I wonder if it is possible to create a many-to-many relationship between an object type and an enum type. There seems to be a more general obstruction with retrieving the list of enum values bound to object, because we cannot have a public constructor in enum type

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My Hibernate version is 3.6.3.

When I access the roles enum set in user object type I get the following error:

Exception in thread "main" org.hibernate.InstantiationException: No default constructor for entity: foo.MyRole
at org.hibernate.tuple.PojoInstantiator.instantiate(PojoInstantiator.java:107)
at org.hibernate.tuple.PojoInstantiator.instantiate(PojoInstantiator.java:123)
at org.hibernate.tuple.entity.AbstractEntityTuplizer.instantiate(AbstractEntityTuplizer.java:603)
at org.hibernate.persister.entity.AbstractEntityPersister.instantiate(AbstractEntityPersister.java:3911)
at org.hibernate.impl.SessionImpl.instantiate(SessionImpl.java:1422)
at org.hibernate.impl.SessionImpl.instantiate(SessionImpl.java:1411)
at org.hibernate.loader.Loader.instanceNotYetLoaded(Loader.java:1446)
at org.hibernate.loader.Loader.getRow(Loader.java:1355)
at org.hibernate.loader.Loader.getRowFromResultSet(Loader.java:611)
at org.hibernate.loader.Loader.doQuery(Loader.java:829)
at org.hibernate.loader.Loader.doQueryAndInitializeNonLazyCollections(Loader.java:274)
at org.hibernate.loader.Loader.loadCollection(Loader.java:2166)
at org.hibernate.loader.collection.CollectionLoader.initialize(CollectionLoader.java:62)
at org.hibernate.persister.collection.AbstractCollectionPersister.initialize(AbstractCollectionPersister.java:627)
at org.hibernate.event.def.DefaultInitializeCollectionEventListener.onInitializeCollection(DefaultInitializeCollectionEventListener.java:83)
at org.hibernate.impl.SessionImpl.initializeCollection(SessionImpl.java:1863)
at org.hibernate.collection.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:369)
at org.hibernate.collection.AbstractPersistentCollection.read(AbstractPersistentCollection.java:111)

The object is MyUser and it has a list of MyRole's of enum type.

@Entity
public enum MyRole {
@Enumerated(EnumType.ORDINAL)
USER
,
@Enumerated(EnumType.ORDINAL)
ADMIN;

@Id
public int getRoleId() {
return ordinal();
}

public void setRoleId(int roleId) {
}

}



@Entity
public class MyUser {

private long userId;

@OneToMany(cascade=CascadeType.ALL)
private Set<MyRole> roles = new HashSet<MyRole>();

...

}

I define the many-to-many relationship in the User mapping file:
<set name="roles" table="MYUSER_MYROLE" cascade="all">
<key column="USERID" />
<many-to-many column="ROLEID" class="foo.MyRole" />
</set>

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check it.