EJB - Entity Relationships

EJB 3.0 provides option to define database entity relationships/mappings pke one-to-one, one-to-many, many-to-one, and many-to-many relationships.

Following are the relevant annotations −

One-to-One − Objects have one-to-one relationship. For example, a passenger can travel using a single ticket at a time.

One-to-Many − Objects have one-to-many relationship. For example, a father can have multiple kids.

Many-to-One − Objects have many-to-one relationship. For example, multiple kids having a single mother.

Many-to-Many − Objects have many-to-many relationship. For example, a book can have multiple authors and an author can write multiple books.

We will demonstrate use of ManyToMany mapping here. To represent ManyToMany relationship, three following tables are required −

Book − Book table, having records of books.

Author − Author table, having records of author.

Book_Author − Book Author table, having pnkage of above mentioned Book and Author table.

Create Tables

Create a table book author, book_author in default database postgres.

CREATE TABLE book (
   book_id     integer,   
   name   varchar(50)      
);

CREATE TABLE author (
   author_id   integer,
   name   varchar(50)      
);

CREATE TABLE book_author (
   book_id     integer,
   author_id   integer 
);

Create Entity Classes

@Entity
@Table(name="author")
pubpc class Author implements Seriapzable{
   private int id;
   private String name;
   ...   
}

@Entity
@Table(name="book")
pubpc class Book implements Seriapzable{
   private int id;
   private String title;
   private Set<Author> authors;
   ...   
}

Use ManyToMany annotation in Book Entity.

@Entity
pubpc class Book implements Seriapzable{
   ...
   @ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE}
      , fetch = FetchType.EAGER)
   @JoinTable(table = @Table(name = "book_author"),
      joinColumns = {@JoinColumn(name = "book_id")},
      inverseJoinColumns = {@JoinColumn(name = "author_id")})
   pubpc Set<Author> getAuthors() {
      return authors;
   }
   ...
}

Example Apppcation

Let us create a test EJB apppcation to test entity relationships objects in EJB 3.0.

EJBComponent (EJB Module)

Author.java

package com.tutorialspoint.entity;

import java.io.Seriapzable;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name="author")
pubpc class Author implements Seriapzable{
    
   private int id;
   private String name;

   pubpc Author() {}

   pubpc Author(int id, String name) {
      this.id = id;
      this.name = name;
   }
   
   @Id  
   @GeneratedValue(strategy= GenerationType.IDENTITY)
   @Column(name="author_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;
   }

   pubpc String toString() {
      return id + "," + name;
   }    
}

Book.java

package com.tutorialspoint.entity;

import java.io.Seriapzable;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Table;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;

@Entity
@Table(name="book")
pubpc class Book implements Seriapzable{

   private int id;
   private String name;
   private Set<Author> authors;

   pubpc Book() {        
   }

   @Id  
   @GeneratedValue(strategy= GenerationType.IDENTITY)
   @Column(name="book_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;
   }

   pubpc void setAuthors(Set<Author> authors) {
      this.authors = authors;
   }    
   
   @ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE}
      , fetch = FetchType.EAGER)
   @JoinTable(table = @Table(name = "book_author"),
      joinColumns = {@JoinColumn(name = "book_id")},
      inverseJoinColumns = {@JoinColumn(name = "author_id")})
   pubpc Set<Author> getAuthors() {
      return authors;
   }
}

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 using this lookup string to get remote business object of type − com.tutorialspoint.interceptor.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.interceptor.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: com.tutorialspoint.interceptor.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.interceptor.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.stateful.LibraryBeanRemote;

import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStreamReader;

import java.util.*;

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.testEmbeddedObjects();
   }
   
   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 testEmbeddedObjects() {

      try {
         int choice = 1; 

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

         while (choice != 2) {
            String bookName;
            String authorName;
            
            showGUI();
            String strChoice = brConsoleReader.readLine();
            choice = Integer.parseInt(strChoice);
            if (choice == 1) {
               System.out.print("Enter book name: ");
               bookName = brConsoleReader.readLine();
               System.out.print("Enter author name: ");
               authorName = brConsoleReader.readLine();               
               Book book = new Book();
               book.setName(bookName);
               Author author = new Author();
               author.setName(authorName);
               Set<Author> authors = new HashSet<Author>();
               authors.add(author);
               book.setAuthors(authors);

               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());
            System.out.print("Author: ");
            Author[] authors = (Author[])books.getAuthors().toArray();
            for(int j=0;j<authors.length;j++) {
               System.out.println(authors[j]);
            }
            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 testInterceptedEjb() method, jndi lookup is done with name - "LibraryPersistenceBean/remote" to obtain the remote business object (stateless EJB).

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 saves the book using stateless session bean addBook() method. Session Bean is storing the book in database.

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

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 html5
Enter Author name: Robert
**********************
Welcome to Book Store
**********************
Options 
1. Add Book
2. Exit 
Enter Choice: 2
Book(s) entered so far: 1
1. learn html5
Author: Robert
BUILD SUCCESSFUL (total time: 21 seconds)