java - sbt 汇编有效，但仍然是 "no main manifest attribute"

Question

我正在使用 sbt 1.0.2。我的 build.sbt 是

name := "myprogram"

version := "0.1"

scalaVersion := "2.12.3"

libraryDependencies += "org.scalatest" %% "scalatest" % "3.0.1" % "test"

// https://mvnrepository.com/artifact/net.sf.py4j/py4j
libraryDependencies += "net.sf.py4j" % "py4j" % "0.10.6"

scalacOptions ++= Seq(
  "-unchecked",
  "-deprecation",
  "-feature",
  "-encoding", "UTF-8",
  "-Xfuture",
  "-Xlint:unsound-match",      // Pattern match may not be typesafe.
  "-Yno-adapted-args",         // Do not adapt an argument list (either by inserting () or creating a tuple) to match the receiver.
  "-Ypartial-unification",     // Enable partial unification in type constructor inference
  "-Ywarn-dead-code",          // Warn when dead code is identified.
  "-Ywarn-inaccessible",       // Warn about inaccessible types in method signatures.
  "-Ywarn-infer-any",          // Warn when a type argument is inferred to be `Any`.
  "-Ywarn-nullary-override",   // Warn when non-nullary `def f()' overrides nullary `def f'.
  "-Ywarn-nullary-unit",       // Warn when nullary methods return Unit.
  "-Ywarn-unused",             // Warn if a method or value is unused.
  "-Ywarn-unused:implicits",   // Warn if an implicit parameter is unused.
  "-Ywarn-unused:imports",     // Warn if an import selector is not referenced.
  "-Ywarn-unused:locals",      // Warn if a local definition is unused.
  "-Ywarn-unused:params",      // Warn if a value parameter is unused.
  "-Ywarn-unused:patvars",     // Warn if a variable bound in a pattern is unused.
  "-Ywarn-unused:privates",    // Warn if a private member is unused.
  "-Ywarn-value-discard"       // Warn when non-Unit expression results are unused.
)

javacOptions ++= Seq("-source", "1.8", "-target", "1.8")

test in assembly := {}
assemblyJarName in assembly := "myprogram.jar"

exportJars := true

mainClass in Compile := Some("com.company.MyClass")
mainClass in assembly := Some("com.company.MyClass")
mainClass in(Compile, run) := Some("com.company.MyClass")
mainClass in(Compile, packageBin) := Some("com.company.MyClass")

它似乎可以使用sbt assembly编译，其中一些输出是

[warn] Multiple main classes detected.  Run 'show discoveredMainClasses' to see the list
[info] Including: py4j-0.10.6.jar
[info] Including: scala-library.jar
[info] Checking every *.class/*.jar file's SHA-1.
[info] Merging files...
[warn] Merging 'META-INF/MANIFEST.MF' with strategy 'discard'
[warn] Merging 'META-INF/maven/net.sf.py4j/py4j/pom.properties' with strategy 'discard'
[warn] Merging 'META-INF/maven/net.sf.py4j/py4j/pom.xml' with strategy 'discard'
[warn] Strategy 'discard' was applied to 3 files

一旦我进入命令行，我就会得到

$ java -jar myprogram.jar
no main manifest attribute, in myprogram.jar

我打开我的 jar 并查看MANIFEST.MF，但找不到主要内容:

Manifest-Version: 1.0
Implementation-Title: myprogram
Implementation-Version: 0.1
Specification-Vendor: default
Specification-Title: myprogram
Implementation-Vendor-Id: default
Specification-Version: 0.1
Implementation-Vendor: default

值得一提的是，我的程序主要是用 Scala 编写的，除了我的主类是用 Java 编写的。

我需要改变什么？

Answer 1

从您显示的日志中，我的猜测是 - 在其中一个包含的 jar 的内容中 - 已经有一个 META-INF/MANIFEST.MF ，以及您的/由 sbt 创建的那个程序集 - 声明主类的地方 - 被丢弃(但我不能 100% 确定，因为您显示的 list 内容似乎包含来自代码的内容)。

我认为这表明您的 list 被丢弃:

[warn] Merging 'META-INF/MANIFEST.MF' with strategy 'discard'

我敢打赌 py4j-0.10.6.jar 中已经有一个“META-INF/MANIFEST.MF”。

如何解决这个问题？

1. 避免使用 fat jar，例如使用 http://www.scala-sbt.org/sbt-native-packager ，但我想这不是重点
2. 调整合并策略，详情请参见 https://github.com/sbt/sbt-assembly#merge-strategy 。更具体地说，您可以尝试设置用于合并 META-INF/MANIFEST.MF 的 MergeStrategy.concat 策略。

大致:

assemblyMergeStrategy in assembly := {
  case PathList("META-INF", "MANIFEST.MF")         => MergeStrategy.concat
  case x =>
    val oldStrategy = (assemblyMergeStrategy in assembly).value
    oldStrategy(x)
}