EJB Generic DAO

Step 1: Declare a GenericDao interface

public interface GenericDao<T> {
    public void save(T entity);
    public void update(T entity);
    public T find(Serializable entityId);
    public void delete(Serializable... ids);
}

Step 2: Create an abstract GenericDaoSupport Class

@SuppressWarnings("unchecked")
public abstract class GenericDaoSupport<T>
    implements GenericDao<T> {

    private Class<T> entityClass 
        = GenericsUtils.getSuperClassGenericType
        (this.getClass());
    
    @PersistenceContext(unitName="ejbPU")
    protected EntityManager em;
    
    public void delete(Serializable... ids) {
        for(Serializable id: ids){
            em.remove(em.getReference(entityClass, id));
        }
    }

    public T find(Serializable entityId) {
        return em.find(entityClass, entityId);
    }

    public void save(T entity) {
        em.persist(entity);
    }

    public void update(T entity) {
        em.merge(entity);
    }
}

We need a utility class here to get the entity type
e.g. For GenericDaoSupport<Order>,
GenericsUtils.getSuperClassGenericType() will return Order class

public class GenericsUtils {
    
    @SuppressWarnings("unchecked")
    public static Class getSuperClassGenericType(Class clazz){
        return getSuperClassGenericType(clazz, 0);
    }
    
    /**
     * 
     * @param clazz
     * @param index
     * @return
     */
    @SuppressWarnings("unchecked")
    public static Class getSuperClassGenericType
        (Class clazz, int index){
        //Get super class's generic Type
        //e.g. GenericDaoSupport<Order>
        Type genericType = clazz.getGenericSuperclass();
        //If not a parameterized type, e.g. GenericDaoSupport
        if (!(genericType instanceof ParameterizedType)){
            //Just return the Object class
            return Object.class;
        }
        //Get actual types
        //e.g. for GenericDaoSupport<Order, Customer, Item>
        //params = {Order, Customer, Item}
        Type[] params = ((ParameterizedType)genericType)
            .getActualTypeArguments();
        if (index >= params.length || index < 0){
            throw new IllegalArgumentException
            ("The index is invalid:"+index);
        }
        return (Class)params[index];
    }
}

Step 3: Create an entity

@Entity
@Table(name="T_ORDER")
public class Order implements Serializable{
    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private int id;
    
    @Column(name="CREATE_DATE")
    @Temporal(TemporalType.DATE)
    private Date createDate = new Date();

    public int getId() {
        return id;
    }

    public Date getCreateDate() {
        return createDate;
    }    
}

Step 4: Declare an enity Dao interface

public interface OrderDao 
    extends GenericDao<Order> {
}

Step 5: Implement the entity Dao interface

@Stateless(name="orderDao")
@Remote(OrderDao.class)
public class OrderDaoImpl 
    extends GenericDaoSupport<Order> 
    implements OrderDao {
}

So whenever we need to create a new entity Dao, we just need to have it extend GenericDaoSupport so that we don't have to duplicate the code of CRUD operations.