Problem: When including several projects that include log4j2 plugins, sbt-assembly will encounter multiple versions of the Log4j2Plugins.dat cache file that it won't be able to reconcile.

Solution: Log4j2 provides a PluginCache class that creates consistent, serialisable cache from multiple different ones.

This plugin provides a simple function to merge those files in a safe way.

For sbt 1.x add sbt-assembly-log4j2 as a dependency in project/assembly.sbt alongside sbt-assembly:

addSbtPlugin("com.eed3si9n" % "sbt-assembly" % "<version>")
addSbtPlugin("com.github.nomadblacky" % "sbt-assembly-log4j2" % "0.1.1")

In your merge strategy, add the following case:

import sbtassembly.Log4j2MergeStrategy

assemblyMergeStrategy in assembly := {
    //...
    case PathList(ps @ _*) if ps.last == "Log4j2Plugins.dat" => Log4j2MergeStrategy.plugincache
    //...
}