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log4j2 + sbt assembly plugin

Problem

When packaging your application as a "fat jar" with sbt's assembly plugin, you may see the following error:

[error] java.util.zip.ZipException: duplicate entry: META-INF/MANIFEST.MF

You might therefore decide to drop your dependencies' META-INF files in an attempt to fix this error. However, depending on the rules configured in build.sbt's assemblyMergeStrategy, and how aggressive you went, critical log4j2 files might be dropped, resulting in errors like:

ERROR StatusLogger Log4j2 could not find a logging implementation. Please add log4j-core to the classpath. Using SimpleLogger to log to the console...

Solution

Configure your build.sbt to have:

assemblyMergeStrategy in assembly := {
  case PathList("META-INF", "MANIFEST.MF") => MergeStrategy.discard
  case _ => MergeStrategy.first
}

Run:

$ sbt clean assembly
$ java -jar target/scala-2.12/log4j2-sbt-assembly-assembly-0.1.0-SNAPSHOT.jar
{"thread":"main","level":"INFO","loggerName":"example.Hello$","message":"hello","endOfBatch":false,"loggerFqcn":"org.apache.logging.log4j.scala.Logger$","instant":{"epochSecond":1525709528,"nanoOfSecond":179000000},"threadId":1,"threadPriority":5,"timestamp":"2018-05-07T17:12:08.179+0100"}