Scala/Mill: Step 5, Specifying a Main Class in the REPL

If you’re using the Scala Mill build tool and have a project that has multiple classes with main methods, this tutorial shows how to choose a main method in the Mill REPL.

Starting Mill’s interactive mode

Start Mill’s interactive mode from your operating system command-line with this mill command in the root directory of your project:

$ mill -i

That command starts the Mill REPL, where you can work with Mill interactively.

Selecting a main method

Inside the REPL you can choose a main method to run with this syntax:

RssAtomReader.runMain("hello.Hello")()

This example assumes:

My project subdirectory is named RssAtomReader
I have multiple main methods in my project
The main method I want to run is in a class named Hello
The Hello class is in a package named hello

In this case I’m using a main method in a Scala class, but it could also be a main method in a Java class.

Mill’s interactive mode

You can see this in the Mill REPL by typing the name of your project and then hitting the [Enter] key. For example, my project is named RssAtomReader, so I type that in the Mill REPL and then hit [Enter], and see this:

@ RssAtomReader

res0: RssAtomReader = ammonite.predef.build#RssAtomReader:4
Children:
    .test
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:
    .allSourceFiles()

The Mill output shows this as part of its output:

.runMain(mainClass: String, args: String*)()

That output is what tells me that I can choose which main class to run:

@ RssAtomReader.runMain("hello.Hello")()

If I instead wanted to run the main method in a class named Foo in a package named foo, I’d use this syntax:

@ RssAtomReader.runMain("foo.Foo")()

Passing command-line arguments to your main class

I haven’t tested this next part yet, but if you want to pass command-line arguments to that class, it looks like this is the correct syntax for that:

@ RssAtomReader.runMain("foo.Foo", "bar", "baz")()

this post is sponsored by my books:
#1 New Release	FP Best Seller	Learn Scala 3	Learn FP Fast

Step 6

Here’s a link to Step 6, Creating a Fat Jar with Mill (assembly). (For some reason the link isn’t showing up below, so I added it here manually.)