DAO and Factory Design Patterns

As already said in the previous post the purpose of the DAO patter is to decouple the persistence logic from the business logic. So that future changes to the persistence tier will not affect the business tier, therefore changing the persistence device will not affect the business logic.


From now on I explain how to use the Factory pattern to implement the DAO pattern:

• a good idea is to start creating an interface containing the methods that we want implement in our daos, like the following one:   

•          public interface GenericDAO {
•                 public void save(Object object) throws DAOException;
•                 public void update(Object object) throws DAOException;
•                 public void remove(Object object) throws DAOException;
•                 public Object findByPrimaryKey(Object pk) throws DAOException;
•                 public Collection findAll() throws DAOException;
•         }

• and if we want we can extend the java.lang.Exception to implement our specific exception (DAOException) 

•    public class DAOException extends Exception {
•           public DAOException(String message) {
•              super(message);    
•           }
•           public DAOException(Throwable e) {
•              super(e);    
•           }
•      }

Then we need to implement the core class which implements the factory and singleton patterns: in this manner we will use  use always only one instance of our DAO (singleton pattern); while the factory pattern returns an interface, in this case the GenericDAO, that at run time will be a real class that implements that interface. Then we create a properties file which will be sued by the DAOFactory to discover the real implementations for DAOs that will be requested at run time by the application.

public class DAOFactory { 
 
    private static DAOFactory factory = null; 
    private Properties props = null; 
 
    private DAOFactory() {
        try {
            props = new Properties();
            props.load(DAOFactory.class.getResourceAsStream("daos.properties"));
        } catch (IOException e) {
            e.printStackTrace();
        }
    } 
 
    public static DAOFactory getInstance() {
        if (factory == null) {
            factory = new DAOFactory();
        }
        return factory;
    } 
 
    public GenericDAO getDAO(String name) {
        GenericDAO dao = null;
        try {
            dao = (GenericDAO) Class.forName(props.getProperty(name))
                .newInstance();
        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
        return dao;
    }
}