How to Deserialize Kotlin Enum While Ignoring Unknown Values Using kotlinx.serialization

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Deserializing Kotlin Enums While Ignoring Unknown Values

When working with JSON data in Kotlin, it’s common to encounter scenarios where you might want to deserialize enums while gracefully handling unknown values. Imagine you have an OperatingMode enum consisting of specific modes—Off, On, and Auto. However, the incoming JSON can contain values that are not represented in this enum, and you want to ignore these unknown values rather than throwing errors during deserialization.

The Problem

Let's take a look at the challenge. Here is the enum in question:

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You might receive JSON that looks like this:

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Or, in a list format:

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In both cases, the modes that aren't defined in the enum should be simply ignored, resulting in:

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or

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To achieve this, one approach is to write custom serializers. However, this often results in verbose and repetitive code.

Initial Solution

Initially, you might write something like this:

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And while this could work, it leads to significant code duplication, especially if you have several enums that require the same deserialization behavior.

A Refactored Approach: Reducing Code Duplication

Luckily, there is a more efficient way! Below, I've detailed a solution that focuses on reducing that redundancy by creating reusable base serializers.

Step 1: Create a Generic Safe Serializer

You can create a generic serializer that can be reused for any enum type:

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Step 2: Create a Non-Null List Serializer

To handle lists of these enums, you can also create a non-null serializer for lists:

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Step 3: Apply to Your Enum

Now, for each enum, applying it is straightforward. You just need to declare:

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This encapsulates the deserialization behavior in your enum, ensuring that any deserialization will now automatically ignore unknown values.

Limitations and Considerations

It is important to note that this approach works well for enums and basic types. If you apply SafeSerializer to complex types, it may throw exceptions that could leave the decoder in an inconsistent state. As a best practice, always ensure that your serializers are tested to handle the types you expect.

Conclusion

With the above method, you can enjoy a cleaner, more structured deserialization process while effectively ignoring unknown values. Happy coding!