Deriving Class from Generic T

问题

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.

回答1:

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;
    }

    // ....

}

回答2:

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

来源：https://stackoverflow.com/questions/801940/deriving-class-from-generic-t

标签

java

generics

reflection

derived-class