I want to do a mapping like:

<set name="myEnums">
<key column="parent_id"/>
<element type="com.foo.MyEnum"/>
</set>

where MyEnum implements PersistentEnum and is modeled like the example in the ref manual.

Assuming that the rules for <composite-element> inside a collection also applies to <element> inside a collection, will I get issues when using the MyEnum class in different sets in different classes due to sharing of the MyEnum instances?? (the instances actually used are singletons defined inside the MyEnum class itself and therefore shared between all sets)..

Not as far as I am aware.
Hibernate will resolve the object reference to the singleton instance using static fromInt() provided by you, in your class implementing the PersistentEnum interface - (similar to that of the serialisation readResolve() method). Note that is method is not part of the interface as the return value is dependent on the class implementing the method.

Specifically which rules are you referring to?
Justin

section 9.5 of the ref. manual states the following:
"composite elements may not own collections, and they should not be the child of any entity other than the unique parent"...

using singletons (which the PersistentEnum implementations are) means breaking the second rule.. unless "they" refers to the database rows and not the objects....

PersistentEnums are a value type. They are not entities. The manual uses very strict language, you should read it again.

right...but as far as I remember, it does not say anything about the composite elements being value types, and the quote from section 9.5 only talks about the parent being an entity. But what is the conclusion here? That composite elements are always a value type?

thanks,
Hoover

All components are always value types, no matter how they appear.