Implementing Data Access Object pattern
An example DAO code for a persistent object that represents Customer information is shown in Example 9.4. The CloudscapeCustomerDAO creates a Customer Transfer Object when the
findCustomer() method is invoked.The sample code to use the DAO is shown in Example 9.6. The class diagram for this example is shown in Figure 9.6.
Figure 9.6 Implementing the DAO pattern
Implementing Factory for Data Access Objects Strategy
Using Factory Method Pattern
Consider an example where we are implementing this strategy in which a DAO factory produces many DAOs for a single database implementation (e.g., Oracle). The factory produces DAOs such as CustomerDAO, AccountDAO, OrderDAO, and so forth. The class diagram for this example is shown in Figure 9.7.
Figure 9.7 Implementing the Factory for DAO strategy using Factory Method
The example code for the DAO factory (CloudscapeDAOFactory) is listed in Example 9.2.
Using Abstract Factory Pattern
Consider an example where we are considering implementing this strategy for three different databases. In this case, the Abstract Factory pattern can be employed. The class diagram for this example is shown in Figure 9.8. The sample code in Example 9.1 shows code excerpt for the abstract DAOFactory class. This factory produces DAOs such as CustomerDAO, AccountDAO, OrderDAO, and so forth. This strategy uses the Factory Method implementation in the factories produced by the Abstract Factory.
Figure 9.8 Implementing the Factory for DAO strategy using Abstract Factory Example 9.1 Abstract DAOFactory Class
// Abstract class DAO Factory
public abstract class DAOFactory {
// List of DAO types supported by the factory
public static final int CLOUDSCAPE = 1;
public static final int ORACLE = 2;
public static final int SYBASE = 3;
...
// There will be a method for each DAO that can be
// created. The concrete factories will have to
// implement these methods.
public abstract CustomerDAO getCustomerDAO();
public abstract AccountDAO getAccountDAO();
public abstract OrderDAO getOrderDAO();
...
public static DAOFactory getDAOFactory(
int whichFactory) {
switch (whichFactory) {
case CLOUDSCAPE:
return new CloudscapeDAOFactory();
case ORACLE :
return new OracleDAOFactory();
case SYBASE :
return new SybaseDAOFactory();
...
default :
return null;
}
}
} |
The sample code for CloudscapeDAOFactory is shown in Example 9.2. The implementation for OracleDAOFactory and SybaseDAOFactory are similar except for specifics of each implementation, such as JDBC driver, database URL, and differences in SQL syntax, if any.
Example 9.2 Concrete DAOFactory Implementation for Cloudscape
// Cloudscape concrete DAO Factory implementation
import java.sql.*;
public class CloudscapeDAOFactory extends DAOFactory {
public static final String DRIVER=
"COM.cloudscape.core.RmiJdbcDriver";
public static final String DBURL=
"jdbc:cloudscape:rmi://localhost:1099/CoreJ2EEDB";
// method to create Cloudscape connections
public static Connection createConnection() {
// Use DRIVER and DBURL to create a connection
// Recommend connection pool implementation/usage
}
public CustomerDAO getCustomerDAO() {
// CloudscapeCustomerDAO implements CustomerDAO
return new CloudscapeCustomerDAO();
}
public AccountDAO getAccountDAO() {
// CloudscapeAccountDAO implements AccountDAO
return new CloudscapeAccountDAO();
}
public OrderDAO getOrderDAO() {
// CloudscapeOrderDAO implements OrderDAO
return new CloudscapeOrderDAO();
}
...
} |
The CustomerDAO interface shown in Example 9.3 defines the DAO methods for Customer persistent object that are implemented by all concrete DAO implementations, such as CloudscapeCustomerDAO, OracleCustomerDAO, and SybaseCustomerDAO. Similar, but not listed here, are AccountDAO and OrderDAO interfaces that define the DAO methods for Account and Order business objects respectively.
Example 9.3 Base DAO Interface for Customer
// Interface that all CustomerDAOs must support
public interface CustomerDAO {
public int insertCustomer(...);
public boolean deleteCustomer(...);
public Customer findCustomer(...);
public boolean updateCustomer(...);
public RowSet selectCustomersRS(...);
public Collection selectCustomersTO(...);
...
} |
The CloudscapeCustomerDAO implements the CustomerDAO as shown in Example 9.4. The implementation of other DAOs, such as CloudscapeAccountDAO, CloudscapeOrderDAO, OracleCustomerDAO, OracleAccountDAO, and so forth, are similar.
Example 9.4 Cloudscape DAO Implementation for Customer
// CloudscapeCustomerDAO implementation of the
// CustomerDAO interface. This class can contain all
// Cloudscape specific code and SQL statements.
// The client is thus shielded from knowing
// these implementation details.
import java.sql.*;
public class CloudscapeCustomerDAO implements
CustomerDAO {
public CloudscapeCustomerDAO() {
// initialization
}
// The following methods can use
// CloudscapeDAOFactory.createConnection()
// to get a connection as required
public int insertCustomer(...) {
// Implement insert customer here.
// Return newly created customer number
// or a -1 on error
}
public boolean deleteCustomer(...) {
// Implement delete customer here
// Return true on success, false on failure
}
public Customer findCustomer(...) {
// Implement find a customer here using supplied
// argument values as search criteria
// Return a Transfer Object if found,
// return null on error or if not found
}
public boolean updateCustomer(...) {
// implement update record here using data
// from the customerData Transfer Object
// Return true on success, false on failure or
// error
}
public RowSet selectCustomersRS(...) {
// implement search customers here using the
// supplied criteria.
// Return a RowSet.
}
public Collection selectCustomersTO(...) {
// implement search customers here using the
// supplied criteria.
// Alternatively, implement to return a Collection
// of Transfer Objects.
}
...
} |
The Customer Transfer Object class is shown in Example 9.5. This is used by the DAOs to send and receive data from the clients. The usage of Transfer Objects is discussed in detail in the Transfer Object pattern.
Example 9.5 Customer Transfer Object
public class Customer implements java.io.Serializable {
// member variables
int CustomerNumber;
String name;
String streetAddress;
String city;
...
// getter and setter methods...
...
} |
Example 9.6 shows the usage of the DAO factory and the DAO. If the implementation changes from Cloudscape to another product, the only required change is the
getDAOFactory() method call to the DAO factory to obtain a different factory.Example 9.6 Using a DAO and DAO Factory - Client Code
...
// create the required DAO Factory
DAOFactory cloudscapeFactory =
DAOFactory.getDAOFactory(DAOFactory.DAOCLOUDSCAPE);
// Create a DAO
CustomerDAO custDAO =
cloudscapeFactory.getCustomerDAO();
// create a new customer
int newCustNo = custDAO.insertCustomer(...);
// Find a customer object. Get the Transfer Object.
Customer cust = custDAO.findCustomer(...);
// modify the values in the Transfer Object.
cust.setAddress(...);
cust.setEmail(...);
// update the customer object using the DAO
custDAO.updateCustomer(cust);
// delete a customer object
custDAO.deleteCustomer(...);
// select all customers in the same city
Customer criteria=new Customer();
criteria.setCity("New York");
Collection customersList =
custDAO.selectCustomersTO(criteria);
// returns customersList - collection of Customer
// Transfer Objects. iterate through this collection to
// get values.
... |
Related Patterns
- Transfer Object
A DAO uses Transfer Objects to transport data to and from its clients. - Factory Method [GoF] and Abstract Factory [GoF]
The Factory for Data Access Objects Strategy uses the Factory Method pattern to implement the concrete factories and its products (DAOs). For added flexibility, the Abstract Factory pattern may be employed as discussed in the strategies. - Broker [POSA1]
The DAO pattern is related to the Broker pattern, which describes approaches for decoupling clients and servers in distributed systems. The DAO pattern more specifically applies this pattern to decouple the resource tier from clients in another tier, such as the business or presentation tier
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