Tiger powered hibernate template

[titiwangsa]

hello to all
since 90% of my dao are sort of boiler plate codes,
i'm trying to use Generics

my attempt to add

	public K add(final T entity) throws DataIntegrityViolationException {
		try {
			final Serializable id = getHibernateTemplate().save(entity);
			getHibernateTemplate().flush();
			return (K) id;
		} catch (InvalidStateException e) {
			getHibernateTemplate().clear();
			throw new DataIntegrityViolationException("failed validation", e);
		}
	}

this looks ok to me.

question is i'm having some problems with get

	public T get(final K key) {
		final Class<T> entityClass = T;
		return (T) getHibernateTemplate().get(entityClass, key);
	}

this does not compile

note:
T is Type of entity
K is Key for entity

for example
T = Student
K = String, (student's id class)

[karldmoore]

There is a JIRA issue about a 1.5 HibernateTemplate, it might be worth reading this however. Is it possible to see your exception?
http://blog.interface21.com/main/200...e-of-required/

[lloyd.mcclendon]

	public T get(final K key) {
		final Class<T> entityClass = T;
		return (T) getHibernateTemplate().get(entityClass, key);
	}

yeah T really means nothing. are you at all familiar with what generics really are and the concept of erasure? when you compile this, T and K get text substitued with Object. At runtime all generic information is erased, it's only a compile time facility. If the program compiles it must be safe so they throw out all that stuff. As a result you can't instiantate T or refer to it like that - it is not a Class, just a marker for an unknown (possibly bound) type reference.

if you need to reference the class of T, you have two options: pass it in as a constructor or always subclass this thing like this

option 1:

class GenericDAOImpl<E extends Entity> implements DAO<E>  {
    private Class<E> entityClass;

    public GenericDAOImpl(Class<E> entityClass)  {
        this.entityClass = entityClass;
    }

    public <PK extends Serializable> E get(PK pk)  {
        @SuppressWarnings("unchecked") // hibernates fault, they are still jdk 1.4
        E entity = (E) getHibernateTemplate().get(getEntityClass(), pk);
        return entity;
    }

option 2, a clever little obscure hack, i'm not even sure why this works:

class GenericDAOImpl<E extends Entity> implements DAO<E>  {
    private Class<E> entityClass;

    public GenericDAOImpl(Class<E> entityClass)  {
        this.entityClass = ((ParameterizedType) getClass().getGenericSuperclass())
                 .getGenericTypeArguments()[0];  // pull it with reflection
    }

    public <PK extends Serializable> E get(PK pk)  {
        @SuppressWarnings("unchecked") // hibernates fault, they are still jdk 1.4
        E entity = (E) getHibernateTemplate().get(getEntityClass(), pk);
        return entity;
    }
}

class BookDAOImpl extends GenericDAOImpl<Book>  { }

// you can also use anonymous classes but if you try to use it directly it will break
new GenericDAOImpl<Book>()  { }

have you seen this article http://www.ibm.com/developerworks/ja...enericdao.html

if you search this & the hibernate forum for generic dao there is TONS of various implementations out there. we have ours very well figured out now it is great. Try to design it so that the client applications have to do minimal typing to talk to the database. also i have found the concept of java's generics absolutely fascinating and have embraced them a good bit. They are quite tricky but once you know what you are doing amazingly powerful. One of these days they will erase the concept of erasure and even more cool stuff will be possible. There's lots of good reading out there. This blog has some good stuff, lots of links to good reading too http://blogs.sun.com/ahe/

[karldmoore]

Here's the link to the JIRA issue.
http://opensource.atlassian.com/proj...rowse/SPR-3128