使用Hibernate模板插入数据

Question

Consider the following:

User.java

class User{
    ...
    private Set<Community> communities = new HashSet<Community>();
    ...
}

Community.java

class Community{
    ...
    private Set<User> users = new HashSet<User>();
    ...
}

User.hbm.xml

<set name="communities" cascade="save-update" lazy="false" table="tbl_user_community">
        <key column="user_id" />
        <many-to-many class="Community" column="community_id"/>
     </set> 

Community.hbm.xml

<set name="users" lazy="false" cascade="save-update" table="tbl_user_community" inverse="true">
    <key column="community_id" />
    <many-to-many class="User" column="user_id"/>
</set>

both has many-to-many relationship.

Code for adding community to user:

HibernateTemplate template = null;
SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
template = new HibernateTemplate(sessionFactory);

User user = template.get(User.class, "100");
Community community = template.get(Community.class,1);



user.getCommunities().add(community);


template.saveOrUpdate(user);

Scenario:

1. community1 is assigned to user1 (inserted into database)
2. community2 is assigned to user1 (inserted into database)
3. community1 is assigned to user2 (inserted into database)
4. community2 is assigned to user2 (not inserted into database, throws NonUniqueObjectException )
5. if i use template.merge(user) instead of template.saveOrUpdate(user) .. it works .. why???

Answer 1

saveOrUpdate() and update() take a detached object and attach this object instance to the session. So if an entity with the same type and ID is already attached to the session, Hibernate can't attach another instance: it would break the guarantee that only one entity with a given ID exists in a session. So it throws this exception.

merge() is different. It takes an detached entity, loads the entity with the same ID in the session if not already loaded, and copies the state (ie the fields) of the detached entity to the attached one. The detached entity stays deteached and unmodified.

So, in general, merge() should be preferred. BTW, the other methods don't exist in the JPA API. If you use saveOrUpdate(), you should generally call it as the first instruction of the transaction, to avoid being in the situation causing the NonUniqueException.