I write Struts action with some utils class from http://caveatemptor.hibernate.org/
HibernateUtil and DAO pattern
I use session-per-request session with Hibernate filter as follow

<filter>
<filter-name>hibernateFilter</filter-name>
<filter-class>jq.com.vn.bean.util.HibernateFilter</filter-class>
</filter>

filter method

public void doFilter(ServletRequest request,
ServletResponse response,
FilterChain chain)
throws IOException, ServletException {

// There is actually no explicit "opening" of a Session, the
// first call to HibernateUtil.beginTransaction() in control
// logic (e.g. use case controller/event handler) will get
// a fresh Session.
try {
chain.doFilter(request, response);

// Commit any pending database transaction.
HibernateUtil.commitTransaction();

} finally {

// No matter what happens, close the Session.
HibernateUtil.closeSession();

}
}



Hibernate version:2.1.6

Mapping documents:
<class name="Contents" table="T_CONTENTS" proxy="Contents">
<id name="id" column="CONTENTS_ID" unsaved-value="null">
<generator class="uuid.hex"/>
</id>
<property name="title" not-null="true" column="F_TITLE"/>
<property name="brief" column="F_BRIEF"/>
<property name="content" column="F_CONTENT"/>
<property name="createDate"
column="F_CREATED"
type="java.util.Date"
not-null="true"/>
<property name="status" column="F_STATUS"/>
<many-to-one
name="category"
column="CATEGORY_ID"
class="Category"
not-null="true"
/>
<set name="images"
lazy="true"
table="T_IMAGES"
order-by="F_TITLE asc">
<key column="CONTENTS_ID"/>
<composite-element class="Image">
<property name="title" column="F_TITLE"/>
<property name="description" column="F_DESCRIPTION"/>
<property name="path" column="F_PATH"/>
<property name="thumbnail" column="F_THUMBNAIL"/>
<property name="createDate"
column="F_CREATED"
type="java.util.Date"
not-null="true"/>
</composite-element>
</set>
</class>


DAO method
public void makePersistent(Contents contents)
throws InfrastructureException {

try {
contents.setCreateDate(new Date());
HibernateUtil.getSession().saveOrUpdate(contents);
} catch (HibernateException ex) {
throw new InfrastructureException(ex);
}
}
Struts Action
private ActionForward updateContents(ActionMapping mapping, ActionForm form, HttpServletRequest request, HttpServletResponse response) {
ContentsForm contentsForm=(ContentsForm) form;
ContentsDAO contentsDAO=new ContentsDAO();
Contents contents=new Contents().copyFrom(contentsForm);
contentsDAO.makePersistent(contents);
return mapping.findForward(Constant.SUCCESS);
}

when i new instant of object, it's work
but if I delete or update, thrown exception:
net.sf.hibernate.NonUniqueObjectException: a different object with the same identifier value was already associated with the session


Anyone give the solution?

not sure if this helps you out ... from hibernate API for the exception you are getting ...

This exception is thrown when an operation would break session-scoped identity. This occurs if the user tries to associate two different instances of the same Java class with a particular identifier, in the scope of a single Session.

Thank for reply soon
I know that is common problem, the reason I known, but the solution for Struts action not work yet