Kotlin Metadata, Jackson and Proguard

If like me you are having issues deserializing json with Jackson into Kotlin Data classes, then this post is for you!

FasterXml provides a jackson-kotlin-module that supports deserializing kotlin classes and data classes, which is great!

But if you are using proguard, you may still run into issues.

After banging my head against a wall all day, and a lot of trial and error, I discovered the problem: kotlin.Metadata

What is kotlin.Metadata?

As explained in this stackoverflow question, kotlin.Metadata is an annotation that contains extra information about a class that is not present in JVM signatures.

It makes sense that Jackson would need this information in order to deserialize data classes!

What’s the fix?

Simply add the following rule to your proguard file to prevent proguard stripping the kotlin.Metadata annotation:

-keep class kotlin.Metadata { *; }

Simple!

Bonus

I also discovered I needed to add a rule to keep synthetic methods on kotlin classes I wanted to use for deserialization. Maybe it will help you too!

-keepclassmembers public class com.mypackage.** {
    public synthetic <methods>;
}

If you liked this article make sure to 😍 it, and follow me on twitter.