EVALUATION

JConsole should display the operation with null as return value such that
it doesn't prevent the user from invoking the operation however it should
log a warning about this fact.
                                     

WORK AROUND

Just use Void.TYPE.getName() instead of <null>
                                     

SUGGESTED FIX

It appears that the <null> value is not supported by JMX specifications for the returnType element of one MBeanOperationInfo.

Two solutions:

1. The JConsole handles this special case and consider the <null> value as a Void type

2. The JConsole tests the returnType element and throw an Exception if it is <null>

The second approach seems best since it would trigger the developer to write a better MBeanInfo while first solution will somewhat "hide" the real problem form the developer.