Composition using JPA and deletion

I have one Role that can refer to a Set of Permissions.

How can I annotate both classes to fit composition (not aggregation) where:

A Permission continues to exist if a Role is deleted ?
A Role continues to exist if it has no Permissions.

P.S: How should this be handled with merging/persisting ?

Solution

This is a ManyToMany relationship annotated with @ManyToMany. If it is required to have a unidirectional ManyToMany then just annotate the member field or getter method (depending on the access type field or property respectively) of the Role class related to Permission. If you need bidirectional relationship then you will have to annotate both classes and specify the mappedBy annotation attribute on the non-owning/target side, the Permission class.

To achieve this relationship a join table will be created if not already exists ( if it exists a @JoinTable may be required to specify exact table name and column names).

Regarding persistence and merging, when a Role persists/merges the associated Permissions will also be persisted/merged and the associated entries in the join table will be created as in a many-to-many relationship the owner side actions are cascaded by default (the owner side is the side without the mappedBy attribute).

So the setup for a bidirectional many-to-many would be something like,

@Entity
public class Role {
....
 @ManyToMany
    private Set<Permission> permissions;
....


@Entity
public class Permission{
.....
@ManyToMany(mappedBy="permissions")
    private List<Role> roles;
....

also check out the spec http://download.oracle.com/otndocs/jcp/persistence-2_1-fr-spec/index.html and this wonderful book http://www.apress.com/9781430219569