Built-in Commands

Mill comes with a number of useful commands out of the box. These are listed in the Scaladoc:

mill.main.MainModule

Mill’s built-in commands are typically not directly related to building your application code, but instead are utilities that help you understand and work with your Mill build.

build.sc (download, browse)

import mill._, javalib._

trait MyModule extends JavaModule{
  object test extends JavaTests with TestModule.Junit4
}

object foo extends MyModule{
  def moduleDeps = Seq(bar)
  def ivyDeps = Agg(
    ivy"net.sourceforge.argparse4j:argparse4j:0.9.0",
  )
}

object bar extends MyModule{
  def ivyDeps = Agg(
    ivy"net.sourceforge.argparse4j:argparse4j:0.9.0",
    ivy"org.apache.commons:commons-text:1.12.0"
  )
}

resolve

resolve lists the tasks that match a particular query, without running them. This is useful for "dry running" an mill 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
bar
clean
inspect
path
plan
resolve
show
shutdown
version
visualize
visualizePlan

> mill resolve _.compile
foo.compile

> mill resolve foo._
foo.allSourceFiles
foo.allSources
foo.artifactId
foo.artifactName
...

You can also use the special wildcards _ as a placeholder for a single segment and __ as a placeholder for many segments. Lists within curly braces ({, }) are also supported.

> 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

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.

> mill inspect foo.run
foo.run(RunModule...)
    Runs this module's code in a subprocess and waits for it to finish
Inputs:
    foo.finalMainClass
    foo.runClasspath
    foo.forkArgs
    foo.forkWorkingDir
    foo.runUseArgsFile

While inspect also works with the same _/__ wildcard/query syntax that resolve do, the most common use case is to inspect one task at a time.

show

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:

> mill show foo.sources
[
  ".../foo/src"
]

> mill show foo.allSourceFiles
[
  ".../foo/src.../Foo..."
]

show is not just for showing configuration values. All tasks return values that can be shown with show. E.g. compile returns the paths to the classes folder and analysisFile file produced by the compilation:

> mill show foo.compile
{
  "analysisFile": ".../out/foo/compile.dest/zinc",
  "classes": ".../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
[
  ".../foo/src"
]

> mill show foo.compileClasspath
[
  ...
  ".../foo/compile-resources"
]

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

When show is used with multiple targets, its output will slightly change to a JSON array, containing all the results of the given targets.

> mill show "foo.{sources,compileClasspath}"
{
  "foo.sources": [
    ".../foo/src"
  ],
  "foo.compileClasspath": [
    ...
    ".../foo/compile-resources"
  ]
}

showNamed

Same as show, but the output will always be structured in a JSON dictionary, whether there is one or more targets in the selection

> mill showNamed "foo.{sources,compileClasspath}"
{
  "foo.sources": [
    ".../foo/src"
  ],
  "foo.compileClasspath": [
    ...
    ".../foo/compile-resources"
  ]
}

path

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, or trying to figure out why running mill foo ends up running another task bar that you didn’t expect it to.

> mill path foo.assembly foo.sources
foo.sources
foo.allSources
foo.allSourceFiles
foo.compile
foo.finalMainClassOpt
foo.prependShellScript
foo.assembly

If there are multiple possible dependency chains, one of them is picked arbitrarily.

plan

mill plan foo shows which tasks would be evaluated if you ran mill foo, and in what order, 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.

> mill plan foo.compileClasspath
foo.transitiveCompileClasspath
foo.compileResources
foo.unmanagedClasspath
...
foo.compileIvyDeps
...
foo.ivyDeps
foo.transitiveIvyDeps
foo.compileClasspath

clean

clean deletes all the cached outputs of previously executed tasks.

> mill clean

clean without arguments cleans the entire project. It also accepts arguments to clean 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 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 versioning schemes, so it may not work as expected for dependencies with particularly weird version numbers.

> mill mill.scalalib.Dependency/showUpdates

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

Current limitations:

Only works for JavaModule modules (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.

visualize

$ mill show visualize foo._
[
    ".../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:

visualize can be very handy for trying to understand the dependency graph of tasks within your Mill build.

visualizePlan

$ mill show visualizePlan foo.compile
[
    ".../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:

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, one per line.

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.

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