Author: fix

As I wrote earlier, last week I did a presentation on Hibernate as a good speaker I investigate my subject. During my investigation I came a cross the project that is being done for the Hibernate in Action second edition. In this book they used some generics for creating the CRUD DAO objects and it really opened my eyes. For me generics was a nice to have gadget for typecasting my collections, I had no idea this could be done with it. So read and be amazed!!

You have an interface like this:

public interface GenericDAO<T , ID extends Serializable>{
    T findById(ID id, boolean lock);
    List<T> findAll();
    List<T> findByExample(T exampleInstance, String... excludeProperty);
    T makePersistent(T entity);
    void makeTransient(T entity);
    void flush();
}

you have an basic implementation of this interface:

public abstract class GenericHibernateDAO<t , ID extends Serializable> implements GenericDAO<t , ID> {
    private Class<T> persistentClass;
    private Session session;
    public GenericHibernateDAO() {
        this.persistentClass = (Class<T>) ((ParameterizedType) getClass()
                                .getGenericSuperclass()).getActualTypeArguments()[0];
     }
    @SuppressWarnings("unchecked")
    public void setSession(Session s) {
        this.session = s;
    }
    protected Session getSession() {
        if (session == null)
            throw new IllegalStateException("Session has not been set on DAO before usage");
        return session;
    }
    public Class<T> getPersistentClass() {
        return persistentClass;
    }
    @SuppressWarnings("unchecked")
    public T findById(ID id, boolean lock) {
        T entity;
        if (lock)
            entity = (T) getSession().load(getPersistentClass(), id, LockMode.UPGRADE);
        else
            entity = (T) getSession().load(getPersistentClass(), id);

        return entity;
    }
    @SuppressWarnings("unchecked")
    public List<T> findAll() {
        return findByCriteria();
    }
    @SuppressWarnings("unchecked")
    public List<T> findByExample(T exampleInstance, String[] excludeProperty) {
        Criteria crit = getSession().createCriteria(getPersistentClass());
        Example example =  Example.create(exampleInstance);
        for (String exclude : excludeProperty) {
            example.excludeProperty(exclude);
        }
        crit.add(example);
        return crit.list();
    }
    @SuppressWarnings("unchecked")
    public T makePersistent(T entity) {
        getSession().saveOrUpdate(entity);
        return entity;
    }
    public void makeTransient(T entity) {
        getSession().delete(entity);
    }
    public void flush() {
        getSession().flush();
    }
    /**
     * Use this inside subclasses as a convenience method.
     */
    @SuppressWarnings("unchecked")
    protected List<T> findByCriteria(Criterion... criterion) {
        Criteria crit = getSession().createCriteria(getPersistentClass());
        for (Criterion c : criterion) {
            crit.add(c);
        }
        return crit.list();
   }

}

And you have a descendent of this base class

public class BillingDetailsDAOHibernate
        extends     GenericHibernateDAO<Billingdetails , Long>
        implements  BillingDetailsDAO {

    @SuppressWarnings("unchecked")
    public List<Billingdetails> findConcrete(Class concreteClass) {
        return getSession().createCriteria(concreteClass).list();
    }
}

In short the descendent tells his parent Substitute all you T with BillingDetails and your Id with Long and abracadabra you have a simple CRUD dao for your billingDetails. Now how’s that for some fancy programming.

At the moment I’m rewritting my DAO objects to return generic collections, maybe in the next refactoring cycle I’ll use this design pattern.

My bet is that with the rise of generics, we’ll see more and more design patterns that are based on generics as this is very powerfull. The example above will surely be used in more projects to come!!

No reaction about my JUG, guess nobody is interested in learning Java. 🙁

ps sorry if wordpress f*cked up the layout, but still check it out

Yesterday evening we did an internal workshop. I explained Hibernate a bit Jacob (a collegeau) also shared his experiences on the Hibernate topic as we implemented Hibernate in a totally different way. Afterwards Jacob gave a session on Spring Webflow. I have to say it’s been a while since I went to programmers workshop. Our l2u meetings are not actually coding, more setting up and explaing new stuff. But hey I’m a coder, not an administrator but as most geeks I know my way around administration as well but it’s not my core business 🙂

I spend quite some time preparing the workshop, I even listened to a session of gavin himself recorded on a mp3 player. (I’ll put it up later.) I made a small example and then I did a pojo with 2 relationships to other tables. While preparing this workhop it gave me the opportunity to take a closer look at the things we don’t use in our project. .e.g. we always use HQL to retrieve our objects. Never do we use the Criteria classes way, it comes down to the same thing but it’s more Object oriented then an EJBQL like query 🙂

Although Jacob his session was rather short (due to our enthousiasm) it was really interesting. I had read the spring book, but I can’t remember much from the web flow framework besides it being a continution framework. I have to say it was a lot to take at once but it’s really really nice.
As most of you might know one of the common problems in J2EE web application is that if a user starts at page A, go’s to page B, go’s to page C and then hits the back button 2 times, changes something in form A and submits it. We actually have a merge of page C with page A. This is a problem and this is where a continuation framework comes to the rescue.
It holds some kind of version of the submited paged and if you your back button a couple of times and then submit the serverside object’s state will correspond as if you had never gone any further in your workflow. You actually went back in your workflow.
He also explained that you can define flows. Let’s say you have a OrderFlow,SearchProductFlow and SearchCustomerFlow. You can start your OrderFlow and when you come to the state where the product should get added you leavy your OrderFlow and start a subflow, the SearchProductFlow. Once this flow has finished (and the user has his selected products) we return those products into the OrderFlow and we go on from the point we left it. You then go a couple of steps further and come to the point you have to select a Customer, then you start another subflow, SearchCustomerFlow,… you get the point.

I’m not quite sure, but I believe Ruby on Rails also has this feature. I bet there are more projects out there that do this, but in Java this was rather hard to do. until now offcourse

On another note I was talking to a friend of mine, I told him I was going to give a talk on Hibernate. He told me would have loved to join, but it was an internal workshop. I talked to my boss about this and kinda proposed to make a more open thing. Something like a jug. I know we have Bejug but I haven’t seen much events off them and on their site isn’t much info about them, therefor I would like to ask if any of the readers of this entry who are interested in joining such a jug would send me an email (fix dot pczone dot be). who knows if we can get a small community together we could actually do a session at least once a month. Probably in the future 1 topic per session will be done as 2 topics were really to much 🙂

So Axel or Tom who’s next 😉