How can a duplicate class be excluded from sbt assembly?

Question

We have a situation in which two dependencies have exactly the same class (because one of the dependencies copied it and included in their own source).

This is causing sbt assembly to fail its deduplication checks.

How can I exclude a class from a particular jar?

Answer 1

You need a mergeStrategy, which will take one of the files.

mergeStrategy in assembly := {
    case PathList("path", "to", "your", "DuplicatedClass.class") => MergeStrategy.first
    case x => (mergeStrategy in assembly).value(x)
}

Update

If you want to handle the file depending on the JAR which it came from, I don't think you can with the merge strategies that assembly plugin defines. What you could do you could define your own strategy.

I would invert your condition though. I think the question should be "How can I include a class from a particular JAR?". The reason is that there can be more than two JARs having the same class, and you can only include one in the end.

You can tell from where the file comes by using AssemblyUtils.sourceOfFileForMerge.

project/IncludeFromJar.scala

import sbtassembly._
import java.io.File
import sbtassembly.Plugin.MergeStrategy

class IncludeFromJar(val jarName: String) extends MergeStrategy {

  val name = "includeFromJar"

  def apply(args: (File, String, Seq[File])): Either[String, Seq[(File, String)]] = {
    val (tmp, path, files) = args
    val includedFiles = files.flatMap { f =>
      val (source, _, _, isFromJar) = sbtassembly.AssemblyUtils.sourceOfFileForMerge(tmp, f)
      if(isFromJar && source.getName == jarName) Some(f -> path) else None
    }
    Right(includedFiles)
  }

}

build.sbt

mergeStrategy in assembly := {
    case PathList("path", "to", "your", "DuplicatedClass.class") => new IncludeFromJar("jarname.jar")
    case x => (mergeStrategy in assembly).value(x)
}