Java(TM) Boutique - Using DAOs in Apache Struts

Tutorials : Using DAOs in Apache Struts :

DAO factories

In order to be able to select in a simple way among the different DAO implementations that we may end up with, we'll need a "DAO factory", another popular design pattern. There's a choice here when implementing such a factory. You could code a single factory which takes some input parameter which specifies the DAO implementation to use. Or you could code an interface which a factory must implement, and then code several factories. I'll use the latter alternative:

package dk.hansen;

/*
* This a factory for the DVDManager DAO
*/
public interface DVDManagerFactoryIF {

  public DVDManagerIF createDVDManager();

}

Usually a factory will be able to create several DAOs, but in our simple example there's only the DVDManager DAO. A factory class for the SimpleDVDManager can now be coded:

package dk.hansen;

import org.apache.log4j.Logger;

public class SimpleDVDManagerFactory implements DVDManagerFactoryIF {

  private static Logger logger = 
    Logger.getLogger(SimpleDVDManagerFactory.class.getName());

  public DVDManagerIF createDVDManager() {
    logger.info("SimpleDVDManagerFactory creates a SimpleDVDManager");
    return new SimpleDVDManager();
  }

}

Since I've promised not to skip proper logging and error handling I don't use System.out.println for output, but Log4J.

The relations between the classes are now:

Unit testing the code

At this stage we should make a unit test program for the factory and DAO interface. Here's a JUnit test program that uses the SimpleDVDManager and its factory class. It tests all the methods in the DAO interface.

The factory is selected by giving the name of the class as a string:

. . .
// Create a factory
String managerName = "dk.hansen.SimpleDVDManagerFactory";
DVDManagerFactory factory;
factory = (DVDManagerFactory)Class.forName(managerName).newInstance();

// Create a DVDManager
manager = factory.createDVDManager();
. . .

The application which will use the DAO can also instantiate the factory by first reading the class name from--for example--a properties file, through JNDI, or some other source. Test programs for the application can therefore easily select simpler DAO implementations than for example the database implementation to be used in production. If new factories are developed, they're also simple to plug in.

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