These automations do not resolve all the problems, but we are able to prioritize the most common ones. Meta has open sourced a number of those refactorings to help other developers to accomplish the same tasks. The right approach to handle this cases consisted in a three-step pipeline to first prepare Java code, then automatically run J2K in a headless Android Studio instance, and finally postprocess the generated files to apply all required refactoring and fixes. Some are easy to do (such as replacing isEmpty), some require research to figure out the first time (as in the case of JUnit rules), and a few are workarounds for actual J2K bugs that can result in anything from a build error to different runtime behavior.

We have found many instances of these small fixes. This worked quite well except for a number of specific frameworks, including JUnit, for which the tool lacks sufficient knowledge to be able to produce correct conversions.

On the front of code conversion proper, Meta engineers opted to use Kotlin official converter J2K, available as a compiler plugin. For example, Meta had to update the ReDex Android bytecode optimizer and the lexer component of syntax highlighter Pygments, and built a Kotlin symbol processing (KSP) API, used to create Kotlin compiler plugins. This was no easy task and required quite some investigation at start.Ī first obstacle Meta engineers had to overcome came from several internal optimization tools in use at Meta that did not work properly with Kotlin. It was also clear to Kotlin engineers that they had to port to Kotlin as much of their Java codebase as possible, mostly to prevent Java null pointers from creeping into the Kotlin codebase and to reduce the remaining Java code requiring maintenance. Meta's decision to adopt Kotlin for their Android apps was motivated by Kotlin advantages over Java, including nullability and functional programming support, shorter code, and the possibility of creating domain specific languages. In the process, they have learned a number of lessons of general interest and developed a few useful approaches, explains Meta engineer Omer Strulovich. Meta has been at work to port their Android codebase from Java to Kotlin.