scalaintellij-ideamill

scala, the mill build tool, and the IntelliJ debugger

I'm writing Scala code and using the Mill build tool. I'd like to be able to use IntelliJ's debugger to debug my code but have not been able to convince it to stop at breakpoints.

I have set up a remote debugging configuration in IntelliJ. I've copy/pasted the suggested Java command line arguments into a script that I use to run Mill:

#!/bin/zsh

export JAVA_OPTS="-agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=*:5005"
mill $@

When I run the program using Mill, the program stops and waits for the debugger to connect. It then runs to completion in spite of having breakpoints set.

breakpoint

The println at line 17 is executed.

The build.sc file for Mill is simplicity itself:

import mill._, scalalib._

object foo extends RootModule with ScalaModule {
  def scalaVersion = "3.3.3"
  def ivyDeps = Agg(
    ivy"com.lihaoyi::os-lib:0.9.3",
    ivy"com.github.scopt::scopt:4.1.0",
    ivy"ca.uwaterloo::da_solver:0.4.0-SNAPSHOT"
  )

  object test extends ScalaTests {
    def ivyDeps       = Agg(ivy"com.lihaoyi::utest:0.7.11")
    def testFramework = "utest.runner.Framework"
  }
}

One of the things I wonder about is the setting for "Use module classpath:"

classpath dropdown

It defaults to <no module>. I've tried that and the other options, all with the same results (breakpoints skipped).

Any suggestions?

I found this StackOverflow post on using the IntelliJ debugger really helpful, except for the above. User @user3416742 asked a similar question in Oct. 2023 but focused on running a unit test. I have that same question; unfortunately no one has answered it yet.

Solution

  • The key insight, from @CrazyCoder, is that mill starts up a subprocess to run the code. That's the process that needs to be connected.

    So, here are the steps:

    In IntelliJ:

    In your mill build file:

    To do the debugging:

    Addendum:

    It would be nice to be able to toggle the debug setting from the mill command line. So far, I have not figured out how to do that. Ideas welcome.