In JBoss 4.0.4.GA it took me awhile to figure out how to enable trace level debugging.

Typically you could do something like this to enable trace level debugging for a category of classes.

<category  name="com.betweengo.app">
  <priority  value="TRACE"/>
</category>

However JBoss 4.0.4.GA has an older log4j implementation so you need to use JBoss’s custom TRACE level.

<category  name="com.betweengo.app">
  <priority  value="TRACE" class="org.jboss.logging.XLevel"/>
</category>

This is documented in the release notes for JBoss-4.2.1.GA.

Since the latest log4j includes a trace level, there is no need to reference the custom jboss TRACE level in conf/jboss-log4j.xml configs, JBAS-4163.

There is additional information on trace level debugging in the articles Enabling TRACE logging on server and Using Logging.