Learning Spring with Kotlin – Sprint 0
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In short, if you are a conservative institution like a bank and evolving your tech stack at a glacial pace; yes you can still use Java. Everybody else, you probably should be using Kotlin.
My advice to anyone looking to do this is:
1) Do it, it's easy and you end up with better code. You can start simple and mix Java and Kotlin code. I would advice starting with some tests or some new features.
2) Beware of the java idioms and cruft in Spring and do things the Kotlin way. So, DSLs instead of annotations + AOP, co-routines instead of spring flux, immutable data classes instead of java beans, etc.
3) Spring has a lot of stuff and not all of it is great or even good. IMHO the added advantage of e.g. spring-cloud or spring-data is very limited and sometimes it's just very limiting/backwards. I ripped out spring-data on several projects where people naively supposed it was the easiest/best way to interact with Elasticsearch. It turns out it is neither. Spring cloud sounds nice until you realize most of the stuff you need is supported in the amazon driver but yet not exposed in spring-cloud. Also all the code samples on Amazon are for their own driver and not for spring-cloud.
4) Re-assess if you actually want to go anywhere near hibernate. IMHO it made a lot of sense when java beans were hard to write. Kotlin data classes take away most of that pain and there are some nice alternatives. Also, you should be using non blocking IO for talking to your DB and hibernate is still stuck on blocking IO. That in itself is a good reason to not use it on new projects.
Once you get a bit further, you'll find yourself wondering if ktor would work. The answer is probably yes. It's actually pretty good. Spring does a lot of things but ultimately do you really need all of that and the associated complexity?
Scripting is better because Kotlin doesn't have simple dependency management like Grape and compilation speed is way faster than Kotlin's. One of Kotlin's downsides is compilation speed and kts (kotlin script) is even slower. Also, Groovy still has ton of utility methods in GDK not matched in kotlin stdlib.
Other than that, I'm sad to say, Groovy is dead or in process of dying. I'm seeing companies rewriting Grails apps to Spring Boot and switching to Kotlin dsl in Gradle. It's a shame really, such a fun language to program in.
I’ll have to check out Kotlin though, seems like the new wave for sure when it comes to simple readable jvm code.
Maven is a bit easier to Google; mainly because it stopped evolving in any meaningful way about a decade ago and nothing is really changing. The XML feels really backwards at this point It seems attempts to move away from that have stalled. It's also a lot slower to run. We're using it on the project I'm currently on and it eats up a lot of my time. Every time I do an mvn spring-boot:run, it goes off and compiles stuff. Even after it just did that and literally nothing changed. Gradle is much better at eliminating unnecessary work.
Huh interesting. Its the other way around for me, Gradle usually being a lot slower. Also, IDE integrations (IntelliJ) is usually better with Maven due to its declarative nature.
But you mention not using Hibernate. I fully agree myself, and used JOOQ, but what is your preference?
Have you tried using Spek?
For database stuff; I've always preferred good old JdbcTemplate in Spring combined with TransactionTemplate. IMHO writing mapping code is not that much of a problem; especially if you stop pretending that your complex object hierarcy maps 1 to 1 to tables and do a proper database design instead.
Writing that code sounds harder than it is. Compared to littering your models with annotations to say "this field is that column and this type", it's about the same lines of code. Except it's in one place where it is easy to maintain. Several frameworks exist to automate this business of course but given how trivially easy it is to do this manually, I don't see the value.
I think it's a good sign when you see it start to creep into random, seemingly disjointed areas, like how Kotlin DSLs are invading a lot of areas of dev-ops/infra-as-code. It speaks to the strengths of the language rather than the passion of the supporting community, imo.
Performance driven purists can continue to doubt, but I'm betting long by a mile on Kotlin, all it will take is time, more and more frameworks and ecosystems will integrate Kotlin, and rough edges can be polished.
>I would advice starting with some tests
The way that Kotlin extension functions and DSLs breathe fresh air into JUnit testing is hard to fully communicate. For me at least, it makes writing test code actually not suck anymore, and even fun in some cases.
You can thank the Oracle lawsuit for this.
Google has been preparing Kotlin as their escape hatch.
It certainly helps them shift away from Java reliance, but have you ever actually coded an Android app? Android is stuck on Java 8 syntax, and Kotlin fixes that. It has a laundry list of QoL improvements over Java (delegation, type alias, extension functions, data classes, better string formatting, operator overloading, smart casting, to name a few) that make it so much more pleasing to use over Java. ESPECIALLY when coupled with the aging Android framework, which it enhances greatly.
If Android had an up to date full implementation of Java, the sales pitch wouldn't work that well.
In fact, there are several people that mistakenly think Android Java is Java and then get positively surprised when they learn about the real deal.
Another aspect is that JVM evolves without taking Kotlin in consideration, so there's always danger of JVM introducing a feature that won't be available in Kotlin (and JVM has several interesting developments namely Loom, Valhalla). Like it or not, Kotlin is second-class citizen on JVM. It made sense in Java 7 era, when Java development seemed stuck and Java thought to be mostly done. Times changed, Java is evolving at much faster rates (I would even say, faster than Kotlin these days).
This is about JVM, of course on Android Kotlin is first-class citizen and everything's good for Kotlin there.
In about 5 years it will be as meanigfull as former guest languages are today.
Without having any language support for what Java 25 will be capable of doing with value types, GPGPU, SIMD, Graal integration, as per JVM/Java roadmap, as it will be jailed by what Android is able to do.
Though there are some experimental non-JDBC drivers using non-blocking I/O to move your blocking code from backend into database. In the end you're still blocking.
If that is because of the memory consumption of kernel threads, be aware you are trading less memory consumption for increased cognitive complexity (if you agree that asynchronicity is more complex than synchronicity).
Admittedly I haven't looked at Spring w/ Kotlin at all, but I can't imagine it would solve fundamental annoyances I have with it (Why can't I capture logs globally while actually handling my exceptions locally?).
IMO the best library for Kotlin web stuff is http4k. Really nice documentation, first class integration for marshalling JSON with Moshi instead of Gson/Jackson, and very good, up-to-date consistent documentation.
It looks like there is a kotlinx-coroutines-reactor module, I'm not sure if that's what you are referring to.
Basically, with co-routines you lose the need for all the container types like Mono, Flux, etc. and replace them with simple suspend functions. This makes code a lot more readable, easier to test, and reason about.
Instead of callbacks for errors, you can rely on the normal try/catch. Any uncaught exception causes the co routine and co routine scope to fail and cancel in a controlled way. The Flow API allows you to do reactive style processing. It also handles back pressure and a few other things.
Basically fully reactive code written using this is structurally very similar to the normal blocking version of the code.
I despise side effects as well as try/catch error handling so these are an especially welcome improvement to java/kotlin.
I don't have much experience with coroutines/flow so I can't actually compare the two approaches but this is my experience with the reactor library. Its unlikely that I will get a chance to continue using Kotlin in the near future but next time I do use Kotlin I will definitely try the coroutine approach and see how it compares.