Translating 10M lines of Java to Kotlin
Meta is translating its Android codebase from Java to Kotlin to boost developer productivity and ensure null safety, using the Kotlinator tool and collaborating with JetBrains to enhance conversion accuracy.
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Meta is undertaking a significant project to translate its extensive Android codebase from Java to Kotlin, aiming to enhance developer productivity and ensure null safety. The transition, which began several years ago, has surpassed the halfway mark, with plans to convert tens of millions of lines of code. While many companies opt to write new code in Kotlin without converting existing Java code, Meta has chosen a comprehensive approach to fully leverage Kotlin's advantages. This includes developing a custom tool called the Kotlinator, which automates the translation process in six phases, addressing challenges such as slow build speeds and the need for compatibility with existing Java code. The Kotlinator incorporates preprocessing and postprocessing steps to ensure that the translated code is functional and idiomatic. Additionally, Meta collaborates with JetBrains to improve the Java-to-Kotlin conversion tool, J2K, and to enhance the accuracy of symbol resolution. The project emphasizes the importance of null safety, as unaddressed nullability can lead to runtime errors. Despite the complexities involved, Meta's commitment to a complete translation reflects its goal of maximizing the benefits of Kotlin while minimizing the drawbacks of a mixed codebase.
- Meta is translating its entire Android codebase from Java to Kotlin.
- The Kotlinator tool automates the translation process in six phases.
- The project aims to enhance developer productivity and ensure null safety.
- Meta collaborates with JetBrains to improve the Java-to-Kotlin conversion tool, J2K.
- The initiative addresses challenges like slow build speeds and compatibility with existing Java code.
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23 commentsI think Kotlin helped push Java in the right direction, but at this point it seems like a weaker choice. Spending years to migrate a massive Java code base like this feels like wasted time.
I learned that social whims (developer fun, preferences , dopamin) are weighted as much as technical rationales (performance, maintenance)
Personally I find that it's an interesting indicator of the capability of the programming languages. Moving from language A to B can be extremely easy if B is as powerful or more powerful in terms of expressiveness than A. It can be an absolute horror if it is less powerful.
Being not null-safe in fact brings additional expressiveness. I guess most would argue that it's not a good type expressiveness. Nonetheless it is a type of expressiveness that can cause trouble during such a transition.
In general it feels like Java is making faster progress than Kotlin nowadays. I wonder where Kotlin would be if it weren't for Android. I hope those folks don't have to migrate back before they even finished.
The only reason fb is able to do this, is the billions of $ behind it... For everyone else this is just pure idiocy
Sure if you like Kotlin, use it for new software, but rewriting milliona loc for some marginal gains... that how businesses fail more often than not
The language's "declarative style lambda syntax" makes code overly-complex. Every library is basically it's own DSL and every lambda context has it's own set of specialized keywords. If I write `test { somthing(value) }`, depending on the context, `somthing` could be invoking a higher-order function, calling a method on an object that the lambda is extending, or calling a static method. The muddling of data/declarations and procedures, makes jumping into new codebases confusing and time-consuming. Rich Hickey has pointed out in numerous talks that syntax itself is complex because rules and order are complex, but in languages we generally trade a little complexity for terseness. Kotlin encourages this trade-off far too much in the direction of complexity, and the language would be almost unusable if not for it's IDE support.
Getting to the root of the previous problem is that method extensions in general feel like an anti-pattern. Without introspecting a method call, it's not possible to tell where that functionality is coming from. It's a huge amount of added cognitive strain to have a classes functionality split into various files, possibly across various code bases.
Another problem is coroutines (oh joy, another new DSL to learn). By now, it should be known, from languages like Go and Erlang, that preemptive lightweight threading is almost always going to be better than cooperative. It avoids function coloring, keeps throughput of applications more consistent, and is generally easier to reason about. Java also now has preemptive lightweight threading via virtual threads, but because much of the Kotlin ecosystem is reliant on the coroutine API, it don't get any of the ergonomic benefits.
Could someone expand on this please.
I mean, with the ressources meta has I’m sure they could have rewritten the tool, made a fork or done any other thing to incorporate their changes (they talk about overrides) or transformed the tool into something better fitting their approach. Maybe it has been done, just not clear from the article
Why? Bit of substance here would be nice. Otherwise it's just another "we migrated to $coolLanguage" post.
>With this in mind, we set out to automate the conversion process and minimize interference with our developers’ daily work. The result was a tool we call the Kotlinator that we built around J2K. It’s now comprised of six phases:
followed by a list of descriptions of the "phases" which only sort of make sense for the name given to them, and are utterly incoherent as actual phases in a process (and grammatically inconsistent). For example, one of the cited "phases" is... "headless J2K". In other words: they have one piece of software that wraps another, and it - gasp - doesn't use the wrapped software's GUI. Aside from being entirely unremarkable, that's neither a phase in a process nor a component of a tool. It's a fact about the component.
LLMs write like this all the time - and it's clear evidence that they do not, in fact, do anything like reasoning, even if they can sometimes be manipulated into generating a second piece of text that resembles an analysis of the first one. The resulting description is so weird that I question whether the authors actually checked the LLM's output for accuracy.
Any human writer who gives a damn about good writing and has any skill, would not allow "it" to refer to "the conversion process" two sentences back when "a tool called the Kotlinator" has been introduced in the interim (or, if that were the intended referent, would notice that tools are not "comprised of phases"). Such a writer would also not come up with abominations like "the conversion process is now comprised of six phases" where "we now use a six-phase conversion process" would be much clearer. Certainly, a six-point bullet list produced by a competent writer would label them in a grammatically consistent way (https://en.wikipedia.org/wiki/Parallelism_(grammar)) - not with an abstract noun describing an action, two participles, two concrete (well, as concrete as software ever is) nouns and a command (who, exactly, is being told to "build" the "error-based fixes" - whatever that means - here?).
I'm starting to feel like Mark Twain.
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On the other hand, I was cynically expecting some mention of using AI for the actual task, and that doesn't seem to be the case.
(Also, the "reactive" web design is broken. The page overflows horizontally for some range of window widths, without causing a horizontal scrollbar to be added.)
Not one link to an opinion piece or two regarding: "kotlin vs java". The nearest thing I found was "What makes Kotlin different".
This sounds somewhat like a debate about which Germanic language is best. German, Dutch and English are all "Germanic" but which is "best"?
Obviously: That's the wrong question to ask and so an answer is doomed to failure.
In the end, does your generated machine code implore the CPU and associated hardware to do what you want it to more efficiently in some way that is not an abstraction?
Pissing contests rarely excite me. Why did you do this?
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