Custom Gradle module in mill.build

[momentary-lapse]

Hello! I have a mill project, and a subproject of it which is being build by gradle. I'm realizing that it's not a perfect approach, but the subproject is in Kotlin and has Springboot dependencies, and I couldn't make mill build it correctly.

Currently its compilation is implemented by an ugly hack using JavaModule:

object `my-gradle-module` extends JavaModule:
  override def moduleDeps = Seq(`some-dep`)

  override def compile = Task {
    os.proc("./gradlew", "build", "-x", "test").call(cwd = BuildCtx.workspaceRoot)
    super.compile()
  }

But it lacks testing stuff and all other mill benefits. I'm wondering, what would be a best approach to handle such subproject? Maybe you have some examples of custom modules for Gradle?

[lefou]

We don't have examples embedding Gradle, so I can't directly point you to one.

May I ask, why you can't migrate the sub-project to Mill? Maybe, converting the project with mill init is an option to you? Once done, you could change the sub-project from build.mill to package.mill and make it a proper sub-module.

• https://mill-build.org/mill/migrating/migrating.html
• https://mill-build.org/mill/migrating/auto-migrating.html

Since you posted your "hack", let me point out some potential issues. Since tasks are cached, you also should capture anything that may lead to an outdated cache. E.g. you could create a gradleSources task to capture them. You should also capture the result of the gradle run and return it from the task, so it blends in transparently to downstream tasks. Either copy it into Task.dest (preferred), so they won't vanish once you run gradle outside again of this task, or set the revalidate-flag of the returned PathRef, so Mill can check that the resource is still unchanged, before using it as the cached result.

def gradleSources = Task.Sources(
  "build.gradle",
  "src/main/java" // or whatever your sources are
  ... // any other paths, that will end up in the compilation
)

def compile = Task {
  gradleSources() // depend on it to properly re-compile when these change
  or.proc(".gradlew", ....
  
  val dummy = Task.dest / "dummy" 
  os.write(dummy, "")

  val classes = PathRef(moduleDir / "target/classes").withRevalidateOnce

  CompilationResult(dummyFile, classes) 
}

[lefou]

You may also look at our example projects that where migrated from Gradle manually.

• https://mill-build.org/mill/comparisons/gradle.html
• https://github.com/com-lihaoyi/mill/blob/main/example/thirdparty/mockito/build.mill

[momentary-lapse]

I tried to set up the subproject in mill in the beginning of this year, but couldn't manage to do that, because it includes complex stuff like springboot starter test (can recall, this was one of the biggest problems), code generation, specific GraphQL plugins, and everything is written in Kotlin. I remember I found examples of configs either for Kotin or for Springboot, but never for both. Tried combining them by myself, but gave up at some point.
Heard the mill support for those things have improved since, so I may give it another shot. Maybe you have specific examples for such kind of setup?

[lefou]

We have some examples for Spring Boot and more for Kotlin. We're currently in the process of extending the Spring Boot documentation.

As I maintain myself a closed-source project with Kotlin and Spring-Boot, I must say, there is no special handling needed to run a Spring-Boot application with Mill, since Spring Boot is all about dynamic runtime setup by inspecting what's on the classpath. So, by adding the right dependencies to mvnDeps a spring boot application should be able to auto-configure the same way as with Gradle or Maven.

[lefou]

Java + Spring Boot

• https://mill-build.org/mill/javalib/web-examples.html#_spring_boot_hello_world_app
• https://mill-build.org/mill/javalib/web-examples.html#_spring_boot_todomvc_app

Kotlin + Spring Boot

• https://mill-build.org/mill/kotlinlib/intro.html
• https://mill-build.org/mill/kotlinlib/web-examples.html#_spring_boot_hello_world_app
• https://mill-build.org/mill/kotlinlib/web-examples.html#_spring_boot_todomvc_app

Open issues / PRs

• Spring Boot: Document getting started docs #6006
• Add spring boot example for mill configuration compatible with spring initializr for a maven kotlin project #6014

[lefou]

• https://mill-build.org/mill/javalib/publishing.html#_building_executable_assemblies_with_repackagemodule

[momentary-lapse]

Thanks much! I think we gonna try the setup based on the new Kotlin + Spring Boot hello world template. Also good to know that my approach of combining build systems is not a good practice :)