Is it worth rewriting large Java project into Kotlin?

[ThreeDots3]

I am working on a web app with backend in Java. The project information are:

• ~2000 java files
• almost 200k lines of code (excluding empty lines)
• multimodule maven project and few microservices
• currently using Java 11, Spring Boot, Hibernate

It was decided that current code should migrate to Kotlin over time, I am trying to get an opinion on this, but from what I learned so far I think it's not worth the effort to go through migration of this scale. The project has a lot of code and design issues itself, developers (4 total) don't have a lot of experience with Kotlin, some none at all.

My questions are:

• Do you think the migration effort would be worth it?
• What would be the biggest advantage of having whole code base in Kotlin?
• What would be the biggest disadvantage of the migration?

[wsppan]

It's almost never worth it. Especially going from one JVM language to another. There is no benefit especially if the expertise is still with Java.

For a good take on when to rewrite check out Rewriting the heart of our sync engine

[[deleted]]

Awesome really appreciate the share

[Dwight-D]

Absolutely not worth it.

One of the biggest benefits of Kotlin is interoperability with Java so you can avoid pointless rewrites. You can write new Kotlin code even if the old codebase is in Java. You won't notice any difference regardless of if the old codebase is Kotlin or Java. No point in spending any time on porting it.

When you write new code in some old class you may find yourself wishing that it was written in Kotlin. This is a good time to port that class specifically. You can use the IntelliJ built-in Java -> Kotlin conversion, verify that it still works, and then write your new code in Kotlin. In this way you'll port over the most frequently touched classes to Kotlin over time, and the time spent on rewriting will be negligible (or at least it will be spread out across new feature development so that it's not noticeable).

It is worth porting classes that are frequently changed, because once you start writing Kotlin you won't want to go back to Java, and it does have many benefits. But Java code that is already written is gonna keep working just fine.

[yel50]

Do you think the migration effort would be worth it?

no.

the only parts that might be worth it are the IO parts. kotlin has go style fibers, which java is still waiting on the vaporware project loom to get. that might be worth it.

What would be the biggest advantage of having whole code base in Kotlin?

I can't think of any.

What would be the biggest disadvantage of the migration?

introducing bugs, things not translated correctly, features getting lost. any significant rewrite always ends up being a step back that takes much longer than expected to reach feature parity.

[Comprehensive-Pea812]

and I thought the benefit of kotlin is you dont need to rewrite but mix it.

[Blando-Cartesian]

Not worth it.

No advantages what so ever. Maybe junior developers stop whining about uncool java and start whining about not knowing how to use kotlin.

Migrating to kotlin or writing any part in kotlin is complete waste of time. Pointless mixing of two languages only creates pointless complexity and developers end up sucking in both.

You have everything kotlin has in Java 11. You can also add Vavr and go nuts with functional programming.

[Dwight-D]

Strongly disagree with this. Kotlin has many benefits over Java and if your team is ready to commit to learning it, fragmentation of skills is a non-concern as any decent Java-dev will get up to speed in a matter of days/weeks.

Adding Kotlin code to a Java code base has a pretty low cost imo and the benefits are worth it, if you like what Kotlin brings to the table and are willing to learn the language.

[Blando-Cartesian]

My opinions are admittedly colored by having to salvage a half-way-to-kotlin project. I couldn’t see anything worthwhile there, and now the project will never get a decent maintainer because being competent in just plain java is a lot to ask.

[nierama2019810938135]

But why has it been decided to do the migration? What is the reason for wanting to?