One of the key features of Model MBeans is the ability to include
extra metadata in the ModelMBeanInfo and related classes
(ModelMBeanAttributeInfo etc) via a Descriptor parameter to those
classes' constructors. It was originally envisaged that Descriptors
would be more generally useful than just for Model MBeans, which is
why the Descriptor interface is in javax.management rather than
javax.management.modelmbean. It would be very useful to allow
Descriptors to be supplied for other types of MBeans, for example:
- to indicate that an attribute or operation is "disabled" (see
  bug 4786777));
- to provide additional information about an attribute that is
  a metric, for example its units, minimum and maximum values, whether
  it is an instantaneous value or is sampled across a period, etc.;
- to specify that the value of an attribute is constant for a given
  MBean instance;
- ...and it is the same as a named property in the MBean's ObjectName
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