Extended mill init to support an existing Maven project

build.mill

@@ -184,6 +184,14 @@ object Deps {
   val sonatypeCentralClient = ivy"com.lumidion::sonatype-central-client-requests:0.3.0"
   val kotlinVersion = "2.0.21"
   val kotlinCompiler = ivy"org.jetbrains.kotlin:kotlin-compiler:$kotlinVersion"
+  val mavenVersion = "3.9.9"
+  val mavenEmbedder = ivy"org.apache.maven:maven-embedder:$mavenVersion"
+  val mavenResolverVersion = "1.9.22"
+  val mavenResolverConnectorBasic = ivy"org.apache.maven.resolver:maven-resolver-connector-basic:$mavenResolverVersion"
+  val mavenResolverSupplier = ivy"org.apache.maven.resolver:maven-resolver-supplier:$mavenResolverVersion"
+  val mavenResolverTransportFile = ivy"org.apache.maven.resolver:maven-resolver-transport-file:$mavenResolverVersion"
+  val mavenResolverTransportHttp = ivy"org.apache.maven.resolver:maven-resolver-transport-http:$mavenResolverVersion"
+  val mavenResolverTransportWagon = ivy"org.apache.maven.resolver:maven-resolver-transport-wagon:$mavenResolverVersion"
 
   object RuntimeDeps {
     val dokkaVersion = "1.9.20"

dist/package.mill

@@ -19,6 +19,7 @@ object `package` extends RootModule with build.MillPublishJavaModule {
 
     def testTransitiveDeps = build.runner.testTransitiveDeps() ++ Seq(
       build.main.graphviz.testDep(),
+      build.main.maven.testDep(),
       build.runner.linenumbers.testDep(),
       build.scalalib.backgroundwrapper.testDep(),
       build.contrib.bloop.testDep(),

docs/modules/ROOT/nav.adoc

@@ -40,6 +40,8 @@
 ** xref:cli/flags.adoc[]
 ** xref:cli/alternate-installation.adoc[]
 ** xref:cli/builtin-commands.adoc[]
+* Migrating to Mill
+** xref:migrating/maven.adoc[]
 // This section gives a tour of the various user-facing features of Mill:
 // library deps, out folder, queries, tasks, etc.. These are things that
 // every Mill user will likely encounter, and are touched upon in the various

docs/modules/ROOT/pages/migrating/maven.adoc

@@ -0,0 +1,155 @@
+= Migrating From Maven to Mill
+:page-aliases: Migrating_A_Maven_Build_to_Mill.adoc
+:icons: font
+
+include::partial$gtag-config.adoc[]
+
+The Mill `init` command can be used to convert a Maven build to Mill. This has
+xref:#limitations[limitations] and is not intended to reliably migrate 100% of
+Maven builds out there in the wild, but is instead meant to provide the basic
+scaffolding of a Mill build for you to further refine and update manually.
+
+Each Maven module with a `pom.xml` is converted to a Mill `build.mill`/`package.mill`
+file containing a top-level `MavenModule`. A nested `test` module is defined if both:
+
+* `src/test` exists
+* a supported xref:javalib/testing.adoc[test framework] is detected (for a _tests only_
+module with test sources in `src/main/java`)
+
+
+Again, note that `mill init` importing Maven builds is best effort.
+This means that while small projects can be expected to complete without issue:
+
+include::partial$example/javalib/migrating/1-maven-complete.adoc[]
+
+More larger projects often require some manual tweaking in order to work:
+
+include::partial$example/javalib/migrating/2-maven-incomplete.adoc[]
+
+Nevertheless, even for larger builds `mill init` automates most of the tedious
+busy-work of writing `build.mill`/`package.mill` files, and makes it much quicker
+to get a working Mill build for any existing Maven project.
+
+
+== Capabilities
+
+The conversion
+
+* handles deeply nested modules
+* captures project metadata
+* configures dependencies for scopes:
+** compile
+** provided
+** runtime
+** test
+* configures testing frameworks:
+** JUnit 4
+** JUnit 5
+** TestNG
+* configures multiple, compile and test, resource directories
+
+=== Command line arguments
+.name of generated base module trait defining project metadata settings
+[source,sh]
+----
+./mill init --base-module MyModule
+----
+.name of generated nested test module (defaults to `test`)
+[source,sh]
+----
+./mill init --test-module test
+----
+.name of generated companion object defining constants for dependencies
+[source,sh]
+----
+./mill init --deps-object Deps
+----
+.capture properties defined in `pom.xml` for publishing
+[source,sh]
+----
+./mill init --publish-properties
+----
+.merge build files generated for a multi-module build
+[source,sh]
+----
+./mill init --merge
+----
+
+.use cache for Maven repository system
+[source,sh]
+----
+./mill init --cache-repository
+----
+.process Maven plugin executions and configurations
+[source,sh]
+----
+./mill init --process-plugins
+----
+
+=== Verified projects
+
+The conversion has been tested with the following projects:
+
+* https://github.com/fusesource/jansi/archive/refs/tags/jansi-2.4.1.zip[jansi]
+[source,sh]
+----
+./mill init --base-module JansiModule --deps-object Deps --cache-repository --process-plugins
+----
+
+* https://github.com/davidmoten/geo/archive/refs/tags/0.8.1.zip[geo] (multi-module build)
+[source,sh]
+----
+./mill init --base-module GeoModule --deps-object Deps --merge --cache-repository --process-plugins
+----
+
+Post `init`, the following tasks were executed successfully:
+
+* `compile`
+* `test`
+* `publishLocal`
+
+[#limitations]
+== Limitations
+
+The conversion does not support
+
+* build extensions
+* build profiles
+* non-Java (native) sources
+
+Maven plugin support is limited to
+
+* https://maven.apache.org/plugins/maven-compiler-plugin/[maven-compiler-plugin]
+
+[TIP]
+====
+These limitations can be overcome by:
+
+* configuring equivalent Mill xref:extending/contrib-plugins.adoc[contrib]
+  or xref:extending/thirdparty-plugins.adoc[third party] plugins
+* defining custom xref:extending/writing-plugins.adoc[plugins]
+* defining custom xref:fundamentals/tasks.adoc[tasks]
+* defining custom xref:fundamentals/cross-builds.adoc[cross modules]
+====
+
+== FAQ
+
+.How to fix compilation errors in generated build files?
+
+This could happen if a module and task name collision occurs. Either rename the module or enclose the name in backticks.
+
+
+.How to fix JPMS `module not found` compilation errors?
+
+Set https://github.com/tfesenko/Java-Modules-JPMS-CheatSheet#how-to-export-or-open-a-package[additional command line options]
+for dependencies.
+
+
+.How to fix test compilation errors?
+
+* The test framework configured may be for an unsupported version; try upgrading the
+  corresponding dependencies.
+* Mill does not add `provided` dependencies to the transitive dependencies of the nested
+  test module; specify the dependencies again, in one of `ivyDeps`, `compileIvyDeps`, `runIvyDeps`, in the test module.
+
+

example/javalib/migrating/1-maven-complete/build.mill

@@ -0,0 +1,29 @@
+/** Usage
+
+> rm build.mill # remove any existing build file
+
+> git init .
+> git remote add -f origin https://github.com/davidmoten/geo.git
+> git checkout 0.8.1 # example multi-module Java project using JUnit4
+
+> ./mill init
+converting Maven build
+writing Mill build file to geo/package.mill
+writing Mill build file to geo-mem/package.mill
+writing Mill build file to build.mill
+init completed, run "mill resolve _" to list available tasks
+
+> ./mill __.compile
+compiling 9 Java sources to .../out/geo/compile.dest/classes ...
+compiling 2 Java sources to .../out/geo-mem/compile.dest/classes ...
+compiling 10 Java sources to .../out/geo/test/compile.dest/classes ...
+done compiling
+compiling 1 Java source to .../out/geo-mem/test/compile.dest/classes ...
+done compiling
+
+> ./mill __.test # all tests pass immediately
+Test run com.github.davidmoten.geo.GeoHashTest finished: 0 failed, 0 ignored, 66 total, ...
+Test run com.github.davidmoten.geo.DirectionTest finished: 0 failed, 0 ignored, 1 total, ...
+...
+
+*/

example/javalib/migrating/2-maven-incomplete/build.mill

@@ -0,0 +1,24 @@
+/** Usage
+
+> rm build.mill # remove any existing build file
+
+> git init .
+> git remote add -f origin https://github.com/dhatim/fastexcel.git
+> git checkout 0.18.4
+> # pom.xml has custom profiles for JPMS that mill init does not support
+> # https://github.com/dhatim/fastexcel/blob/de56e786a1fe29351e2f8dc1d81b7cdd9196de4a/pom.xml#L251
+
+> ./mill init
+converting Maven build
+writing Mill build file to fastexcel-writer/package.mill
+writing Mill build file to fastexcel-reader/package.mill
+writing Mill build file to e2e/package.mill
+writing Mill build file to build.mill
+init completed, run "mill resolve _" to list available tasks
+
+> ./mill -k __.compile # Compilation needs manual tweaking to pass
+error: ...fastexcel-reader/src/main/java/module-info.java:3:32: module not found: org.apache.commons.compress
+error: ...fastexcel-reader/src/main/java/module-info.java:4:27: module not found: com.fasterxml.aalto
+error: ...fastexcel-writer/src/main/java/module-info.java:2:14: module not found: opczip
+
+*/

example/package.mill

@@ -34,6 +34,7 @@ object `package` extends RootModule with Module {
     object dependencies extends Cross[ExampleCrossModuleJava](build.listIn(millSourcePath / "dependencies"))
     object testing extends Cross[ExampleCrossModuleJava](build.listIn(millSourcePath / "testing"))
     object linting extends Cross[ExampleCrossModuleJava](build.listIn(millSourcePath / "linting"))
+    object migrating extends Cross[ExampleCrossModuleJava](build.listIn(millSourcePath / "migrating"))
     object publishing extends Cross[ExampleCrossModuleJava](build.listIn(millSourcePath / "publishing"))
     object web extends Cross[ExampleCrossModule](build.listIn(millSourcePath / "web"))
   }