[hibernate-issues] [Hibernate-JIRA] Closed: (HHH-2477) lazy fetching ManyToOne produces inproper proxies when using single table inheritance strategy

[Chris Bredesen (JIRA)]

     [ http://opensource.atlassian.com/projects/hibernate/browse/HHH-2477?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Chris Bredesen closed HHH-2477.
-------------------------------

    Resolution: Rejected

S. Schnabl correctly quotes HIA and Adam T. correctly quotes the manual.  This is unfixable because at the time the proxy is generated, Hibernate has no knowledge of what the concrete implementation might be.  Once the proxy is hydrated however, the implementation will be the correct concrete class and as such the instances *behavior* will be correct.

If you must have castable/instanceof then you'll need to load() from Session and pass the concrete class so that the resulting instance is what you expect.  Note that a database hit will not occur since the instance is already in Session.

You can also use the Visitor pattern as described here:  http://www.hibernate.org/280.html

> lazy fetching ManyToOne produces inproper proxies when using single table inheritance strategy
> ----------------------------------------------------------------------------------------------
>
>                 Key: HHH-2477
>                 URL: http://opensource.atlassian.com/projects/hibernate/browse/HHH-2477
>             Project: Hibernate3
>          Issue Type: Bug
>          Components: core
>    Affects Versions: 3.2.1
>         Environment: hibernate 3.2.1-GA, postgresql 8.1
>            Reporter: Daniel Nguyen
>         Attachments: bug.zip
>
>   Original Estimate: 1 hour
>  Remaining Estimate: 1 hour
>
> I have entity base class Vehicle and entity class Car which extends Vehicle (using single table inheritance strategy).
> I also have entity class User which have single-ended @ManyToOne relation from User to Vehicle with lazy fetching.
> Now there is a Car instance "c" and User instance "u" with relation to "c". Lazily fetching vehicle from "u" results with proxy object "o" which is instanceof Vehicle but not instanceof Car as expected. So it's impossible cast "o" to Car or at least read any of Car's property from "o".
> This is serious problem when inheritance is extensively used because inproper proxy object remains in cache. To walkaround I'd have to resign with lazy fetching (much performance loss) or manually replace proxy object in cache.
> testing code:
>     SessionFactory sf = null;
>     Session s = null;
>     Transaction tx = null;
>     try
>     {
>       sf = cfg.buildSessionFactory();
>       s = sf.openSession();
>       tx = s.beginTransaction();
>       
>       //create car
>       Car car = new Car();
>       car.setIdVehicle(1);
>       car.setAge(5);
>       s.save(car);
>       
>       //create user of car
>       User user = new User();
>       user.setIdUser(1);
>       user.setVehicle(car);
>       s.save(user);
>       
>       //make sure it is actually added
>       s.flush();
>       // test 1 - works ok, because we loaded vehicle before user
>       s.clear(); //clear cache
>       Vehicle vv = (Vehicle)s.get(Vehicle.class, 1);
>       User uu = (User)s.get(User.class, 1);
>       //we know that user vehicle is actually a Car
>       Car cc = (Car)uu.getVehicle();
>       
>       // test 2 - fails, when using lazy fetching
>       s.clear(); //clear cache
>       User u = (User)s.get(User.class, 1);
>       Vehicle v = u.getVehicle();
>       //we know that user vehicle is actually a Car
>       //so we cast to Car but ClassCastException is raised!!!
>       Car c = (Car)v;
>       //let's check what's actual class of v
>       System.out.println(v.getClass().getName());
>       System.out.println(v.getClass().getSuperclass().getName());
>       //result:
>       //  test.hibernate.Vehicle$$EnhancerByCGLIB$$12ce1883
>       //  test.hibernate.Vehicle
>       //but I expected:
>       //  test.hibernate.Car$$EnhancerByCGLIB$$xxxxx
>       //  test.hibernate.Car
>     }
>     finally
>     {
>       if (tx!=null)
>         tx.rollback();
>       if (s!=null)
>         s.close();
>       if (sf!=null)
>         sf.close();
>     }

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