Mill


Table of Contents

Intro to Mill

Configuring Mill

Mill is your shiny new Java/Scala build tool! Scared of SBT? Melancholy over Maven? Grumbling about Gradle? Baffled by Bazel? Give Mill a try!

Mill aims for simplicity by re-using concepts you are already familiar with, borrowing ideas from modern tools like Bazel, to let you build your projects in a way that's simple, fast, and predictable.

Mill has built in support for the Scala programming language, and can serve as a replacement for SBT, but can also be extended to support any other language or platform via modules (written in Java or Scala) or through external subprocesses.

If you are using Mill, you will probably find the following book by the Author useful in using Mill to the fullest:

Installation

OS X

Installation via homebrew:

brew install mill

Arch Linux

Arch Linux has a Community package for mill:

pacman -S mill

FreeBSD

Installation via pkg(8):

pkg install mill

Windows

To get started, download Mill from: https://github.com/lihaoyi/mill/releases/download/0.9.3/0.9.3-assembly, and save it as mill.bat.

If you're using Scoop you can install Mill via

scoop install mill

Mill also works on a sh environment on Windows (e.g., MSYS2, Cygwin, Git-Bash, WSL; to get started, follow the instructions in the manual section below. Note that:

sed -i '0,/-cp "\$0"/{s/-cp "\$0"/-cp `cygpath -w "\$0"`/}; 0,/-cp "\$0"/{s/-cp "\$0"/-cp `cygpath -w "\$0"`/}' /usr/local/bin/mill

Docker

You can download and run a Docker image containing OpenJDK, Scala and Mill using

docker pull nightscape/scala-mill
docker run -it nightscape/scala-mill

Manual

To get started, download Mill and install it into your system via the following curl/chmod command:

sudo curl -L https://github.com/lihaoyi/mill/releases/download/0.9.3/0.9.3 > /usr/local/bin/mill && sudo chmod +x /usr/local/bin/mill

Bootstrap Scripts (Linux/OS-X Only)

If you are using Mill in a codebase, you can commit the bootstrap launcher as a ./mill script in the project folder:

curl -L https://github.com/lihaoyi/mill/releases/download/0.9.3/0.9.3 > mill && chmod +x mill

Now, anyone who wants to work with the project can simply use the ./mill script directly:

./mill version
./mill __.compile

The mill command will automatically use the version specified by the bootstrap script, even if you installed it via other means. The ./mill file has a version number embedded within it, which you can update simply by editing the script. Note this only works for versions 0.5.0 and above.

Bootstrap scripts are also useful for running Mill in CI, ensuring that your Jenkins/Travis/etc. box has the correct version of Mill present to build/compile/test your code.

millw

Instead of installing mill directly, you can also use lefou/millw as drop-in replacement for mill. It provides a small shell script and also a Windows batch file, that transparently downloads mill and executes it on your behalf. It respects various ways to configure the preferred mill version (MILL_VERSION env var, .mill-version file, --mill-version option) and can also be used as bootstrap script in your project.

Coursier (unsupported)

Installing mill via coursier or cs is currently not officially supported. There are various issues, especially with interactive mode.

Updating Mill

Once installed mill is able to use newer or different versions for each project automatically. You don't need to install multiple versions of mill yourself.

See section Overriding Mill Versions how to do it.

Getting Started

The simplest Mill build for a Java project looks as follows:

// build.sc
import mill._, scalalib._

object foo extends JavaModule {

}

The simplest Mill build for a Scala project looks as follows:

// build.sc
import mill._, scalalib._

object foo extends ScalaModule {
  def scalaVersion = "2.13.1"
}

Both of these would build a project laid out as follows:

build.sc
foo/
    src/
        FileA.java
        FileB.scala
    resources/
        ...
out/
    foo/
        ... 

You can download an example project with this layout here:

The source code for this module would live in the foo/src/ folder, matching the name you assigned to the module. Output for this module (compiled files, resolved dependency lists, ...) would live in out/foo/.

This can be run from the Bash shell via:

$ mill foo.compile                 # compile sources into classfiles

$ mill foo.run                     # run the main method, if any

$ mill foo.runBackground           # run the main method in the background

$ mill foo.launcher                # prepares a foo/launcher/dest/run you can run later

$ mill foo.jar                     # bundle the classfiles into a jar

$ mill foo.assembly                # bundle classfiles and all dependencies into a jar

$ mill -i foo.console              # start a Scala console within your project (in interactive mode: "-i")
 
$ mill -i foo.repl                 # start an Ammonite REPL within your project (in interactive mode: "-i")

You can run mill resolve __ to see a full list of the different tasks that are available, mill resolve foo._ to see the tasks within foo, mill inspect foo.compile to inspect a task's doc-comment documentation or what it depends on, or mill show foo.scalaVersion to show the output of any task.

The most common tasks that Mill can run are cached targets, such as compile, and un-cached commands such as foo.run. Targets do not re-evaluate unless one of their inputs changes, where-as commands re-run every time.

Output

Mill puts all its output in the top-level out/ folder. The above commands would end up in:

out/
    foo/
        compile/
        run/
        runBackground/
        launcher/
        jar/
        assembly/

Within the output folder for each task, there's a meta.json file containing the metadata returned by that task, and a dest/ folder containing any files that the task generates. For example, out/foo/compile/dest/ contains the compiled classfiles, while out/foo/assembly/dest/ contains the self-contained assembly with the project's classfiles jar-ed up with all its dependencies.

Given a task foo.bar, all its output and results can be found be within its respective out/foo/bar/ folder.

Multiple Modules

Java Example

// build.sc
import mill._, scalalib._

object foo extends JavaModule
object bar extends JavaModule {
  def moduleDeps = Seq(foo)
}

Scala Example

// build.sc
import mill._, scalalib._

object foo extends ScalaModule {
  def scalaVersion = "2.13.1"
}
object bar extends ScalaModule {
  def moduleDeps = Seq(foo)
  def scalaVersion = "2.13.1"
}

You can define multiple modules the same way you define a single module, using def moduleDeps to define the relationship between them. The above builds expects the following project layout:

build.sc
foo/
    src/
        Main.scala
    resources/
        ...
bar/
    src/
        Main2.scala
    resources/
        ...
out/
    foo/
        ... 
    bar/
        ... 

And can be built/run using:

$ mill foo.compile        
$ mill bar.compile        

$ mill foo.run            
$ mill bar.run            

$ mill foo.jar            
$ mill bar.jar            

$ mill foo.assembly        
$ mill bar.assembly        

Mill's evaluator will ensure that the modules are compiled in the right order, and re-compiled as necessary when source code in each module changes.

Modules can also be nested:

// build.sc
import mill._, scalalib._

object foo extends ScalaModule {
  def scalaVersion = "2.13.1"
  object bar extends ScalaModule {
    def moduleDeps = Seq(foo)
    def scalaVersion = "2.13.1"
  }
}

Which would result in a similarly nested project layout:

build.sc
foo/
    src/
        Main.scala
    resources/
        ...
    bar/
        src/
            Main2.scala
        resources/
            ...
out/
    foo/
        ...
        bar/
            ...

Where the nested modules can be run via:

$ mill foo.compile        
$ mill foo.bar.compile        

$ mill foo.run            
$ mill foo.bar.run            

$ mill foo.jar            
$ mill foo.bar.jar            

$ mill foo.assembly        
$ mill foo.bar.assembly        

Watch and Re-evaluate

You can use the --watch flag to make Mill watch a task's inputs, re-evaluating the task as necessary when the inputs change:

$ mill --watch foo.compile 
$ mill --watch foo.run 
$ mill -w foo.compile 
$ mill -w foo.run 

Mill's --watch flag watches both the files you are building using Mill, as well as Mill's own build.sc file and anything it imports, so any changes to your build.sc will automatically get picked up.

For long-running processes like web-servers, you can use .runBackground to make sure they re-compile and re-start when code changes, forcefully terminating the previous process even though it may be still alive:

$ mill -w foo.compile 
$ mill -w foo.runBackground 

Parallel Task Execution (Experimental)

By default, mill will evaluate all tasks in sequence. But mill also has support to process tasks in parallel. This feature is currently experimental and we encourage you to report any issues you find on our bug tracker.

To enable parallel task execution, use the --jobs (-j) option followed by a number of maximal parallel threads.

Example: Use up to 4 parallel thread to compile all modules:

mill -j 4 __.compile

To use as much threads as your machine has (logical) processor cores use --jobs 0. To disable parallel execution use --jobs 1. This is currently the default.

Please note that the maximal possible parallelism depends on your project. Tasks that depend on each other can't be processes in parallel.

Command-line Tools

Mill comes built in with a small number of useful command-line utilities:

all

mill all foo.{compile,run}
mill all "foo.{compile,run}"
mill all foo.compile foo.run
mill all _.compile # run compile for every top-level module
mill all __.compile  # run compile for every module

all runs multiple tasks in a single command

resolve

$ mill resolve _
[1/1] resolve
all
clean
foo
inspect
par
path
plan
resolve
show
shutdown
version
visualize
visualizePlan

$ mill resolve _.compile
[1/1] resolve
foo.compile

$ mill resolve foo._
[1/1] resolve
foo.allSourceFiles
foo.allSources
foo.ammoniteReplClasspath
foo.ammoniteVersion
foo.artifactId
foo.artifactName
...

resolve lists the tasks that match a particular query, without running them. This is useful for "dry running" an mill all command to see what would be run before you run them, or to explore what modules or tasks are available from the command line using resolve _, resolve foo._, etc.

mill resolve foo.{compile,run}
mill resolve "foo.{compile,run}"
mill resolve foo.compile foo.run
mill resolve _.compile          # list the compile tasks for every top-level module
mill resolve __.compile         # list the compile tasks for every module
mill resolve _                  # list every top level module and task
mill resolve foo._              # list every task directly within the foo module
mill resolve __                 # list every module and task recursively

inspect

$ mill inspect foo.run
[1/1] inspect
foo.run(JavaModule.scala:442)
    Runs this module's code in a subprocess and waits for it to finish

Inputs:
    foo.finalMainClass
    foo.runClasspath
    foo.forkArgs
    foo.forkEnv
    foo.forkWorkingDir

inspect is a more verbose version of resolve. In addition to printing out the name of one-or-more tasks, it also displays its source location and a list of input tasks. This is very useful for debugging and interactively exploring the structure of your build from the command line.

inspect also works with the same _/__ wildcard/query syntaxes that all/resolve do:

mill inspect foo.compile
mill inspect foo.{compile,run}
mill inspect "foo.{compile,run}"
mill inspect foo.compile foo.run
mill inspect _.compile
mill inspect __.compile
mill inspect _
mill inspect foo._
mill inspect __

show

$ mill show foo.scalaVersion
[1/1] show
"2.13.1"

By default, Mill does not print out the metadata from evaluating a task. Most people would not be interested in e.g. viewing the metadata related to incremental compilation: they just want to compile their code! However, if you want to inspect the build to debug problems, you can make Mill show you the metadata output for a task using the show command:

All tasks return values that can be shown, not just configuration values. e.g. compile returns that path to the classes and analysisFile that are produced by the compilation:

$ mill show foo.compile
[1/1] show
[10/25] foo.resources
{
    "analysisFile": "/Users/lihaoyi/Dropbox/Github/test//out/foo/compile/dest/zinc",
    "classes": "ref:07960649:/Users/lihaoyi/Dropbox/Github/test//out/foo/compile/dest/classes"
}

show is generally useful as a debugging tool, to see what is going on in your build:

$ mill show foo.sources
[1/1] show
[1/1] foo.sources
[
    "ref:8befb7a8:/Users/lihaoyi/Dropbox/Github/test/foo/src"
]

$ mill show foo.compileClasspath
[1/1] show
[2/11] foo.resources
[
    "ref:c984eca8:/Users/lihaoyi/Dropbox/Github/test/foo/resources",
    ".../org/scala-lang/scala-library/2.13.1/scala-library-2.13.1.jar"
]

show is also useful for interacting with Mill from external tools, since the JSON it outputs is structured and easily parsed & manipulated.

path

$ mill path foo.assembly foo.sources
[1/1] path
foo.sources
foo.allSources
foo.allSourceFiles
foo.compile
foo.localClasspath
foo.assembly

mill path prints out a dependency chain between the first task and the second. It is very useful for exploring the build graph and trying to figure out how data gets from one task to another. If there are multiple possible dependency chains, one of them is picked arbitrarily.

plan

$ mill plan foo.compileClasspath
[1/1] plan
foo.transitiveLocalClasspath
foo.resources
foo.unmanagedClasspath
foo.scalaVersion
foo.platformSuffix
foo.compileIvyDeps
foo.scalaOrganization
foo.scalaLibraryIvyDeps
foo.ivyDeps
foo.transitiveIvyDeps
foo.compileClasspath

mill plan foo prints out what tasks would be evaluated, in what order, if you ran mill foo, but without actually running them. This is a useful tool for debugging your build: e.g. if you suspect a task foo is running things that it shouldn't be running, a quick mill plan will list out all the upstream tasks that foo needs to run, and you can then follow up with mill path on any individual upstream task to see exactly how foo depends on it.

visualize

$ mill show visualize foo._
[1/1] show
[3/3] visualize
[
    ".../out/visualize/dest/out.txt",
    ".../out/visualize/dest/out.dot",
    ".../out/visualize/dest/out.json",
    ".../out/visualize/dest/out.png",
    ".../out/visualize/dest/out.svg"
]

mill show visualize takes a subset of the Mill build graph (e.g. core._ is every task directly under the core module) and draws out their relationships in .svg and .png form for you to inspect. It also generates .txt, .dot and .json for easy processing by downstream tools.

The above command generates the following diagram:

VisualizeFoo.svg

visualizePlan

$ mill show visualizePlan foo.compile
[1/1] show
[3/3] visualizePlan
[
    ".../out/visualizePlan/dest/out.txt",
    ".../out/visualizePlan/dest/out.dot",
    ".../out/visualizePlan/dest/out.json",
    ".../out/visualizePlan/dest/out.png",
    ".../out/visualizePlan/dest/out.svg"
]

mill show visualizePlan is similar to mill show visualize except that it shows a graph of the entire build plan, including tasks not directly resolved by the query. Tasks directly resolved are shown with a solid border, and dependencies are shown with a dotted border.

The above command generates the following diagram:

VisualizePlan.svg

Another use case is to view the relationships between modules. For the following two modules:

// build.sc
import mill._, scalalib._

object foo extends ScalaModule {
  def scalaVersion = "2.13.1"
}
object bar extends ScalaModule {
  def moduleDeps = Seq(foo)
  def scalaVersion = "2.13.1"
}

mill show visualizePlan _.compile diagrams the relationships between the compile tasks of each module, which illustrates which module depends on which other module's compilation output:

VisualizeCompile.svg

clean

$ mill clean

clean deletes all the cached outputs of previously executed tasks. It can apply to the entire project, entire modules, or specific tasks.

mill clean                     # clean all outputs
mill clean foo                 # clean all outputs for module 'foo' (including nested modules)
mill clean foo.compile         # only clean outputs for task 'compile' in module 'foo'
mill clean foo.{compile,run}
mill clean "foo.{compile,run}"
mill clean foo.compile foo.run
mill clean _.compile
mill clean __.compile

Search for dependency updates

$ mill mill.scalalib.Dependency/showUpdates

Mill can search for updated versions of your project's dependencies, if available from your project's configured repositories. Note that it uses heuristics based on common versionning schemes, so it may not work as expected for dependencies with particularly weird version numbers.

Current limitations: - Only works for JavaModules (including ScalaModules, CrossScalaModules, etc.) and Maven repositories. - Always applies to all modules in the build. - Doesn't apply to $ivy dependencies used in the build definition itself.

mill mill.scalalib.Dependency/showUpdates
mill mill.scalalib.Dependency/showUpdates --allowPreRelease true # also show pre-release versions

IDE Support

Mill supports any IDE that is compatible with BSP, such as IntelliJ.
Use mill mill.bsp.BSP/install to generate the BSP project config for your build.

It also enables Intellij to provide navigation & code-completion features within your build file itself.

IntelliJ Support (legacy)

Mill supports IntelliJ configuration generation. Use mill mill.scalalib.GenIdea/idea to generate an IntelliJ project config for your build.

This also configures IntelliJ to allow easy navigate & code-completion within your build file itself.

The Build REPL

$ mill --repl
Loading...
@ foo
res0: foo.type = ammonite.predef.build#foo:4
Commands:
    .ideaJavaModuleFacets(ideaConfigVersion: Int)()
    .ideaConfigFiles(ideaConfigVersion: Int)()
    .ivyDepsTree(inverse: Boolean, withCompile: Boolean, withRuntime: Boolean)()
    .runLocal(args: String*)()
    .run(args: String*)()
    .runBackground(args: String*)()
    .runMainBackground(mainClass: String, args: String*)()
    .runMainLocal(mainClass: String, args: String*)()
    .runMain(mainClass: String, args: String*)()
    .console()()
    .repl(replOptions: String*)()
Targets:
...

@ foo.compile
res1: mill.package.T[mill.scalalib.api.CompilationResult] = foo.compile(ScalaModule.scala:143)
    Compiles the current module to generate compiled classfiles/bytecode

Inputs:
    foo.upstreamCompileOutput
    foo.allSourceFiles
    foo.compileClasspath
...
    
@ foo.compile()
[25/25] foo.compile
res2: mill.scalalib.api.CompilationResult = CompilationResult(
  /Users/lihaoyi/Dropbox/Github/test/out/foo/compile/dest/zinc,
  PathRef(/Users/lihaoyi/Dropbox/Github/test/out/foo/compile/dest/classes, false, -61934706)
)

You can run mill --repl to open a build REPL; this is a Scala console with your build.sc loaded, which lets you run tasks interactively. The task-running syntax is slightly different from the command-line, but more in-line with how you would depend on tasks from within your build file.

You can use this REPL to interactively explore your build to see what is available.

Deploying your code

The two most common things to do once your code is complete is to make an assembly (e.g. for deployment/installation) or publishing (e.g. to Maven Central). Mill comes with both capabilities built in.

Mill comes built-in with the ability to make assemblies. Given a simple Mill build:

// build.sc
import mill._, scalalib._

object foo extends ScalaModule {
  def scalaVersion = "2.13.1"
}

You can make a self-contained assembly via:

$ mill foo.assembly

$ ls -lh out/foo/assembly/dest/out.jar
-rw-r--r--  1 lihaoyi  staff   5.0M Feb 17 11:14 out/foo/assembly/dest/out.jar

You can then move the out.jar file anywhere you would like, and run it standalone using java:

$ java -cp out/foo/assembly/dest/out.jar foo.Example
Hello World!

To publish to Maven Central, you need to make foo also extend Mill's PublishModule trait:

// build.sc
import mill._, scalalib._, publish._

object foo extends ScalaModule with PublishModule {
  def scalaVersion = "2.13.1"
  def publishVersion = "0.0.1"

  def pomSettings = PomSettings(
    description = "Hello",
    organization = "com.lihaoyi",
    url = "https://github.com/lihaoyi/example",
    licenses = Seq(License.MIT),
    versionControl = VersionControl.github("lihaoyi", "example"),
    developers = Seq(
      Developer("lihaoyi", "Li Haoyi","https://github.com/lihaoyi")
    )
  )
}

You can change the name of the published artifact (artifactId in the Maven POM) by overriding artifactName in the module you want to publish.

You can download an example project with this layout here:

Which you can then publish using the mill foo.publish command, which takes your sonatype credentials (e.g. lihaoyi:foobarbaz) and GPG password as inputs:

$ mill foo.publish
Missing arguments: (--sonatypeCreds: String, --release: Boolean)

Arguments provided did not match expected signature:

publish
  --sonatypeCreds   String (format: "username:password")
  --signed          Boolean (default true)
  --gpgArgs         Seq[String] (default Seq("--batch", "--yes", "-a", "-b"))
  --readTimeout     Int (default 60000)
  --release         Boolean (default true)
  --connectTimeout  Int (default 5000) 
  --awaitTimeout    Int (default 120000)
  --stagingRelease  Boolean (default true)

You also need to specify release as true or false, depending on whether you just want to stage your module on oss.sonatype.org or you want Mill to complete the release process to Maven Central.

If you are publishing multiple artifacts, you can also use mill mill.scalalib.PublishModule/publishAll as described here

Structure of the out/ folder

The out/ folder contains all the generated files & metadata for your build. It is structured with one folder per Target/Command, that is run, e.g.:

There are also top-level build-related files in the out/ folder, prefixed as mill-*. The most useful is mill-profile.json, which logs the tasks run and time taken for the last Mill command you executed. This is very useful if you want to find out exactly what tasks are being run and Mill is being slow.

Each folder currently contains the following files:

The out/ folder is intentionally kept simplistic and user-readable. If your build is not behaving as you would expect, feel free to poke around the various dest/ folders to see what files are being created, or the meta.json files to see what is being returned by a particular task. You can also simply delete folders within out/ if you want to force portions of your project to be re-built, e.g. deleting the out/main/ or out/main/test/compile/ folders.

Overriding Mill Versions

Apart from downloading and installing new versions of Mill globally, there are a few ways of selecting/updating your Mill version:

echo "0.5.0" > .mill-version

.mill-version takes precedence over the version of Mill specified in the ./mill script.

MILL_VERSION=0.5.0-3-4faefb mill __.compile

or

MILL_VERSION=0.5.0-3-4faefb ./mill __.compile

to override the Mill version manually. This takes precedence over the version specified in ./mill or .mill-version

Note that both of these overrides only work for versions 0.5.0 and above.

Development Releases

In case you want to try out the latest features and improvements that are currently in master, unstable versions of Mill are available as binaries named #.#.#-n-hash linked to the latest tag. Installing the latest unstable release is recommended for bootstrapping mill.

The easiest way to use a development release is by updating the Bootstrap Script, or Overriding Mill Versions via an environment variable or .mill-version file.

Come by our Gitter Channel if you want to ask questions or say hi!

Running Mill with custom JVM options

It's possible to pass JVM options to the Mill launcher. To do this you need to create a .mill-jvm-opts file in your project's root. This file should contain JVM options (strings, starting with -X), one per line. All other lines will be ignored.

For example, if your build requires a lot of memory and bigger stack size, your .mill-jvm-opts could look like this:

-Xss10m
-Xmx10G

The file name for passing JVM options to the Mill launcher is configurable. If for some reason you don't want to use .mill-jvm-opts file name, add MILL_JVM_OPTS_PATH environment variable with any other file name.


This documentation was build from mill master branch.

About Mill: Mill is an Open Source Project created by Li Haoyi. It is actively maintained and the git repository has more that 100 individual contributors around the world.

About Mills Creator: Li Haoyi is a software engineer, an early contributor to Scala.js, and the author of many open-source Scala tools such as Mill, the Ammonite REPL and FastParse. If you've enjoyed using Mill, or enjoyed using Haoyi's other open source libraries, please chip in (or get your Company to chip in!) via Patreon so he can continue his open-source work.


Configuring Mill