Let's take a look at the contents of the Data Model represented by the Todo class in the Todo.java file. Each instance of the Todo class corresponds to a row in the relational database table. The 'Todo' class has three properties: id, title and description. Each of these correspond to a column in the database table.

The 'Entity class' to 'Database Table' mapping information is specified using EJB3 Annotations in the 'Todo' class. This eliminates the need for XML configuration and makes it a lot clearer. The @Entity annotation defines the Todo class as an Entity Bean. The @Id and @GeneratedValue annotations on the id property indicate that the id column is the primary key and that the server automatically generates its value for each Todo object saved into the database.

@Entity
public class Todo implements Serializable {

  private long id;
  private String title;
  private String description;

  public Todo () {
    title ="";
    description ="";
  }

  @Id @GeneratedValue
  public long getId() { return id;}
  public void setId(long id) { this.id = id; }

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

  public String getDescription() { return description; }
  public void setDescription(String description) {
    this.description = description;
  }

}