Setting logLevel throws an exception

Question

I attempted to start sbt with sbt -Dpact.logLevel=DEBUG and it threw a class-cast exception.

[error] (pactVerify) java.lang.ClassCastException: class org.apache.logging.slf4j.Log4jLogger cannot be cast to class ch.qos.logback.classic.Logger (org.apache.logging.slf4j.Log4jLogger is in unnamed module of loader sbt.internal.MetaBuildLoader @2641e737; ch.qos.logback.classic.Logger is in unnamed module of loader sbt.internal.PluginManagement$PluginClassLoader @ee2ae9a)
[error]     at au.com.dius.pact.provider.sbt.SbtProviderPlugin$autoImport$.$anonfun$pactProvidersConfig$2(SbtProviderPlugin.scala:26)
[error]     at au.com.dius.pact.provider.sbt.SbtProviderPlugin$autoImport$.$anonfun$pactProvidersConfig$2$adapted(SbtProviderPlugin.scala:22)
[error]     at scala.Function1.$anonfun$compose$1(Function1.scala:49)
[error]     at sbt.internal.util.$tilde$greater.$anonfun$$u2219$1(TypeFunctions.scala:63)
[error]     at sbt.std.Transform$$anon$4.work(Transform.scala:69)
[error]     at sbt.Execute.$anonfun$submit$2(Execute.scala:283)
[error]     at sbt.internal.util.ErrorHandling$.wideConvert(ErrorHandling.scala:24)
[error]     at sbt.Execute.work(Execute.scala:292)
[error]     at sbt.Execute.$anonfun$submit$1(Execute.scala:283)
[error]     at sbt.ConcurrentRestrictions$$anon$4.$anonfun$submitValid$1(ConcurrentRestrictions.scala:265)
[error]     at sbt.CompletionService$$anon$2.call(CompletionService.scala:65)
[error]     at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
[error]     at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:539)
[error]     at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
[error]     at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136)
[error]     at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635)
[error]     at java.base/java.lang.Thread.run(Thread.java:840)

It looks like it is expecting the log4j logging system, but it is getting the logback system. (My build.sbt does include logback in the dependencies.). Is there a way to set the log level in this case?

project/plugins.sbt includes

libraryDependencySchemes += "org.scala-lang.modules" %% "scala-xml"  % VersionScheme.Always
addSbtPlugin("au.com.dius" %% "pact-jvm-provider-sbt" % "3.5.12")

The library schemes is needed to avoid conflicts in org.scala-lang.modules:scala-xml_2.12 which appeared when I added the pact plugin.

Joe · Accepted Answer · 2025-11-20 00:37:03Z

It looks like it is expecting the log4j logging system, but it is getting the logback system.

I believe it's the opposite; the Pact code in question will only work with a Logback backend:

        if (System.getProperty("pact.logLevel") != null) {
          LoggerFactory.getLogger(org.slf4j.Logger.ROOT_LOGGER_NAME).asInstanceOf[LogbackLogger].setLevel(
            Level.toLevel(System.getProperty("pact.logLevel"))
          )
        }

When Log4j is in use, the cast fails.

However, you may not need to use pact.logLevel here: it's just a Logback-specific shortcut for configuring the root level. You can configure Log4j directly (e.g. this question) with the appropriate root logger level.

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