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The cart Example

The cart session bean represents a shopping cart in an online bookstore. The bean's client can add a book to the cart, remove a book, or retrieve the cart's contents. To assemble cart, you need the following code:

Session bean class (CartBean)
Remote business interface (Cart)

All session beans require a session bean class. All enterprise beans that permit remote access must have a remote business interface. To meet the needs of a specific application, an enterprise bean may also need some helper classes. The CartBean session bean uses two helper classes (BookException and IdVerifier) which are discussed in the section Helper Classes.

The source code for this example is in the <INSTALL>/javaeetutorial5/examples/ejb/cart/ directory.

The Business Interface

The Cart business interface is a plain Java interface that defines all the business methods implemented in the bean class. If the bean class implements a single interface, that interface is assumed to the business interface. The business interface is a local interface unless it is annotated with the javax.ejb.Remote annotation; the javax.ejb.Local annotation is optional in this case.

The bean class may implement more than one interface. If the bean class implements more than one interface, the bean class's business interfaces must be explicitly annotated either @Local or @Remote. However, the following interfaces are excluded when determining if the bean class implements more than one interface:

java.io.Serializable
java.io.Externalizable
Any of the interfaces defined by the javax.ejb package.

The source code for the Cart business interface follows:

package com.sun.tutorial.javaee.ejb;

import java.util.List;
import javax.ejb.Remote;

@Remote
public interface Cart {
  public void initialize(String person) throws BookException;
  public void initialize(String person, String id) 
    throws BookException;
  public void addBook(String title);
  public void removeBook(String title) throws BookException;
  public List<String> getContents();
  public void remove();
} 

Session Bean Class

The session bean class for this example is called CartBean. Like any stateful session bean, the CartBean class must meet these requirements:

It implements the business interface, a plain Java interface.
The class is annotated @Stateful.
The class implements the business methods defined in the business interface.

Stateful session beans also may:

Implement any optional lifecycle callback methods, annotated @PostConstruct, @PreDestroy, @PostActivate, and @PrePassivate.
Implement any optional business methods annotated @Remove.

The source code for the CartBean class follows.

package com.sun.tutorial.javaee.ejb;

import java.util.ArrayList;
import java.util.List;
import javax.ejb.Remove;
import javax.ejb.Stateful;

@Stateful
public class CartBean implements Cart {
  String customerName;
  String customerId;
  List<String> contents;

  public void initialize(String person) throws BookException {
    if (person == null) {
      throw new BookException("Null person not allowed.");
    } else {
      customerName = person;
    }

    customerId = "0";
    contents = new ArrayList<String>();
  }

  public void initialize(String person, String id) 
        throws BookException {
    if (person == null) {
      throw new BookException("Null person not allowed.");
    } else {

      customerName = person;
    }

    IdVerifier idChecker = new IdVerifier();

    if (idChecker.validate(id)) {
      customerId = id;
    } else {
      throw new BookException("Invalid id: " + id);
    }

    contents = new ArrayList<String>();
  }

  public void addBook(String title) {
    contents.add(title);
  }

  public void removeBook(String title) throws BookException {
    boolean result = contents.remove(title);
    if (result == false) {
      throw new BookException(title + " not in cart.");
    }
  }

  public List<String> getContents() {
    return contents;
  }

  @Remove
  public void remove() {
    contents = null;
  }
} 

Lifecycle Callback Methods

Methods in the bean class may be declared as a lifecycle call back method by annotating the method with the following annotations:

javax.annotation.PostConstruct
javax.annotation.PreDestroy
javax.ejb.PostActivate
javax.ejb.PrePassivate

Lifecycle callback methods must be public, return void, and have no parameters.

@PostConstruct methods are invoked by the container on newly constructed bean instances after all dependency injection has completed and before the first business method is invoked on the enterprise bean.

@PreDestroy methods are invoked after any method annotated @Remove has completed, and before the container removes the enterprise bean instance.

@PreActivate methods are invoked by the container before the container passivates the enterprise bean, meaning the container temporarily removes the bean from the environment and saves it to secondary storage.

@PostActivate methods are invoked by the container after the container moves the bean from secondary storage to active status.

Business Methods

The primary purpose of a session bean is to run business tasks for the client. The client invokes business methods on the object reference it gets from dependency injection or JNDI lookup. From the client's perspective, the business methods appear to run locally, but they actually run remotely in the session bean. The following code snippet shows how the CartClient program invokes the business methods:

cart.create("Duke DeEarl", "123");
...
cart.addBook("Bel Canto"); 
...
List<String> bookList = cart.getContents();
...
cart.removeBook("Gravity's Rainbow"); 

The CartBean class implements the business methods in the following code:

public void addBook(String title) {
   contents.addElement(title);
}

public void removeBook(String title) throws BookException {
   boolean result = contents.remove(title);
   if (result == false) {
      throw new BookException(title + "not in cart.");
   }
}

public List<String> getContents() {
   return contents;
} 

The signature of a business method must conform to these rules:

The method name must not begin with ejb to avoid conflicts with callback methods defined by the EJB architecture. For example, you cannot call a business method ejbCreate or ejbActivate.
The access control modifier must be public.
If the bean allows remote access via a remote business interface, the arguments and return types must be legal types for the Java RMI API.
If the bean is a web service endpoint, the arguments and return types for the methods annotated @WebMethod must be legal types for JAX-WS.
The modifier must not be static or final.

The throws clause can include exceptions that you define for your application. The removeBook method, for example, throws the BookException if the book is not in the cart.

To indicate a system-level problem, such as the inability to connect to a database, a business method should throw the javax.ejb.EJBException. The container will not wrap application exceptions such as BookException. Because EJBException is a subclass of RuntimeException, you do not need to include it in the throws clause of the business method.

The Remove Method

Business methods annotated with javax.ejb.Remove in the stateful session bean class can be invoked by enterprise bean clients to remove the bean instance. The container will remove the enterprise bean after a @Remove method completes, either normally or abnormally.

In CartBean, the remove method is a @Remove method:

  @Remove
  public void remove() {
    contents = null;
  } 

Helper Classes

The CartBean session bean has two helper classes: BookException and IdVerifier. The BookException is thrown by the removeBook method, and the IdVerifier validates the customerId in one of the create methods. Helper classes may reside in the EJB JAR file that contains the enterprise bean class, or in an EAR that contains the EJB JAR.

Building and Packaging the CartBean Example

Now you are ready to compile the remote interface (Cart.java), the home interface (CartHome.java), the enterprise bean class (CartBean.java), the client class (CartClient.java), and the helper classes (BookException.java and IdVerifier.java).

In a terminal window, go to this directory:

<INSTALL>/javaeetutorial5/examples/ejb/cart/

Type the following command:

ant

This command calls the default target, which builds and packages the application into an EAR file, cart.ear, located in the dist directory.

Deploying the Enterprise Application

Now that the Java EE application contains the components, it is ready for deployment. To deploy cart.ear, run the deploy task.

ant deploy 

cart.ear will be deployed to the Application Server.

Running the Application Client

When you run the client, the application client container injects any component references declared in the application client class, in this case the reference to the Cart enterprise bean. To run the application client, perform the following steps.

In a terminal window, go to this directory:

<INSTALL>/javaeetutorial5/examples/ejb/cart/

Type the following command:

ant run

This task will retrieve the application client JAR, cartClient.jar and run the application client. cartClient.jar contains the application client class, the helper class BookException, and the Cart business interface.

This is the equivalent of running:

appclient -client cartClient.jar

In the terminal window, the client displays these lines:

...

The all Task

As a convenience, the all task will build, package, deploy, and run the application. To do this, enter the following command:

ant all 

Undeploying cart

To undeploy cart.ear, enter the following command:

ant undeploy 

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