Tutorials : Using DAOs in Apache Struts :

Using DAOs in Apache Struts

Introduction

One of the most popular design patterns is the Data Access Object (DAO) pattern. It's used when you want to separate your presentation code from the code that accesses the back-end containing your data. The back-end is typically a database, but it could be anything: a flat file on your hard disk, a message queue connecting to a mainframe, or a web service interface to some server.

It you're working with an application managing customers, then a DAO module would typically offer functions for "create customer", "update customer", "delete customer", etc.

When the client uses the DAO pattern it does not know how the back-end is implemented. This makes it possible to replace a back-end using a database with for example a web service, without changing one line of code in the client. This is probably not something you do very often, but when you're developing and testing an application, the DAO pattern makes it possible to replace the back-end with something that is simpler to test against--for example a simulated database. Of course this only tests the front-end logic. The back-end logic will also have to be tested.

Another benefit of using the DAO pattern is that you can use the DAO modules for many types of clients: web browsers, Swing clients, web services, and background tasks.

In this article we'll start with a very simple application, and build several DAOs using various types of back-end implementations.

The Case

We need a case that is simple, so we can keep the focus on the DAO technique, but at the same time it shouldn't be so simple so that it neglects to demonstrate how DAOs are used in real life projects. I've used the DVD library example in many of my other articles, so why not once more? Since the example is simple, I'll include code for complete logging and error handling.

We'd like to create an application built on DAOs that can manage a collection of DVDs. A DVD is represented as simply as possible by a unique "id" and a title:

id	A string that uniquely identifies the DVD. This id can be used as the key in a database table.
title	A string that contains the title of the DVD, typically the title of a movie. This title represents the "data part" of a database record.

One DVD can be implemented in Java as a bean:

package dk.hansen;

public class DVD {

  private String id;
  private String title;

  public DVD(String id, String title) {
    setId(id);
    setTitle(title);
  }

  public String getId() {return id;}

  public void setId(String id) {this.id = id;}

  public String getTitle() {return title;}

  public void setTitle(String title) {this.title = title;}

}

The DVD class can, without problem, be mapped to a database table with two columns.

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