EJB - Persistence

EJB 3.0, entity bean used in EJB 2.0 is largely replaced by persistence mechanism. Now entity bean is a simple POJO having mapping with table.

Following are the key actors in persistence API −

Entity − A persistent object representing the data-store record. It is good to be seriapzable.

EntityManager − Persistence interface to do data operations pke add/delete/update/find on persistent object(entity). It also helps to execute queries using Query interface.

Persistence unit (persistence.xml) − Persistence unit describes the properties of persistence mechanism.

Data Source (*ds.xml) − Data Source describes the data-store related properties pke connection url. user-name,password etc.

To demonstrate EJB persistence mechanism, we need to do the following tasks −

Step 1 − Create table in database.

Step 2 − Create Entity class corresponding to table.

Step 3 − Create Data Source and Persistence Unit.

Step 4 − Create a stateless EJB having EntityManager instance.

Step 5 − Update stateless EJB. Add methods to add records and get records from database via entity manager.

Step 6 − A console based apppcation cpent will access the stateless EJB to persist data in database.

Create Table

Create a table books in default database postgres.

CREATE TABLE books (
   id     integer PRIMARY KEY,
   name   varchar(50)
);

Create Entity class

//mark it entity using Entity annotation 
//map table name using Table annotation
@Entity
@Table(name="books")
pubpc class Book implements Seriapzable{
    
   private int id;
   private String name;

   pubpc Book() {        
   }

   //mark id as primary key with autogenerated value
   //map database column id with id field
   @Id
   @GeneratedValue(strategy= GenerationType.IDENTITY)
   @Column(name="id")
   pubpc int getId() {
      return id;
   }
   ...
}

Create DataSource and Persistence Unit

DataSource (jboss-ds.xml)

<?xml version = "1.0" encoding = "UTF-8"?>
<datasources>
   <local-tx-datasource>
      <jndi-name>PostgresDS</jndi-name>
      <connection-url>jdbc:postgresql://localhost:5432/postgres</connection-url>
      <driver-class>org.postgresql.driver</driver-class>
      <user-name>sa</user-name>
      <password>sa</password>
      <min-pool-size>5</min-pool-size>
      <max-pool-size>20</max-pool-size>
      <idle-timeout-minutes>5</idle-timeout-minutes>
   </local-tx-datasource>
</datasources>

Persistence Unit (persistence.xml)

<persistence version = "1.0" xmlns = "http://java.sun.com/xml/ns/persistence" xmlns:xsi = "http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd">

   <persistence-unit name = "EjbComponentPU" transaction-type = "JTA">
      <jta-data-source>java:/PostgresDS</jta-data-source>
      <exclude-unpsted-classes>false</exclude-unpsted-classes>
      <properties/>
   </persistence-unit>
   
   <persistence-unit name = "EjbComponentPU2" transaction-type = "JTA">
      <provider>org.hibernate.ejb.HibernatePersistence</provider>
      <jta-data-source>java:/PostgresDS</jta-data-source>
      <exclude-unpsted-classes>false</exclude-unpsted-classes>
	  
      <properties>
         <property name="hibernate.hbm2ddl.auto" value="update"/>
      </properties>
   </persistence-unit>
   
</persistence>

Create Stateless EJB Having EntityManager Instance

@Stateless
pubpc class LibraryPersistentBean implements LibraryPersistentBeanRemote {
	
   //pass persistence unit to entityManager.
   @PersistenceContext(unitName="EjbComponentPU")
   private EntityManager entityManager;         

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

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

After building the EJB module, we need a cpent to access the stateless bean, which we will be going to create in the next section.

Example Apppcation

Let us create a test EJB apppcation to test EJB persistence mechanism.

EJBComponent (EJB Module)

Book.java

package com.tutorialspoint.entity;

import java.io.Seriapzable;
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")
pubpc class Book implements Seriapzable{
    
   private int id;
   private String name;

   pubpc Book() {        
   }

   @Id
   @GeneratedValue(strategy= GenerationType.IDENTITY)
   @Column(name="id")
   pubpc int getId() {
      return id;
   }

   pubpc void setId(int id) {
      this.id = id;
   }

   pubpc String getName() {
      return name;
   }

   pubpc void setName(String name) {
      this.name = name;
   }    
}

LibraryPersistentBeanRemote.java

package com.tutorialspoint.stateless;

import com.tutorialspoint.entity.Book;
import java.util.List;
import javax.ejb.Remote;

@Remote
pubpc interface LibraryPersistentBeanRemote {

   void addBook(Book bookName);

   List<Book> getBooks();
    
}

LibraryPersistentBean.java

package com.tutorialspoint.stateless;

import com.tutorialspoint.entity.Book;
import java.util.List;
import javax.ejb.Stateless;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;

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

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

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

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

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 - com.tutorialspoint.stateless.LibraryPersistentBeanRemote

JBoss Apppcation 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-com.tutorialspoint.stateless.LibraryPersistentBeanRemote - EJB3.x Remote Business Interface
16:30:02,723 INFO  [SessionSpecContainer] Starting jboss.j2ee:jar=EjbComponent.jar,name=LibraryPersistentBeanRemote,service=EJB3
16:30:02,723 INFO  [EJBContainer] STARTED EJB: com.tutorialspoint.stateless.LibraryPersistentBeanRemote ejbName: LibraryPersistentBean
16:30:02,731 INFO  [JndiSessionRegistrarBase] Binding the following Entries in Global JNDI:

   LibraryPersistentBean/remote - EJB3.x Default Remote Business Interface
   LibraryPersistentBean/remote-com.tutorialspoint.stateless.LibraryPersistentBeanRemote - EJB3.x Remote Business Interface
...   

EJBTester (EJB Cpent)

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 initiapze the InitialContext object of java naming service.

InitialContext object will be used to lookup stateless session bean.

EJBTester.java

package com.tutorialspoint.test;
   
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.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.testEntityEjb();
   }
   
   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 testEntityEjb() {

      try {
         int choice = 1; 

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

         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 name - "LibraryStatefulSessionBean/remote" to obtain the remote business object (stateful ejb).

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

If user enters 1, system asks for book name and saves the book using stateless session bean addBook() method. Session Bean is persisting the book in database via EntityManager call.

If user enters 2, system retrieves books using stateful session bean getBooks() method and exits.

Then another jndi lookup is done with name - "LibraryStatelessSessionBean/remote" to obtain the remote business object (stateless 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 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: 15 seconds)

Run Cpent Again to Access EJB

Restart the JBoss before accessing the 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 Spring
**********************
Welcome to Book Store
**********************
Options 
1. Add Book
2. Exit 
Enter Choice: 2
Book(s) entered so far: 2
1. learn java
2. Learn Spring
BUILD SUCCESSFUL (total time: 15 seconds)

The output shown above states that books are getting stored in persistent storage and are retrieved from database.