Using Hibernate to read DTOs directly

PeterNilsson · **Posted:** Fri Aug 20, 2004 10:56 am

I am evaluating Hibernate to see if we can use it instead of our current ORM tool. One feature we use today that I haven't so far been able to get to work in Hibernate is to read DTOs directly from the database instead of reading entities and then copying data to the DTOs.
I know there has been a lot of discussion on whether to use DTOs or not but skipping the DTOs is not an option for me for legacy reasons.
The DTOs are similar to the entity classes but lack some properties which are only kept in the domain layer.

Example:
There is an entity Car which has a one-to-many association to entity Seat.
These are mapped to one table each, eg Car_Tb and Seat_Tb respectively.

Besides the classes Car and Seat which only exist in the domain layer I need two DTOs: CarTO and SeatTO.
CarTO and SeatTO are mapped to the same tables as Car and Seat, but only the properties needed in the DTOs are mapped.

When loading a DTO I immediately evict it to avoid any flushing:

Code between sessionFactory.openSession() and session.close():

Code:

      Session session = sessionFactory.openSession();
      CarTO dto = (CarTO) session.get(CarTO.class, id);
      session.evict(dto);
      session.close();
      return dto;
 

This works fine besides the collections. The DTOs use Hibernates own collection implementations which make the DTOs unusable in layers that don't know about Hibernate, such as desktop clients.

I have tried to use a PropertyAccessor to replace the collection implementations with the ones from java.util, ie HashSet and ArrayList, but then the evict will not work properly as it seems to rely on Hibernates collection implementations.

What I need is a Session.get() method that automatically evicts the entities and therefore don't need to use special collection implementations.

Any hints on how this can be done?

Peter

Hibernate version: 2.1.4
Mapping documents:

<class name="Car" table="Car_Tb">
<id name="Id" column="CAR_ID" type="int">
<generator class="assigned" />
</id>
<version name="UpdateId" column="Update_Id" type="long" />
<property name="Name" type="string" />
<property name="Type" type="long" column="Type" />
<property name="CreatedWhen" type="timestamp" column="Created_When"/>
<property name="CreatedBy" type="long" column="Created_By" />
<property name="UpdatedBy" type="long" column="Updated_By" />
<property name="UpdatedWhen" type="timestamp" column="Updated_When" />
<set name="Seats" cascade="all-delete-orphan" >
<key column="CAR_FK" />
<one-to-many class="Seat" />
</set>
</class>

<class name="Seat" table="Seat_Tb">
<id name="Id" column="SEA_ID" type="int">
<generator class="assigned" />
</id>
<version name="UpdateId" column="Update_Id" type="long" />
<property name="Color" type="string" />
<property name="Size" type="string" />
<property name="CreatedBy" type="long" column="Created_By" />
<property name="CreatedWhen" type="timestamp" column="Created_When"/>
<property name="UpdatedBy" type="long" column="Updated_By" />
<property name="UpdatedWhen" type="timestamp" column="Updated_When" />
</class>

<class name="CarTO" table="Car_Tb">
<id name="Id" column="CAR_ID" type="int">
<generator class="assigned" />
</id>
<version name="UpdateId" column="Update_Id" type="long" />
<property name="Name" type="string" />
<property name="Type" type="long" column="Type" />
<set name="Seats" cascade="all-delete-orphan">
<key column="CAR_FK" />
<one-to-many class="SeatTO" />
</set>
</class>

<class name="SeatTO" table="Seat_Tb">
<id name="Id" column="SEA_ID" type="int">
<generator class="assigned" />
</id>
<version name="UpdateId" column="Update_Id" type="long" />
<property name="Color" type="string" />
<property name="Size" type="string" />
</class>

anthony · **Posted:** Fri Aug 20, 2004 10:59 am

take a look at projection i.e. "select new DTO(myObj.prop1,myObj.prop2) from Object myObj"

Teg · **Posted:** Fri Aug 20, 2004 11:43 am

Hi Anthony,
is also possible to use collections in the constructor of the DTO?

I have used a HQL

Code:

"select new DataModuleInfo(dm.dmc, dm.type, elements(dm.items)) from DataModuleInfo dm"

where dmc is String, type is char and items is a Set but Hibernate send a

Code:

net.sf.hibernate.PropertyNotFoundException: no appropriate constructor in class: com.caliari.nova.ietm.DataModuleInfo

What type of collection is passed to contructor?

anthony · **Posted:** Fri Aug 20, 2004 11:49 am

i've never tried this... i think you've tried the same type as returned by the query.

Teg · **Posted:** Mon Aug 23, 2004 3:00 am

Yes, is the same type. It can be a problem?
I have defined 2 constructor, a default constructor and a constructor for the projection.

Someone can explain how to use collections in this type of projection?

PeterNilsson · **Posted:** Mon Aug 23, 2004 3:55 am

I'm thinking of using a separate read-only session for reading the DTOs and then using a property accessor for swithing collection implementations.
That way I can avoid the problem with evict requiring Hibernate collection implementations. A separate session is needed because I might modify some Hibernate entites in the same transaction and those I want to flush.

Do you see any problem with this approach?

Code:

      Session session = sessionFactory.openSession(); 
      session.setFlushMode(FlushMode.NEVER);
      CarTO dto = (CarTO) session.get(CarTO.class, id); 
      // No evict needed
      session.close(); 
      return dto; 

set-method in Setter returned by the PropertyAccessor:

Code:

    public void set(Object theTarget, Object theValue) throws HibernateException {
      // Switch collections
      Object value = theValue;
      if (theValue instanceof Set) {
        value = new HashSet((Set) theValue);
      }
      else if (theValue instanceof List) {
        value = new ArrayList((List) theValue);
      } else
        value = theValue;
            
      DTO to = (DTO) theTarget;
      // Generic set-method present in all our DTOs, could of course be replaced by reflection or cglib
      to.set(KEY, value);
    }