Java to Kotlin: A Refactoring Guidebook

Refactoring Beyond the Commit

1 June 2024

In our “Mastering Kotlin Refactoring” workshop at KotlinConf 2024, we categorised refactorings by the effort involved to propagate the code improvements beyond your local workspace. We came up with three categories of refactoring, of increasing effort:

Single Commit Refactorings:

For example, renaming a local variable.

Multiple Commit Refactorings:

For example, renaming a widely used function, type or package.

Multiple Deployment Refactorings:

For example, changes to HTTP APIs or database schema used by systems that must be upgraded without downtime.


Multiple Commit refactorings require changes to be made by “expand/contract”, so that the refactoring does not disrupt work in progress by other developers in the team. Multiple Deployment refactorings require changes to be made by “expand/contract” deployments so that they do not break the running system.

Expand/contract delivers a refactoring in phases. First, we “expand” the system, adding features to the codebase that allow both old and new forms of the code to coexist. Then we migrate code that uses the old form to use the new form. Finally, we “contract” the system, removing the now unused code that supports the old form.

During an expand/contract refactoring, the code gets worse before it gets better. We introduce some duplication to allow the code to gradually move from one design to another. During that period, it is important that everyone working on the code knows how the design is changing: what we are moving away from, and what is the new way of doing things. We don’t want new code being written in the old way, creating even more refactoring work.

IDE Support

IntelliJ has excellent support for Single Commit refactorings. It has some support for Multiple Commit refactorings. Occasionally it will offer an option to expand the code, leaving old and new forms side by side – although so occasionally that I could not find an example while writing this article! I think it’s something that IntelliJ does better when refactoring Java than Kotlin.

Kotlin’s deprecation mechanism helps with Multiple Commit refactorings. You can annotate old features in the codebase as @Deprecated and provide the new form of the code in the annotation. Other developers can then migrate code to the new form automatically in the IDE when it is convenient for them to do so. The @Deprecated annotation can also be used hide old forms from IDE autocompletion, so that developers don’t use old forms accidentally while the refactoring is being propagated across the codebase. We’ll look at these features in more detail in a future post.

IntelliJ doesn’t help at all with Multiple Deployment refactorings. But who knows, maybe in the future the IDE will integrate code editing with deployment orchestration tools, such as Flyway, Liquibase, Kubernetes, Helm, Monopolis, etc., and generate migrations and schedule deployments as you refactor.