Which compiler do you prefer to use in Kotlin projects during the build process—Just-In-Time (JIT) or Ahead-Of-Time (AOT)? Could you briefly explain why? It’s important to consider the project type, performance concerns, or the impact on the development process!
Which do you prefer in the Kotlin compiler: JIT or AOT?
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The Kotlin JIT vs AOT debate isn't really about JVM compilation-time optimizations—Kotlin itself isn't a JIT/AOT compiler; it compiles to JVM bytecode by design. What's being discussed here are the JIT compilation process for Kotlin/JVM (e.g., Kotlin compiler + JVM JIT) and the AOT compilation for Kotlin/Native (LLVM-based).
For Kotlin/JVM projects, I always stick with pure JIT. The reason is simple: JIT's runtime optimizations (like method inlining, loop unrolling, etc.) are far smarter than AOT's. Especially when combined with Kotlin's language features like null safety and coroutines, performance loss without JIT is inevitable. Plus, with no redo (replay) capabilities yet, we have to trust the JVM JIT to optimize things before shipping.
AOT only makes sense for Android release builds (where R8/D8 optimizations differ from AOT anyway). On the Kotlin/Native side, though, AOT is a must since target platforms like iOS and embedded don’t support JIT. The downside? Compilation can be painfully slow. Ultimately, the decision depends on the target platform and how critical build/QA time is.
JIT vs AOT? Sounds interesting! Actually, I prefer AOT because I've seen better startup times and performance on mobile. What kind of project are you working on?
I really struggled with build times after starting a mobile app project on Android. In the prototype phase, every small change would go through JIT’s "hot" compilation, taking 5-6 seconds to go from code to interface tests—perfect for testing ideas on the fly. But as the app grew, especially the build time, it stretched to 15-20 seconds. I kept wondering, "Should I optimize the build or focus on the build system?"
Eventually, I switched to AOT for release builds. While the first build took a while (around 3-4 minutes), every subsequent build produced an instantly runnable APK. I no longer had to worry about performance during live streams or demos. I’ll admit, JIT’s flexibility is nice for small projects, but for larger scales, AOT seems like the only real option.
For a standard Kotlin project (mobile app or backend), I prefer **JIT** during development: it allows faster rebuilds and an efficient feedback loop with tools like Android Studio or IntelliJ. In production, if targeting mobile, I often switch to AOT via the Kotlin/Native compiler or Android’s release mode for runtime optimization. AOT is my preference for performance-critical scenarios, but its longer build time doesn’t justify its use in development.
I prefer AOT in Kotlin, especially for projects targeting platforms outside the JVM (JavaScript, Native). For example, when using the JavaScript target, AOT gives you smaller and faster bundles. In one project, switching the JS build from JIT to AOT reduced startup time by around 30%. While the "run" command is a bit slower during development, the difference really shows in production builds.
But JIT has its advantages too — it's great for quick prototyping and debugging. For instance, when developing an Android project, JIT makes the dev process smoother, but compiling the release APK with AOT significantly boosts performance. Ultimately, I think the most logical approach is to use AOT for production and JIT during development. What scenarios do you prefer each for?