegalArgumentException: Failed requirement.
    at MainKt.main(asd.kt:32)
    at MainKt.main(asd.kt)

Answer 1

在 2023 年 1 月 26 日发布的Kotlinx Serialization 1.5.0-RC中可以对原始编码 JSON 进行编码，并且是实验性的。 这在早期版本中是不可能的。

tl:dr: 跳到此答案底部的“完整示例”

使用JsonDecoder解码

请注意，只有编码需要解决方法 - 只要使用JsonDecoder ，解码BigDecimal和BigInteger就可以直接工作！

private object BigDecimalSerializer : KSerializer<BigDecimal> {

  // ...

  override fun deserialize(decoder: Decoder): BigDecimal =
    when (decoder) {
      // must use decodeJsonElement() to get the value, and then convert it to a BigDecimal
      is JsonDecoder -> decoder.decodeJsonElement().jsonPrimitive.content.toBigDecimal()
      else -> decoder.decodeString().toBigDecimal()
    }
}

使用JsonUnquotedLiteral编码

要编码，必须在编码 JSON 时使用新的JsonUnquotedLiteral() function。

private object BigDecimalSerializer : KSerializer<BigDecimal> {

  // ...

  override fun serialize(encoder: Encoder, value: BigDecimal) =
    when (encoder) {
      // use JsonUnquotedLiteral() to encode the BigDecimal literally
      is JsonEncoder -> encoder.encodeJsonElement(JsonUnquotedLiteral(value.toPlainString()))
      else -> encoder.encodeString(value.toPlainString())
    }
}

使用类型别名的全局配置

Kotlinx Serialization 使用typealias来定义全局可用的序列化策略。 让我们对BigDecimal做同样的事情

typealias BigDecimalJson = @Serializable(with = BigDecimalSerializer::class) BigDecimal

用法示例

创建序列化程序后，可以在typealias中使用FooNumber来自动使用KSerializers 。

@Serializable
data class FooNumbers(
  val decimal: BigDecimalJson,
  val integer: BigIntegerJson,
)

实际的主要 function 没有改变——它和以前一样。

fun main() {
  val fooDecimal = BigDecimal("0.1234567890123456789012345678901234567890")
  val fooInteger = BigInteger("9876543210987654321098765432109876543210")

  val fooNumbers = FooNumbers(fooDecimal, fooInteger)
  println("$fooNumbers")

  val encodedNumbers = Json.encodeToString(fooNumbers)
  println(encodedNumbers)

  val decodedFooNumbers = Json.decodeFromString<FooNumbers>(encodedNumbers)
  println("$decodedFooNumbers")

  require(decodedFooNumbers == fooNumbers)
}

现在BigDecimal和BigInteger可以准确地编码和解码，没有精度损失！

FooNumbers(decimal=0.1234567890123456789012345678901234567890, integer=9876543210987654321098765432109876543210)
{"decimal":0.1234567890123456789012345678901234567890,"integer":9876543210987654321098765432109876543210}
FooNumbers(decimal=0.1234567890123456789012345678901234567890, integer=9876543210987654321098765432109876543210)

完整示例

这是完整的代码：

import java.math.*
import kotlinx.serialization.*
import kotlinx.serialization.descriptors.*
import kotlinx.serialization.encoding.*
import kotlinx.serialization.json.*

@Serializable
data class FooNumbers(
  val decimal: BigDecimalJson,
  val integer: BigIntegerJson,
)

fun main() {
  val fooDecimal = BigDecimal("0.1234567890123456789012345678901234567890")
  val fooInteger = BigInteger("9876543210987654321098765432109876543210")

  val fooNumbers = FooNumbers(fooDecimal, fooInteger)
  println("$fooNumbers")

  val encodedNumbers = Json.encodeToString(fooNumbers)
  println(encodedNumbers)

  val decodedFooNumbers = Json.decodeFromString<FooNumbers>(encodedNumbers)
  println("$decodedFooNumbers")

  require(decodedFooNumbers == fooNumbers)
}

typealias BigDecimalJson = @Serializable(with = BigDecimalSerializer::class) BigDecimal

@OptIn(ExperimentalSerializationApi::class)
private object BigDecimalSerializer : KSerializer<BigDecimal> {

  override val descriptor = PrimitiveSerialDescriptor("java.math.BigDecimal", PrimitiveKind.DOUBLE)

  /**
   * If decoding JSON uses [JsonDecoder.decodeJsonElement] to get the raw content,
   * otherwise decodes using [Decoder.decodeString].
   */
  override fun deserialize(decoder: Decoder): BigDecimal =
    when (decoder) {
      is JsonDecoder -> decoder.decodeJsonElement().jsonPrimitive.content.toBigDecimal()
      else           -> decoder.decodeString().toBigDecimal()
    }

  /**
   * If encoding JSON uses [JsonUnquotedLiteral] to encode the exact [BigDecimal] value.
   *
   * Otherwise, [value] is encoded using encodes using [Encoder.encodeString].
   */
  override fun serialize(encoder: Encoder, value: BigDecimal) =
    when (encoder) {
      is JsonEncoder -> encoder.encodeJsonElement(JsonUnquotedLiteral(value.toPlainString()))
      else           -> encoder.encodeString(value.toPlainString())
    }
}

typealias BigIntegerJson = @Serializable(with = BigIntegerSerializer::class) BigInteger

@OptIn(ExperimentalSerializationApi::class)
private object BigIntegerSerializer : KSerializer<BigInteger> {

  override val descriptor = PrimitiveSerialDescriptor("java.math.BigInteger", PrimitiveKind.LONG)

  /**
   * If decoding JSON uses [JsonDecoder.decodeJsonElement] to get the raw content,
   * otherwise decodes using [Decoder.decodeString].
   */
  override fun deserialize(decoder: Decoder): BigInteger =
    when (decoder) {
      is JsonDecoder -> decoder.decodeJsonElement().jsonPrimitive.content.toBigInteger()
      else           -> decoder.decodeString().toBigInteger()
    }

  /**
   * If encoding JSON uses [JsonUnquotedLiteral] to encode the exact [BigInteger] value.
   *
   * Otherwise, [value] is encoded using encodes using [Encoder.encodeString].
   */
  override fun serialize(encoder: Encoder, value: BigInteger) =
    when (encoder) {
      is JsonEncoder -> encoder.encodeJsonElement(JsonUnquotedLiteral(value.toString()))
      else           -> encoder.encodeString(value.toString())
    }
}