Using DAO Design Pattern

DAO Pattern Definition

Access to data varies depending on the source of the data. Access to persistent storage, such as to a database, varies greatly depending on the type of storage (relational databases, object-oriented databases, flat files, and so forth) and the vendor implementation.
Reference: Blue Prints

Introduction

When you are creating your application framework, you would like to persist your data using smart techniques, and I am not talking about Hibernate, JDO, OJB or anything else, however I can ask for you? Can your application support a storage system replacement without any problem? If your asnwer is NO, this text will be useful for you.

Define a Interface as foundation for DAOs

You can define a simple Interface, describing "WHAT" you would like to do, and not "HOW" to do. Getting this idea, we can to think in Interfaces usage. Take a look in the following code:

package framework.dao; 

import java.util.Collection; 

public interface IGenericDAO {
    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;
}

You can see that this interface can remind you the EJB EntityBeans model, if you are thinking it is correct! EntityBeans is a great idea, so We can continue using nice ideas(concepts) like that.
In fact, we are exposing that this interface says that It can save, update, remove or find results against Information storage.

We need a simple extension from java.lang.Exception called DAOException, which can be throwed by any method, its source can be as simple as in the following code section:

package framework.dao; 

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

The Implementation

We talked previouslly about smart codes, so we need to create and use it automaticlly and easier! You can have two choices:

• Create a DAOFactory class
• Use Spring Framework

DAOFactory

This class will implement a couple of patterns,as such:
Singleton - We will use one and just one instance and
Factory Method - The method returns always an interface, but in execution it will return a concrete class as wich implements this interface, in this case IGenericDAO.
See the following code for DAOFactory:

import java.io.IOException;
import java.util.Properties; 

public class DAOFactory { 

    private static DAOFactory me = 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 (null == me) {
            me = new DAOFactory();
        }
        return me;
    } 

    public IGenericDAO getDAO(String name) {
        IGenericDAO retorno = null;
        try {
            retorno = (IGenericDAO) Class.forName(props.getProperty(name))
                .newInstance();
        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
        return retorno;
    }
}

The DAOFactory is using a properties file to discover the real implementations for DAOs that will be requested by applications.

You can create much better resource of reading, as such to auto-detect changes and update the properties in memory and other stuff.

Using Spring Framework

You can work with Dependency Injection, and you will use the context.xml used in all Spring Applications to describe this dependency.
Other nice Spring's feature is the ability to create DAOs implementations extends the HibernateDAOSupport class, which offer a lot of nice things to make your development easier.

This entry is to make you think that your applications can use dinamic configurations issues, and reduce the coupling between your layers, if you will use Spring or your own IoC framework, the most important is to use some useful DAO strategy as I this text is decribing (more information about DAO in Spring Framework - here).