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https://issues.apache.org/jira/browse/LOG4J2-3593?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17603326#comment-17603326
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Ralph Goers commented on LOG4J2-3593:
-------------------------------------
I am afraid I don't understand. Log4j has 2 versions of the StackLocator class;
one for Java 8 and one for Java 9+. The Java 9 version exists under the
META-INF/versions/9 in accordance with how multi-release jars work. If you are
running in Java 8 the Java 8 version will be used and this error will not
appear. If you are running in Java 9 and multi-release jars are not supported
you will also end up using the Java 8 version and StackLocator will determine
that it cannot use sun.reflect.Reflection because it is not present. That will
cause the warning message and StackLocator will fail to find the caller's
class. Walking the stack trace isn't really an option because that simply
returns the class name whereas the code requires the actual class object.
But you are correct that the message is misleading.
> StackLocator Reflection.getCallerClass warning is misleading / incorrect
> ------------------------------------------------------------------------
>
> Key: LOG4J2-3593
> URL: https://issues.apache.org/jira/browse/LOG4J2-3593
> Project: Log4j 2
> Issue Type: Bug
> Affects Versions: 2.17.2
> Reporter: Marcono1234
> Priority: Major
>
> When the pre-Java 9 {{StackLocator}} is used, but
> {{Reflection.getCallerClass}} is missing, the following warning is printed to
> console:
> {quote}
> WARNING: sun.reflect.Reflection.getCallerClass is not supported. This will
> impact performance.
> {quote}
> This can for example occur when a JAR with dependencies is created but the
> Log4j Java 9 specific code is not included, or {{Multi-Release: true}} is not
> set in the manifest (see also LOG4J2-2834).
> The issue is that the warning is misleading or incorrect. It suggests that
> Log4j is still fully functional, but as seen with LOG4J2-2834, certain
> functionality breaks (such as {{LogManager.getLogger()}}).
> So maybe the underlying issue here is that the warning is correct, but there
> is a bug in {{StackLocator}} and it should actually fall back to using {{new
> Throwable().getStackTrace()}} instead of returning {{null}}?
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