I’ve been working with Kotlin for a while, mainly for side-projects or toy-projects. Since last Google I/O 2017 announcement it has become clear that there are no more reasons or excuses to not use it in production.

One of the big selling points of Kotlin is that you can start small, by converting one class or two, or by creating new ones, while keeping all the remaining code in Java. So, interop between the two languages is almost 100% transparent. Almost.

Working to convert a small project step by step, I started to convert activities into Kotlin. Those activities use ButterKnife (I’m using current version, which is 8.7.0) to inject the views. So, after converting the first activity I stumbled upon a problem with the annotation processor: in Gradle script, you have to use either annotationProcessor or kapt, but not both at the same time. So, you have to choose:

• using annotationProcessor only will not find Kotlin classes, and because of that injection will silently fail at runtime,
• using kapt only will make compilation fail.

The final workaround I found was:

• using kapt3 (by applying kotlin-kapt plugin to the Gradle script) and,
• adding a @JvmField() annotation in addition to ButterKnife annotations so Kotlin compiler generates public fields instead of getters and setters.

By applying kapt3 we fix the compilation error involving “kotlin.jvm.internal.FunctionReference.(ILjava/lang/Object;)V” and by converting Kotlin fields to plain-old Java fields we allow ButterKnife compiler to find the fields to inject, as is unable to find Kotlin fields.

You can find the source code with different options in different branches (the one with the final solution is kotlin-workaround) in this GitHub project.

The project has two activities, one (MainActivity) that is written in Java and kept in this language, and the second one (NextActivity) that is converted to Kotlin. Notice that a simple suite of tests is available to check that both activities are being correctly injected, and that there is a TextView in both activities that has its text replaced by code to prove that the activity has been successfully injected.

Hope this tip is useful!