A key to a piece of metadata associated with a Component at runtime. The key contains type information that can be used to check the type of any metadata value for the key when the value is set on the given Component. MetaDataKey is abstract in order to force the creation of a subtype. That subtype is used to test for identity when looking for the metadata because actual object identity would suffer from problems under serialization. So, the correct way to declare a MetaDataKey is like this: public static MetaDataKey ROLE = new MetaDataKey(Role.class) { }