Wednesday, June 22, 2011

Data Access Object ( DAO ) Pattern

A data access object (DAO) is an object that provides an abstract interface to some type of database or persistence mechanism, providing some specific operations without exposing details of the database. It provides a mapping from application calls to the persistence layer.

A typical DAO implementation has the following components:
  • A DAO interface
  • A concrete class that implements the DAO interface
  • Entities OR Data transfer objects (sometimes called value objects)
Seeing all the components with the Example

public class Order{
    private int id;
    private String customerName;
    private Date date;
    public int getId() { return id; }
    public void setId(int id) { = id; }
    public String getCustomerName() { return customerName; }
    public void setCustomerName(String customerName) {  
          this.customerName = customerName;  
    public Date getDate() { return date; }
    public void setDate(Date date) { = date;}

DAO interface
Get the interface for Create Retrieve Update and Delete operations.

//holds all CRUD behaviours
public interface OrderDao {
      void create(Order entity);
      Order findById(int id) throws OrderDontExistException;
      void update(Order entity);
      void delete(Order entity);      

DAO Implementation
This just takes the datasource object and stores all the queries related to Order object. I am just implementing finById method for simplicity.

public abstract class OrderDaoImpl implements OrderDao {
        DataSource ds;
    public OrderDaoImpl(DataSource ds_) {
        ds = ds_;
    public void create(Order entity) { //do something }
    public E findById(int id) throws OrderDontExistException { 
       String sqlFindById = "select * from ORDERS where id=?";    
       Connection con = getConnectionFromDataSource(ds);
      PreparedStatement ps= con.prepareStatement(sqlFindById);;
         ps.setInt(1, id);
      ResultSet rs = ps.execute(); 
      //process result set 
       Order order = rs.getString("Name");
        //... so on
        return order;   

    public void update(Order entity) { //do something }

    public void delete(Order entity) { //do something }


Exception class

package authordao;

public class OrderDontExistException extends Exception {

    public OrderDontExistException() {

        // TODO Auto-generated constructor stub


    public OrderDontExistException(String message) {


        // TODO Auto-generated constructor stub


    public OrderDontExistException(Throwable cause) {


        // TODO Auto-generated constructor stub


    public DAOException(String message, Throwable cause) {

        super(message, cause);

        // TODO Auto-generated constructor stub


Using the Dao

OrderDao dao = new OrderDaoImpl(ds);
Order order = dao.findById(id);
//Now do some business logic

As this article has shown, implementing the DAO pattern entails more than just writing low-level data access code. You can start building better DAOs today by choosing a transaction demarcation strategy that is appropriate for your application, by incorporating logging in your DAO classes, and by following a few simple guidelines for exception handling.

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