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Update:

So, the issue I'm trying to understand is why I can't have a build.sbt in the root folder. Even if I define a lazy val root = ... in the root build.sbt folder, the main class is not found.

This reddit link:

https://www.reddit.com/r/scala/comments/4igc9x/sbt_playframework_no_main_class_detected/

suggests a solution to the problem. While sbt demo/run does execute the play app, I'd like to eventually just have sbt run from the root project compile and all the projects and then run the server.

Can anyone help out here?

=================================================================

Old question:

I'm trying to understand how build.sbt works, and I'm not sure what's going on. I'd like to build a play project as one of multiple projects being build under a root build.sbt file.

I have the following structure (most files left out for brevity):

.
|-- build.sbt
|-- project/
|   |-- build.properties
|   |-- plugins.sbt
|   |-- project/
|-- demo/
|   |-- build.sbt
|   |-- app/
|   |-- conf/
|   |    |-- application.conf
|   |    |-- routes
|   |-- public/
|   |-- test/

inside the root dir build.sbt file, I have the following.

name := "demo-root"
organization := "com.example"


scalaVersion in ThisBuild := "2.11.11"

lazy val root = (project in file("."))
  .dependsOn(demo)
  .aggregate(demo)

lazy val demo = project
  .enablePlugins(PlayScala)

I've defined addSbtPlugin("com.typesafe.play" % "sbt-plugin" % "2.5.14") inside plugins.sbt in the root project/.

inside the demo/build.sbt I have

name := "demo"

version in ThisBuild := "1.0-SNAPSHOT"

libraryDependencies += filters
libraryDependencies += "org.scalatestplus.play" %% "scalatestplus-play" % "2.0.0" % Test

Now, when I perform sbt run, I receive the following error:

java.lang.RuntimeException: No main class detected.
    at scala.sys.package$.error(package.scala:27)
[trace] Stack trace suppressed: run last root/compile:run for the full output.
[error] (root/compile:run) No main class detected.

However if I put the root build.sbt and root project/ inside of demo/ and try to sbt run, the project compiles successfully and I can access the play server in the regular way.

Why is this error being thrown? Do I need to explicitly declare a main class for the play project in the root build.sbt? I'm following the instructions at http://www.scala-sbt.org/0.13/docs/Multi-Project.html, and I'm just a little lost. Any help in the right direction would be appreciated.

For reference ---

sbt version: 0.13.15

scala version: 2.11.11

play plugin version: 2.5.14

clo_jur
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  • the `libraryDependencies` lines from the root `build.sbt` belong to `demo/build.sbt`, also you do not need to `.enablePlugins(PlayScala)` in the root `build.sbt` as you already do it in the `demo/build.sbt` (where it belongs). How is it going after changing those? – Frederic A. Jun 13 '17 at 03:41

1 Answers1

2

There's no main class in the root project, only in the demo project.

When starting sbt in the root project, you have to use the following commands:

  • project demo
  • run

the equivalent one-line may be: sbt demo/run

To your updates, I don't know how to do that, I would advise against, and I'm pretty sure that's not something sbt will do out of the box for you.

Remember that there could be many sub-projects, each with their own main-class(es). In such situation, what would sbt be supposed to do?

An alternative would be to define the play project as the root/aggregate and have sub-projects there, as illustrated it the play doc: https://www.playframework.com/documentation/2.5.x/SBTSubProjects#Project-structure

Frederic A.
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