I'm not sure I get you. Can you post your custom plugin descriptor? With MBeanResourceComponent, availability is determined by the mere presence of the MBean.

You can look at a trait and then alert if the trait value changes.

If the MBean returns a custom class, the agent will basically turn it into a string, but unless the agent has your class it won't be able to deserialize it from a call.

Have CacheStat implement javax.management.openmbean.CompositeData ; you can then extract a property from it. E.g.

            <metric displayName="Used"

                    property="{Usage.used}"

                    displayType="summary"

                    description="Current memory usage"

                    units="bytes"/>

Where Usage is the MBean attribute 'Usage' and used is the field from it.