DAO layer – Generics to the rescue

Gal LevinskyAugust 8th, 2012Last Updated: October 22nd, 2012

0 55 2 minutes read

Generics can be a powerful tool to create reusable code with the power of compile time verification (type safety..).

Unfortunately I feel the main stream developers still afraid of it.

However, in analogy to Hagrid’s spiders I would say that Generics are seriously misunderstood creatures… :-)

I hope the following example will demonstrate how useful they can be.

The Problem – DAO (Data Access Objects) classes have common methods such as save, update, delete, loadAll.. which are required in every DAO class.

Writing a base class with these common methods and making every DAO object extend it, is simply not enough since each DAO class represents a different domain class and therefore the type used in the common methods’ signature is different (although implementation is similar), for example:

class OrderDAO {
//save method receive an Order
public void save(Order order){....}
//getAll method return Orders List
public List<Order> getAll(){...} 
}
 
class UserDAO{
//save method receive an User
public void save(User user){....}
//getAll method return Users List
public List<User> getAll(){...}
}

How Generics can help us create a base class with a common implementation and yet, keep method signature type-safety?

First, we need to define an interface with the common methods:

/**
 * Base interface for CRUD operations and common queries
 */
public interface IDaoBase<T> {
     
    public List<T> loadAll();
     
    public void save(T domain);
         
    public void update(T domain);
         
    public void delete(T domain);
     
    public T get(Serializable id);
     
    /**
     * Get list by criteria
     * @param detachedCriteria the domain query criteria, include condition and the orders.
     * @return
     * 
     */
    public List<T> getListByCriteria(DetachedCriteria detachedCriteria);
     
    public List<T> getListByCriteria(DetachedCriteria detachedCriteria, int offset, int size);    
}

Please note that we utilize generics so each method signature has a type T, which in the implemented DAO classes, will be a concrete type, per domain.

The second step is to create an abstract class which implements the common functionality:

public abstract class DaoBase<T> extends HibernateDaoSupport implements IDaoBase<T> {
    private Class<T> entityClass;
     
    @Autowired
    public void setSession(SessionFactory sessionFactory){
        this.setSessionFactory(sessionFactory);
    }
         
    public DaoBase() {
         
        entityClass = (Class<T>) ((ParameterizedType) getClass()
                .getGenericSuperclass()).getActualTypeArguments()[0];
    }
 
        public List<T> loadAll(){
        return getHibernateTemplate().loadAll(entityClass);
    }
 
    public void delete(T domain) {
        getHibernateTemplate().delete(domain);
    }
 
    public void save(T domain) {
        getHibernateTemplate().saveOrUpdate(domain);
         
    }
 
    public void update(T domain) {
        getHibernateTemplate().merge(domain);
    }
     
     
        public T get(Serializable id) {
        T o = (T) getHibernateTemplate().get(entityClass, id);
        return o;
    }
 
    public List<T> getListByCriteria(DetachedCriteria detachedCriteria,
            int offset, int size) {
        return getHibernateTemplate().findByCriteria(detachedCriteria, offset, size);
    }
     
    public List<T> getListByCriteria(DetachedCriteria detachedCriteria) {
        return getHibernateTemplate().findByCriteria(detachedCriteria);
    }
}

And that’s it !

Take a minute or two to inspect how the base object implements a generic functionality with a type-safety manner.

All we have to do when implementing a new DAO is:

1. Interface to extend the IDaoBase with a concrete type

public interface DaoUser extends IDaoBase<User> {//<=Notice the User typing
    //Add any additional custom methods..
    public User getbyUsername(String username);
        public User getbyEmail(String email);
}

2. Implementation to extend the DaoBase with a concrete type

//This class has all the common methods, which are type safe for the User class
@Repository("daoUser")
public class DaoUserImpl extends DaoBase<User> implements DaoUser { //<=Notice the User typing
 
    public User getbyUsername(String username) {
// concrete implmentation       ...
    }