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Multiplatform polymorphic serialization

Introduction

Polymorphic serialization is usually a very complicated and dangerous feature due to the amount of reflection it brings and security concerns you should address in your application (like "what if you accidentally load or deserialize a class that is not allowed to be in this part of the program").

To overcome these drawbacks, we've focused our design on a serializers registration. It will eliminate need in cross-platform Class.forName analog and will help avoid security problems.

So, to be able to serialize class hierarchies and restore them using a type's fully-qualified name, you should perform these steps:

  1. Register all subclasses that may appear in serialization and deserialization in some SerialModule.
  2. Pass that serial module to a format instance.
  3. Mark some properties or classes as @Polymorphic.

Step 3 is required if you want to polymorphically serialize an open class. If class is abstract or interface, @Polymorphic annotation inferred automatically (see more in section Differences for interfaces, abstract and open classes). Moreover, if the class is sealed, we do not need to register all subclasses manually – since they are known at compile time, compiler plugin can enumerate them automatically (see more in section Sealed classes).

Table of contents

Basic case

Let's break down a basic case with a simple class hierarchy:

interface Message

@Serializable
data class StringMessage(val message: String): Message

@Serializable
data class IntMessage(val number: Int): Message

To be able to serialize and deserialize both StringMessage and IntMessage, we need the following module:

val messageModule = SerializersModule { // 1
    polymorphic(Message::class) { // 2
        StringMessage::class with StringMessage.serializer() // 3
        IntMessage::class with IntMessage.serializer() // 4
    }
}

Line 1) Creates a DSL builder. Line 2) indicates that the nested block describes subclasses of Message (you can have as many polymorphic blocks as you'd like, for different classes). It means that these subclasses can only be serialized/deserialized when the framework encounters Message class. In other words, this module will work with @Serializable class MessageWrapper(val m: Message) definition and will NOT work with @Serializable class MessageWrapper(val m: Any). This design decision was made for security and type-safety – it encourages you to use more specific types instead of Any.

Lines 3) and 4) register corresponding actual serializers. Again, only classes StringMessage and IntMessage are allowed in the stream – even if you have @Serializable class MyInternalSecretMessage: Message, which should not be exposed to external clients, you shouldn't worry about it. Kotlinx.serialization throws an exception on an attempt to serialize or deserialize a class polymorphically if it is not registered.

The only thing left to do is to create a format instance with messageModule:

val json = Json(context = messageModule)

After that, you can use the json object as usual:

@Serializable
data class MessageWrapper(val m: Message)

json.stringify(MessageWrapper.serializer(), MessageWrapper(StringMessage("string")))
// {"m":{"type":"package.StringMessage","message":"string"}}

json.stringify(MessageWrapper.serializer(), MessageWrapper(IntMessage(121)))
// {"m":{"type":"package.IntMessage","number":121}}

Such an approach works on JVM, JS, and Native without reflection (only with KClass comparison and KClass.isInstance calls)!

Pro tip: to use Message without a wrapper, you can pass PolymorphicSerializer(Message::class) to parse/stringify.

A bit of customizing

By default, encoded type name is equal to class' fully-qualified name. To change that, you can annotate the class with @SerialName annotation:

// If we have...
@Serializable
@SerialName("msg_number")
data class IntMessage(val number: Int): Message

// then...

json.stringify(MessageWrapper.serializer(), MessageWrapper(IntMessage(121)))
// {"m":{"type":"msg_number","number":121}}

JSON with its JsonConfiguration.Stable and JsonConfiguration.Default offers you to store the type name inside the object itself with the key type. You can override key name by creating your own configuration: JsonConfiguration(classDiscriminator = "class") or by copying an existing one: JsonConfiguration.Stable.copy(classDiscriminator = "class").

There is also a possibility to change type name storage location to the first element of wrapping array, i.e. to form [className, object]:

val json = Json(
    configuration = JsonConfiguration(useArrayPolymorphism = true),
    context = messageModule
)

json.stringify(MessageWrapper.serializer(), MessageWrapper(IntMessage(121)))
// {"m":["msg_number",{"number":121}]}

Note: this form is default and can't be changed for formats that do not support polymorphism natively, e.g., Protobuf.

Differences for interfaces, abstract and open classes

As you know, interfaces and abstract classes can't be instantiated. It also means that they can't be deserialized, and therefore, they're by default polymorphic. So if we have

interface Message

or

@Serializable
abstract class Message

These declarations are equivalent:

@Serializable
class MessageWrapper(val message: Message)

@Serializable
class MessageWrapper(@Polymorphic val message: Message)

Open classes have a state, can be instantiated and have a usual serializer. So, for

@Serializable
open class Message

This declaration will use Message.serializer():

@Serializable
class MessageWrapper(val message: Message)

And this will use PolymorphicSerializer:

@Serializable
class MessageWrapper(@Polymorphic val message: Message)

You can also make Message class polymorphic by default by annotating the class itself:

@Serializable
@Polymorphic
open class Message

Sealed classes

Sealed classes in Kotlin use the same inheritance mechanism as abstract and open ones, so kotlinx.serialization works with them in the same way, too.

However, they have one big difference: compiler knows all subclasses of sealed class anyway, therefore, the serialization plugin knows them too, and it is possible to serialize subclasses without a user's intervention correctly. Practically, it means that you do not need @Polymorphic and SerialModule:

@Serializable
sealed class SimpleSealed {
    @Serializable
    public data class SubSealedA(val s: String) : SimpleSealed()

    @Serializable
    public data class SubSealedB(val i: Int) : SimpleSealed()
}

// will perform correct polymorphic serialization and deserialization:
Json.stringify(SimpleSealed.serializer(), SubSealedA("foo"))
// output will be
// {"type":"package.SimpleSealed.SubSealedA", "s":"foo"}

You can use all polymorphism-related settings (see section A bit of customizing) and combine sealed hierarchies with abstract ones (see section Complex hierarchies with several base classes and documentation for SealedClassSerializer).

Note that if you use @Polymorphic on a property with a type of sealed class, it will be serialized using a regular polymorphism mechanism (which requires SerialModule). This was an approach in previous versions of the library (see the next section).

Sealed classes: before 0.14.0

If you, for some reason, use an older version of the library, to serialize sealed hierarchies polymorphically, you have to perform the same steps as for any other class hierarchy: make a SerialModule, register all subclasses, etc., etc. Moreover, you should explicitly mark every usage of sealed class in @Serializable classes as @Polymorphic:

// @Serializable is not needed here
sealed class Message {
    // subclasses...
}

val messageModule = SerializersModule {
    polymorphic<Message>() {
        // subclasses...
    }
}

@Serializable
class MessageWrapper(@Polymorphic val m: Message)
// @Polymorphic is required!

So, we can say that polymorphic serialization (with explicit SerialModule) of sealed classes requires explicit opt-in on use site. To migrate to newer versions, you'll have to remove all @Polymorphic annotations from vals.

Complex hierarchies with several base classes

If you want to register subclasses for multiple base classes, e.g., in a situation like that:

interface Message

@Serializable
abstract class TimestampedMessage : Message {
    abstract val timestamp: Int
}

@Serializable
class Wrapper(
    val request: Message,
    val response: TimestampedMessage
)

You can use polymorphic overload which accepts several base classes:

val messageModule = SerializersModule {
    polymorphic(Message::class, TimestampedMessage::class) {
        // subclasses...
    }
}

You can even add Any::class to this list. By doing this, you'll make possible deserialization of your classes to @Serializable class Wrapper(@Polymorphic val any: Any). Use this feature with caution.

If the base class itself needs serializer (in case it is open), you can use polymorphic overload with KSerializer<BaseClass> in arguments or just register serializer as usual inside DSL block.

A word for multi-project applications and library developers

When the application grows in size, it may become inconvenient to store all class hierarchies in one serial module. And you don't have to: serial modules are composable. They even have plus operator:

val json = Json(context = messageModule + anotherModule)

Or you can use include in the SerializersModule {} DSL.

If you're writing a library or shared module with an abstract class and some implementations of it, you can export your own MyLibrarySerialModule for your clients to use so that a client can combine your module with their modules.

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