如何序列化/反序列化Kotlin密封类？ [英] How to serialize/deserialize Kotlin sealed class?

问题描述

我有以下密封的类：

sealed class ViewModel {

  data class Loaded(val value : String) : ViewModel()
  object Loading : ViewModel()

}

我如何序列化/反序列化ViewModel类的实例，比方说JSON格式？

How can I serialize/deserialize instances of the ViewModel class, let's say to/from JSON format?

我尝试使用Genson序列化器/反序列化器库-它可以处理Kotlin数据类，还可以支持多态类型（例如，使用某些元数据指定具体的类型）。

I've tried to use Genson serializer/deserializer library - it can handle Kotlin data classes, it's also possible to support polymorphic types (eg. using some metadata to specify concrete types).

但是，库在Kotlin object 类型上失败，因为这些都是单例，没有公共构造函数。我想我可以编写一个自定义的Genson转换器来处理它，但是也许有更简单的方法可以做到这一点？

However, the library fails on Kotlin object types, as these are singletons without a public constructor. I guess I could write a custom Genson converter to handle it, but maybe there's an easier way to do it?

推荐答案

我结束了实施自定义Converter以及Factory以将其正确插入Genson。

I ended up implementing a custom Converter plus a Factory to properly plug it into Genson.

它使用Genson的元数据约定将对象表示为：

It uses Genson's metadata convention to represent the object as:

{ 
  "@class": "com.example.ViewModel.Loading" 
}

转换器假定设置了 useClassMetadata 标志，因此序列化只需要标记一个空对象即可。对于反序列化，它从元数据中解析类名称，将其加载并获取 objectInstance 。

The converter assumes useClassMetadata flag set, so serialization just needs to mark an empty object. For deserialization, it resolves class name from metadata, loads it and obtains objectInstance.

object KotlinObjectConverter : Converter<Any> {

override fun serialize(objectData: Any, writer: ObjectWriter, ctx: Context) {
    with(writer) {
        // just empty JSON object, class name will be automatically added as metadata
        beginObject()
        endObject()
    }
}

override fun deserialize(reader: ObjectReader, ctx: Context): Any? =
    Class.forName(reader.nextObjectMetadata().metadata("class"))
        .kotlin.objectInstance
        .also { reader.endObject() }
}

为确保此转换器仅应用于实际的对象，我对其进行了注册

To make sure that this converter is applied only to actual objects, I register it using a factory, that tells Genson when to use it and when to fall back to the default implementation.

object KotlinConverterFactory : Factory<Converter<Any>> {

    override fun create(type: Type, genson: Genson): Converter<Any>? =
        if (TypeUtil.getRawClass(type).kotlin.objectInstance != null) KotlinObjectConverter
        else null

}

工厂可用于通过构建器配置Genson：

The factory can be used to configure Genson via builder:

GensonBuilder()
        .withConverterFactory(KotlinConverterFactory)
        .useClassMetadata(true) // required to add metadata during serialization
        // some other properties
        .create()

使用链式转换器功能，代码可能会更好，但是我还没有时间检查它

The code probably could be even nicer with chained converters feature, but I didn't have time to check it out yet.

这篇关于如何序列化/反序列化Kotlin密封类？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！