DAO Implementation

The Data Access Object (DAO) pattern is a good practice to implement a persistence layer and it encapsulates data access codes from the business tier. A DAO object exposes an interface to a business object and performs persistence operation relating to a particular persistent entity. Now, we can implement persistence operations like CRUD in a DAO class with JPA and EntityManager injected by Spring.

@Repository
public class TodoDao {

    @PersistenceContext
    private EntityManager em;

    @Transactional(readOnly=true)
    public List<Todo> queryAll() {
        Query query = em.createQuery("SELECT t FROM Todo t");
        List<Todo> result = query.getResultList();
        return result;
    }

    ...

    @Transactional
    public Todo save(Todo todo){
        em.persist(todo);
        return todo;
    }

    @Transactional
    public Todo update(Todo todo){
        todo = em.merge(todo);
        return todo;
    }

    @Transactional
    public void delete(Todo todo){
        Todo r = get(todo.getId());
        if(r!=null){
            em.remove(r);
        }
    }
}

• Line 1: We register TodoDao as a Spring bean with @Repository because it is a DAO class according to Spring's suggestion.
• Line 4: As Spring manages our entity manager factory, it can understand @PersistenceContext and inject a transaction scope EntityManager for us. Hence, we don't need to create EntityManager by our own.
• Line 7: We have enabled Spring's declarative transaction management so that we can apply @Transactional on a methods.

After completing DAO classes, we can inject them to our service class with Spring's @Autowired because they are all Spring beans.

@Service("todoListService")
@Scope(value="singleton",proxyMode=ScopedProxyMode.TARGET_CLASS)
public class TodoListServiceImpl implements TodoListService {

    @Autowired
    TodoDao dao;

    public List<Todo>getTodoList() {
        return dao.queryAll();
    }

    ...
}

Completing the above steps, we have created a dependency relationship among the controller, service, and persistence classes as follows:

Each of these classes encapsulates cohesive functions and has decoupled relationships with others. You can easily expand the architecture by adding more classes or create dependencies between two layers.

You can visit http://localhost:8080/zkessentials to see the result.