For people who use the "shorthand" form of xmbean descriptors and embed it directly in their -service.xml descriptor here is a quick hint. This is something that regularly trips me up at least.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE server
    PUBLIC "-//JBoss//DTD MBean Service 3.2//EN"
    "http://www.jboss.org/j2ee/dtd/jboss-service_3_2.dtd">
 
<server>
    <mbean code="org.jboss.mail.MailListenerChainService"
           name="mail.test:service=mlchain">
           <xmbean>
   <class>org.jboss.mail.MailListenerChainService</class>     
      <constructor>
       <description>The no-arg constructor</description>
       <name>org.jboss.mail.MailListenerChainService</name>
      </constructor>
               
   <attribute access="read-write" 
                                         setMethod="setListeners" 
                                         getMethod="getListeners">
     <name>Listeners</name>
 <type>org.w3c.dom.Element</type>
   </attribute>        
           
           <attribute access="read-only" getMethod="getNumberListeners">
            <name>NumberListeners</name>
            <type>org.w3c.dom.Element</type>
           </attribute>
           
           <operation>
            <name>processMail</name>
            <parameter>
            <name>mail</name>
            <type>org.jboss.mail.message.Mail</type>
            </parameter>
            <return-type>org.jboss.mail.message.Mail</return-type>
           </operation>
           
               &defaultOperations;
           </xmbean>          
    </mbean>
</server>

The problem is that for the JBoss deployer to detect that "yes, it is an xmbean" and not a "regular mbean" it requires the xmbean-dd attribute on the "mbean" element even though you've embedded the xmbean element. If you forget it (like I did above) you get a very unsatisfying "org.jboss.deployment.DeploymentException: Class does not expose a management interface: java.lang.Object;".

meaning:

...
<server>
    <mbean code="org.jboss.mail.MailListenerChainService"
           name="mail.test:service=mlchain" xmbean-dd="">
...