Production use of Kotlin



I’d like to ask you for the list of projects in production using Kotlin language. We’re in POC phase deciding whether to use it or not.
Our project has quite simple requirements:

  • OOP
  • JSON serialization
  • Math calculations
  • Kotlin compiled to javascript
  • large codebase (several hundreds thousands lines of code)
  • unit testing

From my tests it seems it’s feasible, however is Kotlin -> Javascript for this kind of project production ready ?

Thank you in advance,


Hello Marek,

I’m the sole developer in a startup creating a web platform for music. Around two to three months ago I invited Kotlin to help write code faster, more effectively. Overall, Kotlin has been a benefit above all else. It take the verbosity of Java and minifies it, functional concepts can be expressed more easily (if you are not using Java 8 Lambdas) and many features of the language make programming a joy.

The Good

- Json Conversion: No problem because you can use a Java library. I used Jackson, but use whatever suits your fancy.

  • OOP: A breeze and joy. No longer having to write equals, hashcode, toString, etc because the data annotation does it for you automagically. Traits are BEAUTIFUL and delegates are a strong capability which I admittedly don’t use enough in my code. Solely the use of multiple inheritance is one that eludes me.

However, I find the single constructor restriction too restrictive. Be aware of that and read this StackOverflow post on how to circumvent this issue.

  • Unit Testing: Doesn’t get in your way, but doesn’t add much. Kotlin has a small unit testing library which is nice because you can write nifty code like:

failsWitch(javaClass<IOException>()) {   // Your failure code here }

but that's about it. It doesn't have it's own mocking library, but Kotlin can very well use an existing one. I use Mockito without problems, and the code ends up nicer than in Java (except that static imports are a pain in Kotlin and something I avoid)

Large Codebase:  Not sure, but positive outlook. Our project is at 10k loc and growing, but not at the scale you’re using it :slight_smile:

The Bad

- Documentation. It's not great. Bad, in fact. For example, the only reference to the splat operator (*) is found in a very old blog post from Kotlin. You need the splat operator when you want to interface with Java vararg... functions

  • The IDE isn’t on par with Java. For example, to write an injectable constructor in Kotlin, do:
class Foo [Inject] (val toInject : Bar)

Fine enough, but if I, say, optimize imports, then IDEA actually removes the import! Not a deal breaker, but definitely a nuisance. For some reason, Kotlin also takes longer to update the code. What I mean is, when I import a class in Java, the change is reflected almost immediately, all red references going away. In Kotlin, this takes up to two seconds. This is not a negligible amount of time.

  • It doesn’t always reduce boilerplate. Instead of Foo.class, you write javaClass<Foo>(). For the most part, however, this is a non-issue. This is honestly the only place I encountered where Kotlin is more verbose than Java.

  • Barely anybody knows how to program in Kotlin. I have to tell most of my potential hires about Kotlin. I usually present them with the choice of continuing to write Java-code since this project is about 50/50 Java and Kotlin. This will change in favor of Kotlin over time, but I’d assume you have to expect some resistance here.

  • Immature plugins. I’ve had more trouble with the maven-kotlin compiler plugin than I’d like to admit. It wasn’t possible for me to use an external jar and Kotlin at the same time. The solution was to cherry-pick the relevant files and discard the .jar afterwards. If you only use maven and no other .jars, great. Otherwise, meh.

  • Sometimes wonky syntax. For example, when I wish to restrict access to a certain method:

public Response getAudioFile(@RestrictedTo({ Authority.Artist, Authority.Approved }) Account account)

in Kotlin:

(default public) fun getAudioFile(RestrictedTo(Authority.Artist, Authority.Approved) account : Account)

or when a Set is necessary

fun getAudioFile(RestrictedTo(setOf(Authority.Artist, Authority.Approved)) account : Account)

Maybe it's just me, but I find it a little counterintuitive.

  • The Java to Kotlin Converter isn’t great. Yes it converts your code, but in doing so makes it horrendously ugly. You will undoubtedly spend time refactoring and beautifying the code. A lot of times it creates noisy code, a lot of which can be deleted.


I heartily recommend Kotlin and will continue to use it in future projects.


Thanks for the kind words!

Most of the issues you point out will be fixed in not so distant future.


Hi Christian.

Thank you for your answer :slight_smile:. I appreciate it a lot.

Have a nice day,


One of the things that I would be really interested in is the what the module system is going to be like. Module system as in "which classes can access which other classes in a defined way" and not in "this is Maven 3.0". This is one of the things that is keeping me from spending more time with Kotlin and it prohibits production use. Java visibilities have always felt way to limited to express larger and more sophisticated concepts...

What’s the current situation in this area?


The curresnt vision is more or less in line with how modules work in IntelliJ IDEA projecrs: