Why use Facade pattern for EJB session bean

问题:

I want to ask what is the reason to use Facade Pattern when access EJB Session Bean. In my Netbeans 6.9.1, if I do New > Sessions Bean for Entity Classes, and let say that I select User entity, then Netbeans would generate this code

AbstractFacade.java
public abstract class AbstractFacade<T> {
    private Class<T> entityClass;

    public AbstractFacade(Class<T> entityClass) {
        this.entityClass = entityClass;
    }

    protected abstract EntityManager getEntityManager();

    public void create(T entity) {
        getEntityManager().persist(entity);
    }

    public T edit(T entity) {
        return getEntityManager().merge(entity);
    }

    public void remove(T entity) {
        getEntityManager().remove(getEntityManager().merge(entity));
    }

    public T find(Object id) {
        return getEntityManager().find(entityClass, id);
    }

    public List<T> findAll() {
        javax.persistence.criteria.CriteriaQuery cq = getEntityManager().getCriteriaBuilder().createQuery();
        cq.select(cq.from(entityClass));
        return getEntityManager().createQuery(cq).getResultList();
    }

    public List<T> findRange(int[] range) {
        ...
    }

    public int count() {
        ...
    }

AND

UserFacade.java    

package com.bridgeye.ejb;

import javax.ejb.Stateless;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;

@Stateless
public class UserFacade extends AbstractFacade<User> {
    @PersistenceContext(unitName = "Bridgeye2-ejbPU")
    private EntityManager em;

    @Override
    protected EntityManager getEntityManager() {
        return em;
    }

    public UserFacade() {
        super(User.class);
    }        
}

I want to ask what is the benefit of this. If I have 10 entities, then Netbeans would generated 10 Facade classes plus the AbstractFacade. This seems to be overkill to me. Let say somewhere inside my managed bean, i have to persist a User and School then I have do this

someManagedBean.java

...
@EJB
private UserFacade userEJB;

@EJB
private SchoolFacade schoolEJB;

...

public void someMethod(User user, School school){
    ...
    userEJB.create(user);
    schoolEJB.create(school);    
}

Is this the right things to do?

回答1:

The point of using EJBs at all is that they provide features such as declarative transactions and security via annotations (or XML configurations). If you don't use those features, there is no point in having EJBs.

Additionally, that autogenerated facade code is jsut a starting point (and a bad one). The EJBs should form a domain API, not a wrapper for all individual CRUD operations. They should only contain the operations you actually want to be performed on your domain model, and many of them should span several CRUD operations that need to be performend within a transaction. For example:

@TransactionAttribute
public void transferUser(User u, School from, School to){
    from.getUsers().remove(u);
    to.getUsers().add(u);
    u.setSchool(to);
    getEntityManager().merge(from);
    getEntityManager().merge(to);
    getEntityManager().merge(u);
}

The app server will execute all operations within a transaction, which ensures, for example, that you cannot have a situation where an exception is thrown for some reason and you end up with User that is removed from one Schoold but not added to the other one.

回答2:

Yes and no. The Facade pattern makes a lot of sense, but having a separate facace per domain object makes no sense.

You will want to group facades per groups of functionality of domain objects. Imagine a billing system. It has bills, items, customer, addresses. So you would there perhaps a Facade for bill handling (adding items, setting customer, printing, marking as paid) and a different facade for creation and update of users, associating with addresses and so on.

回答3:

I think you should have one AbstractFacade for typical CRUD operations as you can see and many SpecificFacade for manipulating a given entity. So you can not reimplement the same basic logic many times just one... and concentrate only on more complex logic (transactions) associated with entities.