JPA Retrieve Data By Using Many-to-Many Relation

In the previous section, you had read about the many-to-many relation.

Here, you will learn how to retrieve data to database table by using the JPA many-to-many relation. @ManyToMany Annotation provides the facility to relate data many-to-many to database table. In this case, you need to insert, update and retrieve many data to many tables. To use java.util.set for this operation.

As follow the following steps:

Table author:

Table book:

Table author_book:

Model Class:

Author.java

/**
*
*/
package roseindia;

import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinTable;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

/**
* @author Administrator
*
*/
@Entity
@Table(name="author")
public class Author {

@Id
@GeneratedValue(strategy=GenerationType.AUTO)
@Column(name="id")
private int id;

/**
* @return the id
*/
public int getId() {
return id;
}

/**
* @param id the id to set
*/
public void setId(int id) {
this.id = id;
}

@Column(name="authorName", nullable=false, length=50, insertable=true)
private String authorName;

/**
* @return the authorName
*/
public String getAuthorName() {
return authorName;
}

/**
* @param authorName the authorName to set
*/
public void setAuthorName(String authorName) {
this.authorName = authorName;
}

//for joing the tables (many-to-many)
@ManyToMany(cascade=CascadeType.ALL)
@JoinTable(name = "author_book",
joinColumns = {
@JoinColumn(name="authorId")
},
inverseJoinColumns = {
@JoinColumn(name="bookId")
}
)
private Set<Book> books;

/**
* @return the books
*/
public Set<Book> getBooks() {
return books;
}

/**
* @param books the books to set
*/
public void setBooks(Set<Book> books) {
this.books = books;
}

}

Book.java

/**
*
*/
package roseindia;

import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

/**
* @author Administrator
*
*/
@Entity
@Table(name="book")
public class Book {

@Id
@GeneratedValue(strategy=GenerationType.AUTO)
@Column(name="id")
private int id;

/**
* @return the id
*/
public int getId() {
return id;
}

/**
* @param id the id to set
*/
public void setId(int id) {
this.id = id;
}

@Column(name="bookName", nullable=false, length=250, insertable=true)
private String bookName;

/**
* @return the bookName
*/
public String getBookName() {
return bookName;
}

/**
* @param bookName the bookName to set
*/
public void setBookName(String bookName) {
this.bookName = bookName;
}

//for joing the tables (many-to-many)
@ManyToMany(cascade=CascadeType.ALL)
@JoinTable(name = "author_book",
joinColumns = {
@JoinColumn(name="bookId")
},
inverseJoinColumns = {
@JoinColumn(name="authorId")
}
)
private Set<Author> authors;

/**
* @return the authors
*/
public Set<Author> getAuthors() {
return authors;
}

/**
* @param authors the authors to set
*/
public void setAuthors(Set<Author> authors) {
this.authors = authors;
}

}

Main Class:

ManyToManyRelation1.java

/**
*
*/
package roseindia;

import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

/**
* @author Administrator
*
*/
public class ManyToManyRelation1 {

/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub

EntityManagerFactory emf=null;
EntityManager em=null;
try{
emf=Persistence.createEntityManagerFactory("default", new HashMap());
em=emf.createEntityManager();
em.getTransaction().begin();

Author author = new Author();
author.setAuthorName("Clifford Geertz");
Author author1 = new Author();
author1.setAuthorName("JP Morgenthal");

Book book = new Book();
book.setBookName("Phoenix");
Book book1 = new Book();
book1.setBookName("Enterprise Applications Integration with XML and Java");

HashSet bookSet = new HashSet();
bookSet.add(book);
bookSet.add(book1);
author.setBooks(bookSet);
author1.setBooks(bookSet);

em.persist(author);
em.persist(author1);

Author authorRecord = em.find(Author.class, author.getId());
System.out.println("Author: "+ authorRecord.getAuthorName());
Set<Book> bookset = authorRecord.getBooks();
Iterator it = bookset.iterator();
while(it.hasNext()){
Book b = (Book)it.next();
System.out.println("Book: "+ b.getBookName());
}
em.getTransaction().commit();
System.out.println("Done");
}
catch(Exception e){
System.out.println(e.getMessage());
}
finally{
em.close();
emf.close();
}

}

}

Output:

log4j:WARN No appenders could be found for logger (org.hibernate.cfg.annotations.Version).

log4j:WARN Please initialize the log4j system properly.

Hibernate: insert into author (authorName) values (?)

Hibernate: insert into book (bookName) values (?)

Hibernate: insert into author (authorName) values (?)

Author: Clifford Geertz

Book: Phoenix

Book: Enterprise Applications Integration with XML and Java

Hibernate: insert into author_book (authorId, bookId) values (?, ?)

Done 0

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JPA Retrieve Data By Using Many-to-Many Relation

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