为何使用Facade模式进行EJB会话bean [英] Why use Facade pattern for EJB session bean

问题描述

我想询问当访问EJB Session Bean时使用Facade Pattern的原因是什么。在我的Netbeans 6.9.1中，如果我执行新的> 实体类的会话Bean ，并且说我选择用户实体，则Netbeans会生成此代码

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);
    }        
}

我想问一下这个的好处。如果我有10个实体，那么Netbeans会生成10个Facade类和AbstractFacade。这似乎对我来说太过分了。让我们说我的托管bean里面的某个地方，我必须坚持 a 用户和学校然后我这样做

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);    
}

这是正确的事吗？ / p>

Is this the right things to do?

推荐答案

使用EJB的一点是它们通过注释（或XML配置）提供了声明性事务和安全性等功能。如果不使用这些功能，那么EJB就没有任何意义。

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.

此外，该自动生成的外观代码是jsut的起点（而不是一个）。 EJB应该形成域API，而不是所有单个CRUD操作的包装器。它们应该仅包含您实际想要在域模型上执行的操作，并且其中许多操作应跨越需要在事务中执行的多个CRUD操作。例如：

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);
}

应用服务器将执行事务中的所有操作，确保例如你不能因为某种原因而抛出异常的情况，最后你会从一个Schoold中删除但不被添加到另一个的用户。

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.

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