Programming languages: Why Meta is shifting its Android apps from Java to Kotlin


Picture: Getty Photos/Nitat Termmee

Engineers at Fb guardian Meta have detailed their multi-year effort to transform the codebases for its quite a few Android apps from the Java programming language over to Kotlin. 

That is a big job for a corporation with hundreds of builders and lots of massive Android apps, together with Fb, Instagram, Messenger, Portal, and its Quest VR headset.   

“Shifting away from Java, which we presently use for Android improvement, and over to Kotlin just isn’t a trivial job,” notes Omer Strulovich, a software program engineer at Fb. 

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The migration to Kotlin is underway, with Meta’s Android app repositories already consisting of 10 million traces of Kotlin code. 

“In the present day, our Android apps for Fb, Messenger, and Instagram every have greater than 1 million traces of Kotlin code, and the speed of conversion is rising. In complete, our Android codebase has greater than 10 hundreds of thousands traces of Kotlin code,” famous Strulovich. 

Java is without doubt one of the prime three programming languages, because of its dominance in enterprise purposes, whereas Kotlin is a much less widespread however nonetheless prime 20 programming language. Meta’s swap away from Java to Kotlin for Android app improvement is a milestone, given Meta’s scale and the scale of its apps. 

However, regardless of the advantages of Kotlin that motivated Meta’s swap, Strulovich outlines a number of important disadvantages that make the transfer dangerous, a minimum of for giant apps. 

Firstly, the big hole in reputation between Java and Kotlin leaves the latter with fewer instruments, and people instruments, having to account for Kotlin-Java interoperability, are advanced. Regardless of the languages’ 100% interoperability, Meta has discovered it could possibly’t take away all Java. 

“Kotlin is a well-liked language, however in contrast with Java, the recognition hole is obvious. Java is the world’s second or third hottest language (relying on how one measures this). This implies fewer instruments can be found. Worse than that, all of the Kotlin instruments must account for Kotlin and Java interoperability, which complicates their implementation,” notes Strulovich.

However Meta’s largest concern is slower construct occasions in Kotlin versus Java. 

“We knew from the beginning that Kotlin’s construct occasions could be longer than Java’s. The language and its ecosystem are extra sophisticated, and Java had twenty years of a head begin to optimize its compiler. Since we personal a number of massive apps, the implications of longer construct occasions may negatively influence our builders’ expertise.” 

On that final level, Strulovich factors to compile occasions after the HTTP shopper undertaking, OkHttp, switched from Java to Kotlin in 2019. OkHttp is small, with solely 24,000 traces of Kotlin. Its compile time in Java was 2.4 seconds in comparison with 10.2 seconds in Kotlin. 

Meta engineers determined to transform nearly all their code into Kotlin somewhat than writing new code utilizing Kotlin and leaving many of the current code in Java (the second method is how the Linux kernel undertaking and the Android Open Supply Mission are introducing Rust into their codebases). 

Doubtlessly excellent news for Kotlin adoption elsewhere is that Meta has launched a number of in-house Kotlin conversion instruments on GitHub that others may be capable to use straight, or at least may encourage others to seek out methods to automate the conversion. 

As Fb engineers detailed final yr, whereas Kotlin adoption may be easy, it’s troublesome at scale. Fb makes use of its personal Buck construct system versus IntelliJ/Android Studio IDE, which use Gradle. 

Strulovich outlines a number of open-source Kotlin instruments that Meta contributed to, such because the “Pygments” library that it is utilizing to “convey the expertise to par with Java”. Meta additionally constructed Ktfmt, a plugin for IntelliJ and Android Studio that gives a “deterministic Kotlin formatter primarily based on the code and philosophy of google-java-format.”

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Strulovich says that, to date, Meta has on common seen an 11% discount within the dimension of its codebase. The Google House crew reported a 33% discount in its app’s codebase dimension when migrating new characteristic improvement to Kotlin.   

“We now have seen a lot greater numbers quoted on-line, however we suspect these numbers are derived from particular examples,” Strulovich stated. “We’re nonetheless completely satisfied about this quantity, because the traces eliminated are often boilerplate code, which is much less implicit than its shorter Kotlin counterpart.”

Kotlin was first developed by JetBrains, the maker of developer instruments or IDEs like IntelliJ. Google has been utilizing Kotlin for Android apps since 2017 and the pair have collectively supported the Kotlin language via the Kotlin Basis since 2018. 

In 2019, Google’s Android crew adopted the “Kotlin-first” method. Google has over 70 Android apps constructed a minimum of partly with Kotlin, together with Maps, House, Play, Drive, and Messages. Google says, in comparison with Java, Kotlin is extra environment friendly, produces safer code and a smaller codebase, and is 100% interoperable with the Java language – making the swap from Java codebases simpler by permitting them to coexist.

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