sbt: run task on subproject

Question

I have the following project structure:

lazy val root = project.aggregate(rest,backend)
lazy val rest = project
lazy val backend = project

When I execute the "run" task from the parent, I want a specific class from the "backend" project to have its main method executed. How would I accomplish this?

Anacrusis

Answer 1

lazy val root = project
  .aggregate(rest, backend)
  .dependsOn(rest, backend) //<- don't forget dependsOn
lazy val rest =
  project
lazy val backend =
  project.settings(
    Compile / run / mainClass := Some("fully.qualified.path.to.MainClass")
  )

Compile / run := (backend / Compile / run).evaluated

Piefer

Why is the dependsOn from root to backend important? The last line seems to work for me, also without it (i.e. sbt clean followed by sbt run does the right thing). – Googly

"<<=" operator was removed from sbt 1.x – Unbolted

Re: the removal of <<=, I've done by best to update the answer to use modern sbt 1 syntax. – Particia