In the JMX 1.1 RI, an MBeanInfo could be constructed with a null value for the various array parameters to the constructor (MBeanAttributeInfo[] etc). This null value was stored in the constructed object, but a zero-sized MBeanAttributeInfo[] was returned from MBeanInfo.getAttributes().

In the JMX 1.2 RI, the behaviour was changed so that a null MBeanAttributeInfo[] parameter was immediately replaced by a zero-sized array in the constructed object.

The problem is that the 1.2 RI therefore no longer expects an MBeanInfo ever to have a null array in its private fields ("attributes", etc). But if you deserialize an object that was serialized with the 1.1 RI, you can in fact have such a beast. This happens, for example, if you connect to a JDMK 5.0 agent through its RMI or HTTP connectors.