As of Mustang b90, in the description of the MXBean annotation, the paragraph
"Reconstructing an instance of Java type J from a CompositeData" lists several cases.
The case #2 is about leveraging the fact some ConstructorProperties annotation have been found.

In case JMX 1.4 would be implemented by a JMX licensee over Java SE 5.0, then ConstrucProperties isn't available. In such a case, it isn't clearly stated what to do.