I am trying to configure a custom layout class to Log4J as described in my previous post. The class uses java.util.regex.Matcher to identify potential credit card numbers in log messages. It works perfectly in unit tests (I can also programmatically configure a logger to use it and produce the expected output). However when I try to deploy it with our app in JBoss, I get the following error:

--- MBEANS THAT ARE THE ROOT CAUSE OF THE PROBLEM ---
ObjectName: jboss.web.deployment:war=MyWebApp-2010_02-SNAPSHOT.war,id=476602902
  State: FAILED
  Reason: java.lang.LinkageError: java/util/regex/Matcher

I couldn't even find any info on this form of the error - typically LinkageError seems to show up with a "loader constrain violation" message, like in here.

Technical details: we use JBoss 4.2, Java 5, Log4J 1.2.12. We deploy our app in an .ear, which contains (among others) the above mentioned .war file, and the custom layout class in a separate jar file. We override the default settings in jboss-log4J.xml with our own log4j.properties located in a different folder, which is added to the classpath at startup, and is provided via Carbon.

I can only guess: are two different Matcher class versions loaded from somewhere, or is Matcher loaded by two different classloaders when it is used from the jar and the war?

What does this error message mean, and how can I fix it?