Deriving Class from Generic T

Question

I have a parameterized hibernate dao that performs basic crud operations, and when parameterized is used as a delegate to fulfil basic crud operations for a given dao.

public class HibernateDao <T, ID extends Serializable> implements GenericDao<T, ID>

I want to be able to derive Class from T at runtime to create criteria queries in Hibernate, such that:

public T findByPrimaryKey(ID id) { return (T) HibernateUtil.getSession().load(T.getClass(), id); }

I know:

T.getClass()

does not exist, but is there any way to derive the correct Class object from T at runtime?

I have looked at generics and reflection but have not come up with a suitable solution, perhaps I am missing something.

Thanks.

Adam Paynter · Accepted Answer · 2009-04-29 11:38:29Z

17

You could have the Class passed as a constructor argument.

public class HibernateDao <T, ID extends Serializable> implements GenericDao<T, ID> { private final Class<? extends T> type; public HibernateDao(Class<? extends T> type) { this.type = type; } // .... }

answered Apr 29, 2009 at 11:38

Adam Paynter

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4 Comments

bowsie Over a year ago

Yes this is one solution I have. I would prefer to be able to derive the class from T if possible, without adding the Class dependency.

Adam Paynter Over a year ago

@bowsie: I understand your pain. Unfortunately, the compiler completes erases all traces of T. It is called type erasure: java.sun.com/docs/books/tutorial/java/generics/erasure.html

Nishant Over a year ago

@AdamPaynter it's really hard to explain erasure to a newbie why can you use the <T> that you see right in front of you.

Adam Paynter Over a year ago

@Nishant: You're right. I was in the same situation when I was learning Java. It seemed like it should be there! A violation of the principle of least astonishment. :)

rdk · Accepted Answer · 2009-05-24 22:28:42Z

There is the way how to figure out class of type argument T using reflection:

private Class<T> persistentClass = (Class<T>) ((ParameterizedType)getClass().getGenericSuperclass()).getActualTypeArguments()[0];

Here is the way how I use it:

public class GenericDaoJPA<T> implements GenericDao<T> { @PersistenceContext protected EntityManager entityManager; protected Class<T> persistentClass = figureOutPersistentClass(); private Class<T> figureOutPersistentClass() { Class<T> clazz = (Class<T>)((ParameterizedType) (getClass().getGenericSuperclass())).getActualTypeArguments()[0]; log.debug("persistentClass set to {}", clazz.getName()); return clazz; } public List<T> findAll() { Query q = entityManager.createQuery("SELECT e FROM " + persistentClass.getSimpleName() + " e"); return (List<T>) q.getResultList(); } }

I suppose this only works when your ConcreteEntityDao is a direct superclass of HibernateDao<ConcreteEntity,...>.

I've found it here: www.greggbolinger.com/blog/2008/04/17/1208457000000.html

this only works in method (stack location) that creates the parameterized object. It doesn't work if you receive the parameterized object from another method.
I'm not sure I understand. Could you explain it in a little bit more detail? Are there any implications for web application development?
This works great when you subclass the generic class directly. If you have multiple levels of inheritance, you'll need to account for them in figureOutPersistentClass by using getSuperClass.

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