Handling Json in Functional Way in Kotlin

Talk at Jax London 2025

Showing the motivations and the features of Kondor-Json library. And a preview of new version features.

Transcript

1. Uberto Barbini Kondor-Json A Functional Kotlin library to handle Json precisely
2. The Usual Problems
3. The Usual Problems • D D DDDD D DD D DDD DDDD D DD D DD D D D D D D D DD D D DD D DD DDDD DD D DDDDD DD
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5. The Usual Problems • D D DDDD D DD D DDD DDDD D DD D DD D D D D D D D DD D D DD D DD DDDD DD D DDDDD DD • L LLL LL L L L L L L LL LL L LLLL LL L L LLLL LL L L L L LLLL L LL LLLL L L LL LL LL LLLL L L L LLL • U UUUU U UUsU UUU U UU U U U UUUU U U U U U UU UU U UU U U U UU U U U UUUUUU UU
6. The Usual Problems • D D DDDD D DD D DDD DDDD D DD D DD D D D D D D D DD D D DD D DD DDDD DD D DDDDD DD • L LLL LL L L L L L L LL LL L LLLL LL L L LLLL LL L L L L LLLL L LL LLLL L L LL LL LL LLLL L L L LLL • U UUUU U UUsU UUU U UU U U U UUUU U U U U U UU UU U UU U U U UU U U U UUUUUU UU • A A A A A A A A A A A A AA A A AA A A AAAAA A A AAA A AAA A A A A AAAA A AAA A AA A A AAAAA
7. The Usual Problems • D D DDDD D DD D DDD DDDD D DD D DD D D D D D D D DD D D DD D DD DDDD DD D DDDDD DD • L LLL LL L L L L L L LL LL L LLLL LL L L LLLL LL L L L L LLLL L LL LLLL L L LL LL LL LLLL L L L LLL • U UUUU U UUsU UUU U UU U U U UUUU U U U U U UU UU U UU U U U UU U U U UUUUUU UU • A A A A A A A A A A A A AA A A AA A A AAAAA A A AAA A AAA A A A A AAAA A AAA A AA A A AAAAA • D D Di D DD D D D DDDDD D DD DDD DDD D DD D i D D D D DDDD D DD D D D DD D D D DDDD D D
8. The Usual Problems • D D DDDD D DD D DDD DDDD D DD D DD D D D D D D D DD D D DD D DD DDDD DD D DDDDD DD • L LLL LL L L L L L L LL LL L LLLL LL L L LLLL LL L L L L LLLL L LL LLLL L L LL LL LL LLLL L L L LLL • U UUUU U UUsU UUU U UU U U U UUUU U U U U U UU UU U UU U U U UU U U U UUUUUU UU • A A A A A A A A A A A A AA A A AA A A AAAAA A A AAA A AAA A A A A AAAA A AAA A AA A A AAAAA • D D Di D DD D D D DDDDD D DD DDD DDD D DD D i D D D D DDDD D DD D D D DD D D D DDDD D D • D D D D D D D D DD DD DDDD DD D DDDDDD D D D D D D DDDD D D DDDDD DDD D D D D DD D D DD DD D D D D
9. The Usual Problems • D D DDDD D DD D DDD DDDD D DD D DD D D D D D D D DD D D DD D DD DDDD DD D DDDDD DD • L LLL LL L L L L L L LL LL L LLLL LL L L LLLL LL L L L L LLLL L LL LLLL L L LL LL LL LLLL L L L LLL • U UUUU U UUsU UUU U UU U U U UUUU U U U U U UU UU U UU U U U UU U U U UUUUUU UU • A A A A A A A A A A A A AA A A AA A A AAAAA A A AAA A AAA A A A A AAAA A AAA A AA A A AAAAA • D D Di D DD D D D DDDDD D DD DDD DDD D DD D i D D D D DDDD D DD D D D DD D D D DDDD D D • D D D D D D D D DD DD DDDD DD D DDDDDD D D D D D D DDDD D D DDDDD DDD D D D D DD D D DD DD D D D D • M MMM MM M MM M MM M M M M MM M MM M M M MMMM M M MM MM MM MMMMMM MM MMMMMMM MMMM
10. The Usual Problems • Domain Objects don’t map exactly on the Json protocol required • Lots of Domain Tiny Types that shouldn’t appear in Json (eg. Ids, Age, Money) • Using re fl ection makes easy to break things when refactoring • Annotations and custom serializers work but they taint the domain • Di ff erent collaborators want di ff erent Json for the same objects • Data Transfer Objects brings a lot of duplication and are a source of bugs • Most of Json libraries need a single shared instance (Singleton) • Solutions based on code generations are hard to maintain
11. Fluent Builders Solution - more code + fully customisable
12. Kotlin DSL Power
13. Functional Design Pure Functions and Immutable Objects interface ToJson<T> { fun toJson(value: T): String } interface FromJson<T> { fun fromJson(json: String): JsonOutcome<T> } interface JsonConverter<T, JN : JsonNode> : Profunctor<T, T>, ToJson<T>, FromJson<T> { val _nodeType: NodeKind<JN> val jsonStyle: JsonStyle get() = JsonStyle.singleLine fun schema(): JsonNodeObject = valueSchema(_nodeType) ... }
14. Talking About Functional Kotlin…
15. Handling Errors Without Exceptions
16. What Else We Found Sealed Classes
17. What Else We Found Flattened Fields
18. What Else We Found Maps to Json Objects
19. What Else We Found Json Schema generation
20. What Else We Found Nested Json
21. Why Open Source it? • Main drive: be able to use it in a new job • Contributions from many people, each with their own interesting issue • More eyes for issues and security • Not a huge e ff ort but still it takes a few hours a month • LLMs are helping a lot with docs and other small issues
22. Version 4.0 • 2x Performance Improvement (skipping JsonNode allocation) • New Auto converters with “TestTime” re fl ection
23. Demo and QA