没有@Serializable 的数据类的自定义序列化程序
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【中文标题】没有@Serializable 的数据类的自定义序列化程序【英文标题】:Custom serializer for data class without @Serializable 【发布时间】:2021-03-24 02:29:04 【问题描述】:我正在尝试将 JSON 文件反序列化为使用 kotlinx.serialization
无法控制的 Kotlin 数据类。
这个类看起来是这样的:
public data class Lesson(
val uid: String,
val start: Instant,
val end: Instant,
val module: String,
val lecturers: List<String>,
val room: String?,
val type: String?,
val note: String?
)
我尝试解析的 JSON 如下所示:
"lessons": [
"uid": "sked.de956040",
"start": "2020-11-02T13:30:00Z",
"end": "2020-11-02T16:45:00Z",
"module": "IT2101-Labor SWE I: Gruppe 1 + 2",
"lecturers": [
"Kretzmer"
],
"room": "-",
"type": "La",
"note": "Prüfung Online"
]
这是通过以下方式尝试的:
@Serializable
data class ExpectedLessons(
val lessons: List<Lesson>
)
val decoded = Json.decodeFromString<ExpectedLessons>(text)
【问题讨论】:
【参考方案1】:由于Lesson
类无法修改,因此无法添加@Serializable
注释以使(反)序列化工作。
因此,您可以创建两个自定义序列化程序以使其工作。
@OptIn(ExperimentalSerializationApi::class)
@Serializer(forClass = Lesson::class)
object LessonSerializer : KSerializer<Lesson>
override val descriptor: SerialDescriptor = buildClassSerialDescriptor("Lesson")
element<String>("uid")
element<String>("start")
element<String>("end")
element<String>("module")
element<List<String>>("lecturers")
element<String?>("room", isOptional = true)
element<String?>("type", isOptional = true)
element<String?>("note", isOptional = true)
override fun serialize(encoder: Encoder, value: Lesson)
encoder.encodeStructure(descriptor)
encodeStringElement(descriptor, 0, value.uid)
encodeSerializableElement(descriptor, 1, InstantSerializer, value.start)
encodeSerializableElement(descriptor, 2, InstantSerializer, value.end)
encodeStringElement(descriptor, 3, value.module)
encodeSerializableElement(descriptor, 4, ListSerializer(String.serializer()), value.lecturers)
encodeNullableSerializableElement(descriptor, 5, String.serializer(), value.room)
encodeNullableSerializableElement(descriptor, 6, String.serializer(), value.type)
encodeNullableSerializableElement(descriptor, 7, String.serializer(), value.note)
override fun deserialize(decoder: Decoder): Lesson
return decoder.decodeStructure(descriptor)
var uid: String? = null
var start: Instant? = null
var end: Instant? = null
var module: String? = null
var lecturers: List<String> = emptyList()
var room: String? = null
var type: String? = null
var note: String? = null
loop@ while (true)
when (val index = decodeElementIndex(descriptor))
DECODE_DONE -> break@loop
0 -> uid = decodeStringElement(descriptor, 0)
1 -> start = decodeSerializableElement(descriptor, 1, InstantSerializer)
2 -> end = decodeSerializableElement(descriptor, 2, InstantSerializer)
3 -> module = decodeStringElement(descriptor, 3)
4 -> lecturers = decodeSerializableElement(descriptor, 4, ListSerializer(String.serializer()))
5 -> room = decodeNullableSerializableElement(descriptor, 5, String.serializer().nullable)
6 -> type = decodeNullableSerializableElement(descriptor, 6, String.serializer().nullable)
7 -> note = decodeNullableSerializableElement(descriptor, 7, String.serializer().nullable)
else -> throw SerializationException("Unexpected index $index")
Lesson(
requireNotNull(uid),
requireNotNull(start),
requireNotNull(end),
requireNotNull(module),
lecturers,
room,
type,
note
)
@OptIn(ExperimentalSerializationApi::class)
@Serializer(forClass = Instant::class)
object InstantSerializer : KSerializer<Instant>
override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("Instant", PrimitiveKind.STRING)
override fun serialize(encoder: Encoder, value: Instant)
encoder.encodeString("$value")
override fun deserialize(decoder: Decoder): Instant
return Instant.parse(decoder.decodeString())
您可以在使用序列化程序之前配置它们,如下所示:
@file:UseSerializers(InstantSerializer::class, LessonSerializer::class)
【讨论】:
完美答案,非常感谢。小提示,为了让它工作,我必须这样做:@Serializable(with = MySerializer::class) data class MyClass
而不是:@file:UseSerializers(MySerializer::class)
不知道为什么。我正在使用kotlinxSerialization = "1.3.1"
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