SBT 运行语句显示：未检测到主类答案

【问题标题】：SBT run statement shows: No main class detectedSBT 运行语句显示：未检测到主类
【发布时间】：2020-05-24 06:20:20
【问题描述】：

我创建了一个网络应用程序，当我尝试使用sbt run 运行时，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

build.sbt的内容是：

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：

我做错了什么？

【问题讨论】：

标签： scala sbt

【解决方案1】：

您没有定义主类，即程序的入口点。在 JVM you have to have 上至少定义一个类

public static void main(args: String[])

在 Scala 的情况下意味着类似

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

如果确实有一个这样的类，sbt 可以检测到它，如果有更多，你必须告诉 sbt 使用哪一个：

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

为了打电话给run。如果你运行 JAR，这个类可以使用 manifest.mf 指定或显式传递。

【讨论】：