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EJB – Callbacks

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EJB - Callbacks

In this guide we will discuss about Callbacks of EJB. Callback is a mechanism by which the life cycle of an enterprise bean can be intercepted. EJB 3.0 specification has specified callbacks for which callback handler methods are created. EJB Container calls these callbacks. We can define callback methods in the EJB class itself or in a separate class. EJB 3.0 has provided many annotations for callbacks.

Following is the list of callback annotations for stateless bean −

AnnotationDescription
@PostConstructInvoked when a bean is created for the first time.
@PreDestroyInvoked when a bean is removed from the bean pool or is destroyed.

Following is the list of callback annotations for stateful bean −

AnnotationDescription
@PostConstructInvoked when a bean is created for the first time.
@PreDestroyInvoked when a bean is removed from the bean pool or is destroyed.
@PostActivateInvoked when a bean is loaded to be used.
@PrePassivateInvoked when a bean is put back to bean pool.

Following is the list of callback annotations for message driven bean −

AnnotationDescription
@PostConstructInvoked when a bean is created for the first time.
@PreDestroyInvoked when a bean is removed from the bean pool or is destroyed.

Following is the list of callback annotations for entity bean −

AnnotationDescription
@PrePersistInvoked when an entity is created in database.
@PostPersistInvoked after an entity is created in database.
@PreRemoveInvoked when an entity is deleted from the database.
@PostRemoveInvoked after an entity is deleted from the database.
@PreUpdateInvoked before an entity is to be updated in the database.
@PostLoadInvoked when a record is fetched from database and loaded into the entity.

Example Application

Let us create a test EJB application to test various callbacks in EJB.

StepDescription
1Create a project with a name EjbComponent under a package in.adglob.stateless as explained in the EJB – Create Application chapter. You can also use the project created in EJB – Persistence chapter as such for this chapter to add various callbacks to EJB.
2Create LibrarySessionBean.java and LibrarySessionBeanRemote as explained in the EJB – Create Application chapter. Keep rest of the files unchanged.
3Use Beans created in the EJB – Persistence chapter. Add callback methods as shown below. Keep rest of the files unchanged.
4Create a java class BookCallbackListener under package in.adglob.callback. This class will demonstrates the separation of callback methods.
5Clean and Build the application to make sure business logic is working as per the requirements.
6Finally, deploy the application in the form of jar file on JBoss Application Server. JBoss Application server will get started automatically if it is not started yet.
7Now create the EJB client, a console based application in the same way as explained in the EJB – Create Application chapter under topic Create Client to access EJB.

EJBComponent (EJB Module)

BookCallbackListener.java

package In.adglob.callback;

import javax.persistence.PrePersist;
import javax.persistence.PostLoad;
import javax.persistence.PostPersist;
import javax.persistence.PostRemove;
import javax.persistence.PostUpdate;
import javax.persistence.PreRemove;
import javax.persistence.PreUpdate;

import In.adglob.entity.Book;

public class BookCallbackListener {
    
   @PrePersist
   public void prePersist(Book book) {
      System.out.println("BookCallbackListener.prePersist:" 
         + "Book to be created with book id: "+book.getId());
   }

   @PostPersist
   public void postPersist(Object book) {
      System.out.println("BookCallbackListener.postPersist::"
         + "Book created with book id: "+((Book)book).getId());
   }

   @PreRemove
   public void preRemove(Book book) {
      System.out.println("BookCallbackListener.preRemove:"
         + " About to delete Book: " + book.getId());
   }

   @PostRemove
   public void postRemove(Book book) {
      System.out.println("BookCallbackListener.postRemove::"
         + " Deleted Book: " + book.getId());
   }

   @PreUpdate
   public void preUpdate(Book book) {
      System.out.println("BookCallbackListener.preUpdate::"
         + " About to update Book: " + book.getId());
   }

   @PostUpdate
   public void postUpdate(Book book) {
      System.out.println("BookCallbackListener.postUpdate::"
         + " Updated Book: " + book.getId());
   }

   @PostLoad
   public void postLoad(Book book) {
      System.out.println("BookCallbackListener.postLoad::"
         + " Loaded Book: " + book.getId());
   }
}

Book.java

package in.adglob.entity;
 
import java.io.Serializable;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.EntityListeners;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Table;
 
@Entity
@Table(name="books")
public class Book implements Serializable{
    
   private int id;
   private String name;
 
   public Book() {        
   }
 
   @Id
   @GeneratedValue(strategy= GenerationType.IDENTITY)
   @Column(name="id")
   public int getId() {
      return id;
   }
 
   public void setId(int id) {
      this.id = id;
   }
 
   public String getName() {
      return name;
   }
 
   public void setName(String name) {
      this.name = name;
   }    
}

LibraryStatefulSessionBean.java

package in.adglob.stateful;

import java.util.ArrayList;
import java.util.List;

import javax.annotation.PostConstruct;
import javax.annotation.PreDestroy;

import javax.ejb.PostActivate;
import javax.ejb.PrePassivate;
import javax.ejb.Stateful;

@Stateful
public class LibraryStatefulSessionBean 
   implements LibraryStatefulSessionBeanRemote {
   List<String> bookShelf;    

   public LibraryStatefulSessionBean() {
      bookShelf = new ArrayList<String>();
   }

   public void addBook(String bookName) {
      bookShelf.add(bookName);
   }    

   public List<String> getBooks() {
      return bookShelf;
   }

   @PostConstruct
   public void postConstruct() {
      System.out.println("LibraryStatefulSessionBean.postConstruct::"
         + " bean created.");
   }

   @PreDestroy
   public void preDestroy() {
      System.out.println("LibraryStatefulSessionBean.preDestroy:"
         + " bean removed.");
   }

   @PostActivate
   public void postActivate() {
      System.out.println("LibraryStatefulSessionBean.postActivate:"
         + " bean activated.");
   }

   @PrePassivate
   public void prePassivate() {
      System.out.println("LibraryStatefulSessionBean.prePassivate:"
         + " bean passivated.");
   }    
}

LibraryStatefulSessionBeanRemote.java 

package in.adglob.stateful;

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

@Remote
public interface LibraryStatefulSessionBeanRemote {
   void addBook(String bookName);
   List getBooks();
}

LibraryPersistentBean.java

package in.adglob.stateless;

import in.adglob.entity.Book;
import java.util.List;

import javax.annotation.PostConstruct;
import javax.annotation.PreDestroy;

import javax.ejb.Stateless;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;

@Stateless
public class LibraryPersistentBean 
   implements LibraryPersistentBeanRemote {
    
   public LibraryPersistentBean() {}

   @PersistenceContext(unitName="EntityEjbPU")
   private EntityManager entityManager;         

   public void addBook(Book book) {
      entityManager.persist(book);
   }    

   public List<Book> getBooks() {     
      return entityManager.createQuery("From Book")
         .getResultList();
   }

   @PostConstruct
   public void postConstruct() {
      System.out.println("postConstruct:: LibraryPersistentBean session bean"
         + " created with entity Manager object: ");
   }

   @PreDestroy
   public void preDestroy() {
      System.out.println("preDestroy: LibraryPersistentBean session"
      + " bean is removed ");
   }
}

LibraryPersistentBeanRemote.java

package in.adglob.stateless;

import in.adglob.entity.Book;
import java.util.List;
import javax.ejb.Remote;

@Remote
public interface LibraryPersistentBeanRemote {

   void addBook(Book bookName);

   List<Book> getBooks();
    
}
  • As soon as you deploy the EjbComponent project on JBOSS, notice the jboss log.
  • JBoss has automatically created a JNDI entry for our session bean – LibraryPersistentBean/remote.
  • We will be using this lookup string to get remote business object of type -in.adglob.stateless.LibraryPersistentBeanRemote

JBoss Application Server Log Output

...
16:30:01,401 INFO  [JndiSessionRegistrarBase] Binding the following Entries in Global JNDI:
   LibraryPersistentBean/remote - EJB3.x Default Remote Business Interface
   LibraryPersistentBean/remote-in.adglob.stateless.LibraryPersistentBeanRemote - EJB3.x Remote Business Interface
16:30:02,723 INFO  [SessionSpecContainer] Starting jboss.j2ee:jar=EjbComponent.jar,name=LibraryPersistentBean,service=EJB3
16:30:02,723 INFO  [EJBContainer] STARTED EJB: in.adglob.stateless.LibrarySessionBeanRemote ejbName: LibraryPersistentBean
...

EJBTester (EJB Client)

jndi.properties

java.naming.factory.initial=org.jnp.interfaces.NamingContextFactory
java.naming.factory.url.pkgs=org.jboss.naming:org.jnp.interfaces
java.naming.provider.url=localhost
  • These properties are used to initialize the InitialContext object of java naming service.
  • InitialContext object will be used to lookup stateless session bean.

EJBTester.java

package in.adglob.test;
   
import in.adglob.stateful.LibrarySessionBeanRemote;

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.naming.InitialContext;
import javax.naming.NamingException;

public 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));
   }
   
   public static void main(String[] args) {

      EJBTester ejbTester = new EJBTester();

      ejbTester.testEntityEjb();
   }
   
   private void showGUI() {
      System.out.println("**********************");
      System.out.println("Welcome to Book Store");
      System.out.println("**********************");
      System.out.print("Options \n1. Add Book\n2. Exit \nEnter Choice: ");
   }
   
   private void testEntityEjb() {    
      try {
      int choice = 1; 

      LibraryPersistentBeanRemote libraryBean = 
      (LibraryPersistentBeanRemote)
      ctx.lookup("LibraryPersistentBean/remote");

      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);
            libraryBean.addBook(book);          
         } else if (choice == 2) {
            break;
         }
      }

      List<Book> booksList = libraryBean.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 initialize the InitialContext object.
  • In testStatelessEjb() method, jndi lookup is done with the name – “LibrarySessionBean/remote” to obtain the remote business object (stateless EJB).
  • Then user is shown a library store User Interface and he/she is asked to enter a choice.
  • If the user enters 1, the system asks for book name and saves the book using stateless session bean addBook() method. Session Bean is storing the book in the database.
  • If user enters 2, system retrieves books using stateless session bean getBooks() method and exits.

Run Client to Access EJB

Locate EJBTester.java in project explorer. Right click 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 Java
**********************
Welcome to Book Store
**********************
Options 
1. Add Book
2. Exit 
Enter Choice: 2
Book(s) entered so far: 1
1. Learn Java
BUILD SUCCESSFUL (total time: 13 seconds)

JBoss Application Server Log Output

You can find the following callback entries in JBoss log

14:08:34,293 INFO  [STDOUT] postConstruct:: LibraryPersistentBean session bean created with entity Manager object
...
16:39:09,484 INFO  [STDOUT] BookCallbackListener.prePersist:: Book to be created with book id: 0
16:39:09,531 INFO  [STDOUT] BookCallbackListener.postPersist:: Book created with book id: 1
16:39:09,900 INFO  [STDOUT] BookCallbackListener.postLoad:: Loaded Book: 1
...

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