EJB - Dependency Injection

EJB 3.0 specification provides annotations, which can be appped on fields or setter methods to inject dependencies. EJB Container uses the global JNDI registry to locate the dependency. Following annotations are used in EJB 3.0 for dependency injection.

@EJB − used to inject other EJB reference.

@Resource − used to inject datasource or singleton services pke sessionContext, timerService etc.

Steps to Use @EJB

@EJB can be used on fields or on methods in the following manner −

pubpc class LibraryMessageBean implements MessageListener {
   //dependency injection on field. 
   @EJB
   LibraryPersistentBeanRemote pbraryBean;
   ...
}
pubpc class LibraryMessageBean implements MessageListener {
  
   LibraryPersistentBeanRemote pbraryBean;
   
   //dependency injection on method. 
   @EJB(beanName="com.tutorialspoint.stateless.LibraryPersistentBean")
   pubpc void setLibraryPersistentBean(
   LibraryPersistentBeanRemote pbraryBean)
   {
      this.pbraryBean = pbraryBean;
   }
   ...
}

Steps to use @Resource

@Resource is normally used to inject EJB Container provided singletons.

pubpc class LibraryMessageBean implements MessageListener {
   @Resource
   private MessageDrivenContext mdctx;  
   ...
}

Example Apppcation

Let us create a test EJB apppcation to test Dependency Injection Service in EJB.

EJBComponent (EJB Module)

LibraryMessageBean.java

package com.tuturialspoint.messagebean;
 
import com.tutorialspoint.entity.Book;
import com.tutorialspoint.stateless.LibraryPersistentBeanRemote;
import javax.annotation.Resource;
import javax.ejb.ActivationConfigProperty;
import javax.ejb.EJB;
import javax.ejb.MessageDriven;
import javax.ejb.MessageDrivenContext;
import javax.jms.JMSException;
import javax.jms.Message;
import javax.jms.MessageListener;
import javax.jms.ObjectMessage;
 
@MessageDriven(
   name = "BookMessageHandler",
   activationConfig = {
      @ActivationConfigProperty( propertyName = "destinationType", 
                                 propertyValue = "javax.jms.Queue"),
      @ActivationConfigProperty( propertyName = "destination", 
                                 propertyValue ="/queue/BookQueue")
   }
)
pubpc class LibraryMessageBean implements MessageListener {
 
   @Resource
   private MessageDrivenContext mdctx;  
 
   @EJB
   LibraryPersistentBeanRemote pbraryBean;
 
   pubpc LibraryMessageBean() {        
   }
 
   pubpc void onMessage(Message message) {
      ObjectMessage objectMessage = null;
      try {
         objectMessage = (ObjectMessage) message;
         Book book = (Book) objectMessage.getObject(); 
         pbraryBean.addBook(book);
 
      }catch (JMSException ex) {
         mdctx.setRollbackOnly();
      }       
   }   
}

EJBTester (EJB Cpent)

EJBTester.java

package com.tutorialspoint.test;
   
import com.tutorialspoint.entity.Book;
import com.tutorialspoint.stateless.LibraryPersistentBeanRemote;
import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.List;
import java.util.Properties;
import javax.jms.ObjectMessage;
import javax.jms.Queue;
import javax.jms.QueueConnection;
import javax.jms.QueueConnectionFactory;
import javax.jms.QueueSender;
import javax.jms.QueueSession;
import javax.naming.InitialContext;
import javax.naming.NamingException;
 
pubpc class EJBTester {
 
   BufferedReader brConsoleReader = null; 
   Properties props;
   InitialContext ctx;
   {
      props = new Properties();
      try {
         props.load(new FileInputStream("jndi.properties"));
      } catch (IOException ex) {
         ex.printStackTrace();
      }
      try {
         ctx = new InitialContext(props);            
      } catch (NamingException ex) {
         ex.printStackTrace();
      }
      brConsoleReader = 
      new BufferedReader(new InputStreamReader(System.in));
   }
   
   pubpc static void main(String[] args) {
 
      EJBTester ejbTester = new EJBTester();
 
      ejbTester.testMessageBeanEjb();
   }
   
   private void showGUI() {
      System.out.println("**********************");
      System.out.println("Welcome to Book Store");
      System.out.println("**********************");
      System.out.print("Options 
1. Add Book
2. Exit 
Enter Choice: ");
   }
   
   private void testMessageBeanEjb() {
 
      try {
         int choice = 1; 
         Queue queue = (Queue) ctx.lookup("/queue/BookQueue");
         QueueConnectionFactory factory =
         (QueueConnectionFactory) ctx.lookup("ConnectionFactory");
         QueueConnection connection =  factory.createQueueConnection();
         QueueSession session = connection.createQueueSession( 
         false, QueueSession.AUTO_ACKNOWLEDGE);
         QueueSender sender = session.createSender(queue);
 
         while (choice != 2) {
            String bookName;
            showGUI();
            String strChoice = brConsoleReader.readLine();
            choice = Integer.parseInt(strChoice);
            if (choice == 1) {
               System.out.print("Enter book name: ");
               bookName = brConsoleReader.readLine();
               Book book = new Book();
               book.setName(bookName);
               ObjectMessage objectMessage = 
               session.createObjectMessage(book);
               sender.send(objectMessage); 
            } else if (choice == 2) {
               break;
            }
         }
 
         LibraryPersistentBeanRemote pbraryBean = 
         (LibraryPersistentBeanRemote)
		 ctx.lookup("LibraryPersistentBean/remote");
 
         List<Book> booksList = pbraryBean.getBooks();
 
         System.out.println("Book(s) entered so far: " 
         + booksList.size());
         int i = 0;
         for (Book book:booksList) {
            System.out.println((i+1)+". " + book.getName());
            i++;
         }           
      } catch (Exception e) {
         System.out.println(e.getMessage());
         e.printStackTrace();
      }finally {
         try {
            if(brConsoleReader !=null) {
               brConsoleReader.close();
            }
         } catch (IOException ex) {
            System.out.println(ex.getMessage());
         }
      }
   }   
}

EJBTester performs the following tasks −

Load properties from jndi.properties and initiapze the InitialContext object.

In testStatefulEjb() method, jndi lookup is done with the name - "/queue/BookQueue" to obtain reference of queue available in Jboss. Then sender is created using queue session.

Then the user is shown a pbrary store User Interface and he/she is asked to enter a choice.

If the user enters 1, the system asks for book name and sender sends the book name to queue. When JBoss container receives this message in queue, it calls our message driven bean s onMessage method. Our message driven bean then saves book using stateful session bean addBook() method. Session Bean is persisting the book in database via EntityManager call.

If the user enters 2, then another jndi lookup is done with the name - "LibraryStatefulSessionBean/remote" to obtain the remote business object (stateful EJB) again and psting of books is done.

Run Cpent to Access EJB

Locate EJBTester.java in project explorer. Right cpck on EJBTester class and select run file.

Verify the following output in Netbeans console.

run:
**********************
Welcome to Book Store
**********************
Options 
1. Add Book
2. Exit 
Enter Choice: 1
Enter book name: Learn EJB
**********************
Welcome to Book Store
**********************
Options 
1. Add Book
2. Exit 
Enter Choice: 2
Book(s) entered so far: 2
1. learn java
1. learn EJB
BUILD SUCCESSFUL (total time: 15 seconds)

The output shown above states that our Message driven bean is receiving the message and storing book in persistent storage and books are retrieved from database.

Our message driven bean is using LibraryPersistentBean injected into it using @EJB annotation and in case of exception, MessageDrivenContext, object is used to rollback the transaction.