Log4j 2 uses a cache to store information about the plugins it discovers during initialization.

The cache file has the .dat extension and contains serialized data related to the plugin classes found in the classpath. This cache file helps Log4j 2 avoid re-scanning the classpath every time the application starts up, resulting in faster startup times.

When building a jar file, these .dat files need to be merged using the log4j-core library. Merging them using the sbt-assembly doesn't work.

This plugin implements a CustomMergeStrategy which uses the log4j2 libraries to merge multiple .dat files.

In project/plugins.sbt add

addSbtPlugin("uk.gov.nationalarchives" % "sbt-assembly-log4j2" % "x.x.x")

Add the merge strategy to your assembly merge strategy in build.sbt

import uk.gov.nationalarchives.sbt.Log4j2MergePlugin

(assembly / assemblyMergeStrategy) := {
  case PathList(ps@_*) if ps.last == "Log4j2Plugins.dat" => Log4j2MergePlugin.log4j2MergeStrategy
  case _ => MergeStrategy.first
}

Run sbt test for the unit tests.

Run sbt scripted for sbt script tests.

The plugin is deployed to Maven central using GitHub actions.