sbt-assembly: Logback does not work with über-JAR - scala

Question

If I run the application within IntelliJ, logging works fine, but if I run the über-JAR, I get the following error:
LF4J: No SLF4J providers were found.
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#noProviders for further details.
I use the following configuration to build my über-JAR with sbt-assembly:
lazy val app = (project in file("."))
.settings(
assembly / mainClass := Some("com.example.app.Main"),
assembly / assemblyJarName := "gcm.jar",
assembly / assemblyMergeStrategy := {
case PathList("META-INF", xs#_*) => MergeStrategy.discard
case PathList("reference.conf") => MergeStrategy.concat
case x => MergeStrategy.first
}
)
The dependencies for Logback and scala-logging would be:
ThisBuild / libraryDependencies += "ch.qos.logback" % "logback-classic" % "1.4.0"
ThisBuild / libraryDependencies += "com.typesafe.scala-logging" %% "scala-logging" % "3.9.5"
The logback.xml:
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration>
<configuration>
<import class="ch.qos.logback.classic.encoder.PatternLayoutEncoder"/>
<import class="ch.qos.logback.core.ConsoleAppender"/>
<import class="ch.qos.logback.core.FileAppender"/>
<appender name="STDOUT" class="ConsoleAppender">
<encoder class="PatternLayoutEncoder">
<pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{0} - %msg%n</pattern>
</encoder>
</appender>
<appender name="FILE" class="FileAppender">
<file>gcm.log</file>
<append>true</append>
<immediateFlush>true</immediateFlush>
<encoder class="PatternLayoutEncoder">
<pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{0} - %msg%n</pattern>
</encoder>
</appender>
<root level="debug">
<appender-ref ref="STDOUT"/>
<appender-ref ref="FILE"/>
</root>
</configuration>
..

Answer 1

This happens because Service Providers (defined in META-INF) are discarded by your merge strategy. I had the same issue when migrating from logback 1.2.x to 1.4.x.
One option I found in sbt-assembly doc is to replace your META-INF case with:
case PathList("META-INF", xs#_*) =>
(xs map {_.toLowerCase}) match {
case "services" :: xs =>
MergeStrategy.filterDistinctLines
case _ => MergeStrategy.discard
}
Is there a way to find out if that is the case - and if - what causes the conflict?
Running in debug mode ./sbt assembly --debug might help identifying which files are discarded:
...
[debug] Merging 'META-INF/services/ch.qos.logback.classic.spi.Configurator' with strategy 'discard'
[debug] Merging 'META-INF/services/jakarta.servlet.ServletContainerInitializer' with strategy 'discard'
[debug] Merging 'META-INF/services/org.slf4j.spi.SLF4JServiceProvider' with strategy 'discard'
...