Generic Hibernate dao

Question

Hi, I have been working with a generic HibernateDao to which I pass the entity type through a constructor parameter, and it works perfectly. I ran into a [SpringSource team blog|http://blog.springsource.com/2006/09/29/exploiting-generics-metadata/] by Rob Harrop that explained how to determine the actual type of the parameter at runtime. I also found a [page on the Hibernate documentation|http://www.hibernate.org/328.html] that explained the same concept. I used Robs version for my own GenericDao:
public class HibernateDao<E> implements GenericDao<E> {
    private SessionFactory sessionFactory;
    private Class<E> entityClass;
    public HibernateDao() {
        entityClass = extractTypeParameter(getClass());
   // All sorts of DAO  methods
   private Class extractTypeParameter(Class<? extends GenericDao> genericDaoType) {
        Type[] genericInterfaces = genericDaoType.getGenericInterfaces();
        // find the generic interface declaration for GenericDao<E>
        ParameterizedType genericInterface = null;
        for (Type t : genericInterfaces) {
            if (t instanceof ParameterizedType) {
                ParameterizedType pt = (ParameterizedType)t;
                if (GenericDao.class.equals(pt.getRawType())) {
                    genericInterface = pt;
                    break;
        if(genericInterface == null) {
            throw new IllegalArgumentException("Type '" + genericDaoType
               + "' does not implement GenericDao<E>.");
        return (Class)genericInterface.getActualTypeArguments()[0];
}and ran into a ClassCastException: sun.reflect.generics.reflectiveObjects.TypeVariableImpl cannot be cast to java.lang.Class for the line with the return statement. It seems they both implement the Type interface, but they are no subclasses of each other. These articles must have been read and tried by numerous people and I found other sites that implement the same thing, what am I doing wrong to get this exception?
Edited by: Peetzore on 9-feb-2009 14:45

Answer 1

OK, I've also encountered the same problem, investigated it and finally got into solution.
The trick with getActualTypeArguments() being of type Class and not TypeVariable works when you instantiate the type argument statically, not dynamically.
Statically here means:
class SomeEntityHibernateDAO extends HibernateDAO<SomeEntity> { ... }
SomeEntityHibernateDAO dao = new SomeEntityHibernateDAO();and dynamically:
HibernateDAO<SomeEntity> dao = new HibernateDAO<SomeEntity>();In the first case, the "actual type argument" (like in getActualTypeArgument) of E in your code gets bound to Class<SomeEntity>, in the second - to TypeVariable.
It may be the case, that the desired Class object is further extractable from the TypeVariable (look at its methods), but simply what you may want is to define separate classes for all entities (statically binding the type parameter) and use these instead of generic declarations.
Hope it helps.
Cheers,
Jarek