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The CartBean Example

The CartBean 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 construct CartBean, you need the following code:

Session bean class (CartBean)
Home interface (CartHome)
Remote interface (Cart)

All session beans require a session bean class. All enterprise beans that permit remote access must have a home and a remote 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>/j2eetutorial14/examples/ejb/cart/ directory.

Session Bean Class

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

It implements the SessionBean interface.
The class is defined as public.
The class cannot be defined as abstract or final.
It implements one or more ejbCreate methods.
It implements the business methods.
It contains a public constructor with no parameters.
It must not define the finalize method.

The source code for the CartBean class follows.

import java.util.*;
import javax.ejb.*;

public class CartBean implements SessionBean {

  String customerName;
  String customerId;
  Vector contents;

  public void ejbCreate(String person) 
    throws CreateException {

    if (person == null) {
      throw new CreateException("Null person not allowed.");
    }
    else {
      customerName = person;
    }

    customerId = "0";
    contents = new Vector();
  }

  public void ejbCreate(String person, String id) 
    throws CreateException {

    if (person == null) {
      throw new CreateException("Null person not allowed.");
    }
    else {
      customerName = person;
    }

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

    contents = new Vector();
  }

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

  public void removeBook(String title) throws BookException {

    boolean result = contents.removeElement(title);
    if (result == false) {
      throw new BookException(title + "not in cart.");
    }
   }

   public Vector getContents() {
      return contents;
   }

   public CartBean() {}
   public void ejbRemove() {}
   public void ejbActivate() {}
   public void ejbPassivate() {}
   public void setSessionContext(SessionContext sc) {}

}  

The SessionBean Interface

The SessionBean interface extends the EnterpriseBean interface, which in turn extends the Serializable interface. The SessionBean interface declares the ejbRemove, ejbActivate, ejbPassivate, and setSessionContext methods. The CartBean class doesn't use these methods, but it must implement them because they're declared in the SessionBean interface. Consequently, these methods are empty in the CartBean class. Later sections explain when you might use these methods.

The ejbCreate Methods

Because an enterprise bean runs inside an EJB container, a client cannot directly instantiate the bean. Only the EJB container can instantiate an enterprise bean. During instantiation, the example program performs the following steps.

The client invokes a create method on the home object:

Cart shoppingCart = home.create("Duke DeEarl","123");

The EJB container instantiates the enterprise bean.
The EJB container invokes the appropriate ejbCreate method in CartBean:

public void ejbCreate(String person, String id)
  throws CreateException {

  if (person == null) {
    throw new CreateException("Null person not allowed.");
  }
  else {
    customerName = person;
  }

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

  contents = new Vector();
}

Typically, an ejbCreate method initializes the state of the enterprise bean. The preceding ejbCreate method, for example, initializes the customerName and customerId variables by using the arguments passed by the create method.

An enterprise bean must have one or more ejbCreate methods. The signatures of the methods must meet the following requirements:

The access control modifier must be public.
The return type must be void.
If the bean allows remote access, the arguments must be legal types for the Java Remote Method Invocation (Java RMI) API.
The modifier cannot be static or final.

The throws clause can include the javax.ejb.CreateException and other exceptions that are specific to your application. The ejbCreate method usually throws a CreateException if an input parameter is invalid.

Business Methods

The primary purpose of a session bean is to run business tasks for the client. The client invokes business methods on the remote object reference that is returned by the create method. 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 shoppingCart = home.create("Duke DeEarl", "123");
...
shoppingCart.addBook("The Martian Chronicles"); 
shoppingCart.removeBook("Alice In Wonderland");
bookList = shoppingCart.getContents(); 

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.removeElement(title);
   if (result == false) {
      throw new BookException(title + "not in cart.");
   }
}

public Vector getContents() {
   return contents;
} 

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

The method name must not conflict with one 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, the arguments and return types must be legal types for the Java RMI API.
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. When a business method throws an EJBException, the container wraps it in a RemoteException, which is caught by the client. 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.

Home Interface

A home interface extends the javax.ejb.EJBHome interface. For a session bean, the purpose of the home interface is to define the create methods that a remote client can invoke. The CartClient program, for example, invokes this create method:

Cart shoppingCart = home.create("Duke DeEarl", "123"); 

Every create method in the home interface corresponds to an ejbCreate method in the bean class. The signatures of the ejbCreate methods in the CartBean class follow:

public void ejbCreate(String person) throws CreateException   
... 
public void ejbCreate(String person, String id) 
   throws CreateException  

Compare the ejbCreate signatures with those of the create methods in the CartHome interface:

import java.io.Serializable;
import java.rmi.RemoteException;
import javax.ejb.CreateException;
import javax.ejb.EJBHome;

public interface CartHome extends EJBHome {
  Cart create(String person) throws 
            RemoteException, CreateException;
  Cart create(String person, String id) throws 
            RemoteException, CreateException; 
} 

The signatures of the ejbCreate and create methods are similar, but they differ in important ways. The rules for defining the signatures of the create methods of a home interface follow.

The number and types of arguments in a create method must match those of its corresponding ejbCreate method.
The arguments and return type of the create method must be valid RMI types.
A create method returns the remote interface type of the enterprise bean. (But an ejbCreate method returns void.)
The throws clause of the create method must include the java.rmi.RemoteException and the javax.ejb.CreateException.

Remote Interface

The remote interface, which extends javax.ejb.EJBObject, defines the business methods that a remote client can invoke. Here is the source code for the Cart remote interface:

import java.util.*;
import javax.ejb.EJBObject;
import java.rmi.RemoteException;

public interface Cart extends EJBObject {
 
   public void addBook(String title) throws RemoteException;
   public void removeBook(String title) throws 
              BookException, RemoteException;
   public Vector getContents() throws RemoteException;
} 

The method definitions in a remote interface must follow these rules:

Each method in the remote interface must match a method implemented in the enterprise bean class.
The signatures of the methods in the remote interface must be identical to the signatures of the corresponding methods in the enterprise bean class.
The arguments and return values must be valid RMI types.
The throws clause must include the java.rmi.RemoteException.

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 ejbCreate methods. Helper classes must reside in the EJB JAR file that contains the enterprise bean class.

Building 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>/j2eetutorial14/examples/ejb/cart/

Type the following command:

asant build

Creating the Application

In this section, you'll create a J2EE application named CartApp, storing it in the file CartApp.ear.

In deploytool, select FileNewApplication.
Click Browse.
In the file chooser, navigate to <INSTALL>/j2eetutorial14/examples/ejb/cart/.
In the File Name field, enter CartApp.
Click New Application.
Click OK.
Verify that the CartApp.ear file resides in <INSTALL>/j2eetutorial14/examples/ejb/cart/.

Packaging the Enterprise Bean

In deploytool, select FileNewEnterprise Bean.
In the EJB JAR screen:
Select Create New JAR Module in Application.
In the Create New JAR Module in Application field, select CartApp.
In the JAR Name field, enter CartJAR.
Click Choose Module File.
Click Edit Contents.
Locate the <INSTALL>/j2eetutorial14/examples/ejb/cart/build/ directory.
Select BookException.class, Cart.class, CartBean.class,
CartHome.class, and IdVerifier.class.
Click Add.
Click OK.
Click Next.
In the General screen:
In the Enterprise Bean Class field, select CartBean.
In the Enterprise Bean Name field, enter CartBean.
In the Enterprise Bean Type field, select Stateful Session.
In the Remote Home Interface field, select CartHome.
In the Remote Interface field, select Cart.
Click Next.
Click Finish.

Packaging the Application Client

To package an application client component, you run the New Application Client wizard of deploytool. During this process the wizard performs the following tasks.

Creates the application client's deployment descriptor
Puts the deployment descriptor and client files into a JAR file
Adds the JAR file to the application's CartApp.ear file

To start the New Application Client wizard, select FileNewApplication Client. The wizard displays the following dialog boxes.

Introduction dialog box
Read the explanatory text for an overview of the wizard's features.
Click Next.
JAR File Contents dialog box
Select the button labeled Create New AppClient Module in Application.
In the combo box below this button, select CartApp.
In the AppClient Display Name field, enter CartClient.
Click Edit Contents.
In the tree under Available Files, locate the <INSTALL>/j2eetutorial14/examples/ejb/cart/build directory.
Select CartClient.class.
Click Add.
Click OK.
Click Next.
General dialog box
In the Main Class combo box, select CartClient.
Click Next.
Click Finish.

Specifying the Application Client's Enterprise Bean Reference

When it invokes the lookup method, the CartClient refers to the home of an enterprise bean:

Object objref =
  initial.lookup("java:comp/env/ejb/SimpleCart"); 

You specify this reference as follows.

In the tree, select CartClient.
Select the EJB Ref's tab.
Click Add.
In the Coded Name field, enter ejb/SimpleCart.
In the EJB Type field, select Session.
In the Interfaces field, select Remote.
In the Home Interface field, enter CartHome.
In the Local/Remote Interface field, enter Cart.
In the JNDI Name field, select CartBean.
Click OK.

Deploying the Enterprise Application

Now that the J2EE application contains the components, it is ready for deployment.

Select CartApp.
Select ToolsDeploy.
Under Connection Settings, enter the user name and password for the Application Server.
Under Application Client Stub Directory, check Return Client Jar.
In the field below the checkbox enter <INSTALL>/j2eetutorial14/examples/ejb/cart/.
Click OK.
In the Distribute Module dialog box, click Close when the deployment completes.
Verify the deployment.
In the tree, expand the Servers node and select the host that is running the Application Server.
In the Deployed Objects table, make sure that CartApp is listed and that its status is Running.
Verify that CartAppClient.jar is in <INSTALL>/j2eetutorial14/examples/ejb/cart/.

Running the Application Client

To run the application client, perform the following steps.

In a terminal window, go to the <INSTALL>/j2eetutorial14/
examples/ejb/cart/ directory.
Type the following command:

appclient -client CartAppClient.jar

In the terminal window, the client displays these lines:

The Martian Chronicles
2001 A Space Odyssey
The Left Hand of Darkness
Caught a BookException: Alice in Wonderland not in cart.

All of the material in The J2EE(TM) 1.4 Tutorial is copyright-protected and may not be published in other works without express written permission from Sun Microsystems.