CODEWITHSUNDEEP
scalasbt
softshipper
softshipper

Reputation: 34099

SBT run statement shows: No main class detected

I have created a web app and when I try to run with sbt run, it shows: 

sbt run    
[warn] No sbt.version set in project/build.properties, base directory: /home/developer/scala/user-svc/target/scala-2.13
[info] Loading global plugins from /home/developer/.sbt/1.0/plugins
[info] Set current project to scala-2-13 (in build file:/home/developer/scala/user-svc/target/scala-2.13/)
[error] java.lang.RuntimeException: No main class detected.
[error]         at scala.sys.package$.error(package.scala:30)
[error] stack trace is suppressed; run last Compile / bgRun for the full output
[error] (Compile / bgRun) No main class detected.
[error] Total time: 0 s, completed May 23, 2020, 11:44:32 PM

The content of build.sbt is: 

val Http4sVersion = "0.21.4"
val CirceVersion = "0.13.0"
val Specs2Version = "4.9.3"
val LogbackVersion = "1.2.3"

lazy val root = (project in file("."))
  .settings(
    organization := "io.example",
    name := "user-svc",
    version := "0.0.1-SNAPSHOT",
    scalaVersion := "2.13.2",
    libraryDependencies ++= Seq(
      "org.http4s" %% "http4s-jetty" % Http4sVersion,
      "org.http4s" %% "http4s-jetty-client" % Http4sVersion,
      "org.http4s" %% "http4s-circe" % Http4sVersion,
      "org.http4s" %% "http4s-dsl" % Http4sVersion,
      "io.circe" %% "circe-generic" % CirceVersion,
      "org.specs2" %% "specs2-core" % Specs2Version % "test",
      "ch.qos.logback" % "logback-classic" % LogbackVersion
    ),
    addCompilerPlugin("org.typelevel" %% "kind-projector" % "0.10.3"),
    addCompilerPlugin("com.olegpy" %% "better-monadic-for" % "0.3.1"),
  )

scalacOptions ++= Seq(
  "-deprecation",
  "-encoding", "UTF-8",
  "-language:higherKinds",
  "-language:postfixOps",
  "-feature",
  "-Xfatal-warnings",
)

The folder structure should be correct:

enter image description here

What am I doing wrong?

Upvotes: 0

Views: 2670

Answers (1)

Mateusz Kubuszok
Mateusz Kubuszok

Reputation: 27595

You don't have a main class, the entrypoint to your program defined. On JVM you have to have at least one class which defines

public static void main(args: String[])

which in Scala's case means something like

object Main {
  def main(args: Array[String]): Unit = {
    ...
  }
}

If there is exactly one such class, sbt can detect it, if there are more, you have to tell sbt which one to use using:

mainClass := Some("name.of.my.Main")

in order to call run. If you run JAR this class can be specified using manifest.mf or passed explicitly.

Upvotes: 2

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