🚉 Metro

Introduction

Metro is a compile-time dependency injection framework that draws heavy inspiration from Dagger, Anvil, and Kotlin-Inject. It seeks to unify their best features under one, cohesive solution while adding a few new features and implemented as a compiler plugin.

Why another DI framework?

It’s felt for some time like the Kotlin community has been waiting for a library at the intersection of these different tools and features. Different tools exist for parts of these, but there’s not yet been a unified solution that checks all these boxes, leaves behind some of these tools’ limitations, and embraces newer features that native compiler plugins offer.

In short, Metro stands on the shoulders of giants. It doesn’t try to reinvent the wheel, it does try to make those wheels work better together.

Note

I’m aware of the XKCD comic 🙂, I think Metro offers a compelling feature set with interop hooks that make it easy to integrate with an existing codebase.

Installation

Apply the gradle plugin.

plugins {
  id("dev.zacsweers.metro") version "x.y.z"
}

And that’s it! The default configuration will add the multiplatform runtime artifact (which has annotations you can use) and wire it all automatically.

You can configure custom behaviors with APIs on the metro DSL extension.

metro {
  // Defines whether or not metro is enabled. Useful if you want to gate this behind a dynamic
  // build configuration.
  enabled = true // Default

  // Enable (extremely) verbose debug logging
  debug = false // Default

  // See the kdoc on MetroPluginExtension for full details
}

Snapshots of the development version are available in Sonatype’s snapshots repository.

Supported platforms

The compiler plugin itself supports all multiplatform project types. The first-party annotations artifact is also multiplatform and supports all common JVM, JS, and native targets.

Caveats

• Kotlin compiler plugins are not a stable API! Compiled outputs from this plugin should be stable, but usage in newer versions of kotlinc are not guaranteed to be stable. That said, Metro will strive to make matching releases available quickly and test against Kotlin prereleases eagerly.

Acknowledgements

Special thanks to the many people that helped advise and offer feedback on Metro’s design and implementation.

• Brian Norman
• Ralf Wondratschek
• Jesse Wilson
• Dany Santiago
• Eva Tatarka
• Bill Phillips
• Bryan Stern
• James Barr
• and many more!

License

Copyright (C) 2025 Zac Sweers

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

   https://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.