SBT运行语句显示。没有检测到主类

Question

我已经创建了一个网络应用，当我尝试使用 sbt run，它显示。

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  

内容是： build.sbt 是。

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",
)

文件夹结构应该是正确的

我到底做错了什么？

Answer 1

你没有定义一个主类，也就是程序的入口点。在JVM上 你要有 至少有一个类，它定义了

public static void main(args: String[])

在Scala的情况下，它的意思是类似于

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

如果正好有一个这样的类，sbt可以检测到它，如果有更多的类，你必须告诉sbt使用哪一个。

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

告诉sbt使用哪个类，以便调用 run. 如果你运行JAR，这个类可以使用 manifest.mf 或明确通过。