Mule boot classloader does not find a custom-written Log4j filter included in our code distribution when Mule is started.
Issues java.lang.ClassNotFoundException and crashes.

DETAIL:

We had a need to write a custom Log4J filter to use in our logging process.
We use SLF4J as a façade for Log4J
We know that Mule is using a static bridge JCL->SLF4J
We wrote the filter, configured it in log4j.xml, deployed the application on Mule.

Here is a snippet of log4j.properties

<appender name="MainSystemAppender" class="org.apache.log4j.RollingFileAppender">
<param name="File" value="system.log"/>
<param name="Append" value="true"/>
<param name="MaxFileSize" value="5MB"/>
<param name="MaxBackupIndex" value="5"/>
<layout class="org.apache.log4j.PatternLayout">
<param name="ConversionPattern" value="%-5p %d [%t] %c: %m\n"/>
</layout>

<filter class="org.integrationlayer.util.logging.GenericOutputFilter">
<param name="objectValue" value="HealthCheck"/>
</filter>
</appender>

We absolutely verified that the filter class file is in the jar. Jar is deployed under $MULE_HOME/lib/user. Mule would not start, issues exception:

log4j:ERROR Could not instantiate class [org.integrationlayer.util.logging.GenericOutputFilter].
java.lang.ClassNotFoundException: org.integrationlayer.util.logging.GenericOutputFilter
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClassInternal(Unknown Source)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Unknown Source)
at org.apache.log4j.helpers.Loader.loadClass(Loader.java:178)
at org.apache.log4j.helpers.OptionConverter.instantiateByClassName(OptionConverter.java:319)
at org.apache.log4j.xml.DOMConfigurator.parseFilters(DOMConfigurator.java:279)
at org.apache.log4j.xml.DOMConfigurator.parseAppender(DOMConfigurator.java:200)
at org.apache.log4j.xml.DOMConfigurator.findAppenderByName(DOMConfigurator.java:150)
at org.apache.log4j.xml.DOMConfigurator.findAppenderByReference(DOMConfigurator.java:163)
at org.apache.log4j.xml.DOMConfigurator.parseChildrenOfLoggerElement(DOMConfigurator.java:425)
at org.apache.log4j.xml.DOMConfigurator.parseRoot(DOMConfigurator.java:394)
at org.apache.log4j.xml.DOMConfigurator.parse(DOMConfigurator.java:829)
at org.apache.log4j.xml.DOMConfigurator.doConfigure(DOMConfigurator.java:712)
at org.apache.log4j.xml.DOMConfigurator.doConfigure(DOMConfigurator.java:618)
at org.apache.log4j.helpers.OptionConverter.selectAndConfigure(OptionConverter.java:470)
at org.apache.log4j.LogManager.<clinit>(LogManager.java:122)
at org.slf4j.impl.Log4jLoggerFactory.getLogger(Log4jLoggerFactory.java:73)
at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:88)
at org.apache.commons.logging.impl.SLF4JLogFactory.getInstance(SLF4JLogFactory.java:155)
at org.apache.commons.logging.impl.SLF4JLogFactory.getInstance(SLF4JLogFactory.java:131)
at org.apache.commons.logging.LogFactory.getLog(LogFactory.java:203)
at org.mule.modules.boot.LicenseHandler.<clinit>(LicenseHandler.java:34)
at org.mule.modules.boot.MuleBootstrap.requestLicenseAcceptance(MuleBootstrap.java:135)
at org.mule.modules.boot.MuleBootstrap.prepareBootstrapPhase(MuleBootstrap.java:99)
at org.mule.modules.boot.MuleBootstrap.main(MuleBootstrap.java:62)

It seems that the boot classloader cannot see our application jars sitting in the $MULE_HOME/lib/user. In fact, I attempted to move the jar with the filter into $MULE_HOME/lib/opt without any result

Any and all advice is greatly appreciated. It has been very frustrating to see Classloader issues here.