Kotlinx Serialization: Handling “Polymorphic Serializer was not found…” Error
Understanding the Error
The error message “Polymorphic serializer was not found for missing class discriminator (‘null’)” arises when using kotlinx.serialization for serializing and deserializing polymorphic data. This occurs when the serializer encounters an object without a valid discriminator value, which is needed to identify the correct concrete type.
Polymorphism and Discriminators
* **Polymorphism** refers to the ability of an object to take on multiple forms or types. In Kotlin, this is achieved using interfaces and abstract classes.
* **Discriminators** are essential for serialization libraries to handle polymorphism. They act as tags that help identify the concrete type of an object during deserialization.
Causes of the Error
The “Polymorphic Serializer was not found…” error can stem from several issues:
* **Missing Discriminator:** The most common cause is the absence of a discriminator value in the serialized data.
* **Incorrect Discriminator Value:** The discriminator might exist but have an invalid or unexpected value.
* **Missing Serializer:** The serializer for a particular concrete type might be missing or not registered.
* **Incorrect Type Hierarchy:** The type hierarchy might be defined incorrectly, leading to ambiguity in type identification.
Resolving the Error
Here’s a breakdown of how to fix this error:
1. Ensuring Discriminator Presence
* **Explicitly Provide Discriminators:** When serializing objects, always include the necessary discriminator value:
“`kotlin
import kotlinx.serialization.Serializable
import kotlinx.serialization.json.Json
@Serializable
data class Animal(val type: String)
@Serializable
data class Dog(val breed: String) : Animal(“Dog”)
@Serializable
data class Cat(val color: String) : Animal(“Cat”)
fun main() {
val dog = Dog(“Golden Retriever”)
val json = Json.encodeToString(dog)
println(json) // {“type”:”Dog”,”breed”:”Golden Retriever”}
val cat = Cat(“Black”)
val json2 = Json.encodeToString(cat)
println(json2) // {“type”:”Cat”,”color”:”Black”}
}
“`
* **Custom Serialization:** For more complex scenarios, you can define custom serializers that handle discriminator generation and parsing.
2. Validating Discriminator Values
* **Enum Discriminators:** When dealing with a limited set of types, using enums for discriminators can ensure correctness.
* **Type Checking:** Implement logic to verify the discriminator value before deserialization, throwing exceptions if necessary.
3. Registering Serializers
* **Polymorphic Serialization:** Use `@Serializable` for the base type and specify `@SerialName` for each subtype, along with the corresponding discriminator values.
* **Custom Serializers:** You can create dedicated serializers for complex type relationships or to implement specific logic.
4. Correct Type Hierarchy
* **Inheritance:** Ensure a clear and well-defined inheritance hierarchy for your polymorphic data.
* **Interface Definitions:** Use interfaces effectively to define common properties and methods.
Example
“`kotlin
import kotlinx.serialization.Serializable
import kotlinx.serialization.json.Json
@Serializable
sealed interface Animal {
val name: String
}
@Serializable
@SerialName(“Dog”)
data class Dog(override val name: String, val breed: String) : Animal
@Serializable
@SerialName(“Cat”)
data class Cat(override val name: String, val color: String) : Animal
fun main() {
val dog = Dog(“Buddy”, “Golden Retriever”)
val json = Json.encodeToString(dog)
println(json) // {“type”:”Dog”,”name”:”Buddy”,”breed”:”Golden Retriever”}
val decodedDog: Animal = Json.decodeFromString(json)
println(decodedDog) // Dog(name=Buddy, breed=Golden Retriever)
}
“`
Table Comparison
| Feature | Kotlinx Serialization |
|—|—|
| Polymorphism | Supported with discriminators |
| Discriminator Handling | Auto-detection (optional) |
| Serializer Registration | Automatic for sealed classes |
| Serialization Format | JSON, Protobuf, CBOR, and more |
Conclusion
Understanding how to correctly handle polymorphism and discriminators within kotlinx.serialization is crucial for successful serialization and deserialization of polymorphic data. By carefully defining your data structures, registering serializers, and validating discriminator values, you can avoid the “Polymorphic Serializer was not found…” error and work with polymorphic data seamlessly.