This may seem like a stupid question, especially after years of carefully creating them. Yet these thoughts about DAO arose in my mind when I watched Adam Bien’s Real World JavaEE rerun on Parley’s.
In his talk, Adam says he doesn’t use DAOs anymore - even though he has one ready so as to please architects (them again). My first reaction was utter rejection: layered architecture is at the root of decoupling and decoupling is a requirement for an evolutive design. Then, I digested the information and thought, why not?
Let’s have a look at the definition of the DAO design pattern from Oracle:
Use a Data Access Object (DAO) to abstract and encapsulate all access to the data source. The DAO manages the connection with the data source to obtain and store data.
Back in the old days, either with plain JDBC or EJB, having DAO was really about decoupling.
Nowadays and when you think about that, isn’t it what JPA’s EntityManager is all about?
In effect, most JPA DAO just delegate their base methods to the EntityManager.
public void merge(Person person) {
em.merge(person);
}So, for basic CRUD operations, the point seems a valid one, doesn’t it?
That would be a fool’s conclusion, since the above snippet doesn’t take into account most cases.
What if a Person has an Address (or more than one)?
In web applications, we don’t have the whole object graph, only the Person to be merged, so we wouldn’t merge the entity but load it by it’s primary key and update fields that were likely changed in the GUI layer.
The above snippet should probably look like the following:
public Person merge(Person person) {
Person original = em.find(Person.class, person.getId());
original.setFirstName(person.getFirstName());
original.setLastName(person.getLastName());
em.flush();
return original;
}But what about queries? Picture the following:
public class PersonService {
public List<Person> findByFirstNameAndLastName(String firstName, String lastName) {
CriteriaBuilder builder = em.getCriteriaBuilder();
CriteriaQuery<Person> select = builder.createQuery(Person.class);
Root<Person> fromPerson = select.from(Person.class);
Predicate equalsFirstName = builder.equal(fromPerson.get(Person_.firstName), firstName);
Predicate equalsLastName = builder.equal(fromPerson.get(Person_.lastName), lastName);
select.where(builder.and(equalsFirstName, equalsLastName));
return em.createQuery(select).getResultList();
}
}The findByFirstNameAndLastName() method clearly doesn’t use any business DSL but plain query DSL.
In this case, and whatever the query strategy used (JPA-QL or Criteria), I don’t think it would be wise to use such code directly in the business layer and the DAO still has uses.
Both those examples lead me to think DAO are still needed in the current state of things. Anyway, asking these kind of questions are of great importance since patterns tend to stay even though technology improvements make them obsolete.