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Chapter 6. Using Seam

JBoss Seam is a framework that provides the glue between the new EJB3 and JSF frameworks that are part of the Java EE 5.0 standard. In fact, the name Seam refers to the seamless manner in which it enables developers to use these two frameworks in an integrated manner. Seam automates many of the common tasks, and makes extensive use of annotations to reduce the amount of xml code that needs to be written. The overall effect is to significantly reduce the total amount of coding that needs to be done.
We have included two versions of the example application, one coded using EJB3 / JSF without using Seam, and one using Seam, to demonstrate clearly the difference in application development using the Seam framework.

Note

Refer to the "Seam Reference Guide" included in the documentation set (JBOSS_DIST/doc/seam/Seam_Reference_Guide.pdf) for important information regarding the deployment of Seam examples and detailed information on developing applications using Seam.

6.1.  Data Model

In the previous chapter we looked at the Data Model used in the EJB3/JSF implementation of this sample application. Let's start off our examination of the Seam implementation in the same way, by examining how the Data Model is implemented. This is done in the Todo.java file.
@Entity
@Name("todo")
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; }

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

  @NotNull
  @Length(max=250) 
  public String getDescription() { return description; }
  public void setDescription(String description) {
    this.description = description;
  }

}
The @Entity annotation defines the class as an EJB3 session bean, and tells the container to map the Todo class to a relational database table. Each property of the class will become a column in the table. Each instance of the class will become a row in this table. Since we have not used the @Table annotation, Seam's "configuration by exception" default will name the table after the class.
@Entity and @Table are both EJB3 annotations, and are not specific to Seam. It is possible to use Seam completely with POJOs (Plain Old Java Objects) without any EJB3-specific annotations. However, EJB3 brings a lot of advantages to the table, including container managed security, message-driven components, transaction and component level persistence context, and @PersistenceContext injection, which we will encounter a little further on.
The @Name annotation is specific to Seam, and defines the string name for Seam to use to register the Entity Bean. This will be the default name for the relational database table. Each component in a Seam application must have a unique name. In the other components in the Seam framework, such as JSF web pages and session beans, you can reference the managed Todo bean using this name. If no instance of this class exists when it is referenced from another component, then Seam will instantiate one.
The @Id annotation defines a primary key id field for the component. @GeneratedValue specifies that the server will automatically generate this value for the component when it is saved to the database.
Seam provides support for model-based constraints defined using Hibernate Validator, although Hibernate does not have to be the object persister used. The @NotNull annotation is a validation constraint that requires this property to have a value before the component can be persisted into the database. Using this annotation allows the validation to be enforced by the JSF code at the view level, without having to specify the exact validation constraint in the JSF code.
At this point the only apparent difference between the Seam version and the EJB3/JSF version of the app is the inclusion of the validator annotation @NotNull, and the @Name annotation. However, while the EJB3/JSF version of this application requires a further TodoBean class to be manually coded and managed in order to handle the interaction between the Todo class and the web interface, when using Seam the Seam framework takes care of this work for us. We'll see how this is done in practice as we examine the implementation of the user interface.

 
 
  Published under the terms of the Open Publication License Design by Interspire