Collection Mapping

[subhashchandra medhiassam]

Hi all,
I am new to Hibernate. I am entangled with Collection-Mappping.I need some good tutorial having complete example program on Collection Mapping.

Also, i tried the following to test Collection mapping.

I wrote the following POJO class:

 
package roseindia.tutorial.hibernatecollection;
 
import java.util.Date;
import java.util.List;
 
public class Book {
 
    private Long id;
    private String isbn;
    private String name;
    private Date publishDate;
    private Integer price;
    private List chapters;
    
    public List getChapters() {
        return chapters;
    }
    public void setChapters(List chapters) {
        this.chapters = chapters;
    }
    public Long getId() {
        return id;
    }
    public void setId(Long id) {
        this.id = id;
    }
    public String getIsbn() {
        return isbn;
    }
    public void setIsbn(String isbn) {
        this.isbn = isbn;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Integer getPrice() {
        return price;
    }
    public void setPrice(Integer price) {
        this.price = price;
    }
    public Date getPublishDate() {
        return publishDate;
    }
    public void setPublishDate(Date publishDate) {
        this.publishDate = publishDate;
    }
    
    
 
}
   

I wrote the following class to persist an object of type Book into the MYSQL database running on local host:

  package roseindia.tutorial.hibernatecollection;
 

import java.util.Date;
import java.util.LinkedList;
import java.util.List;
 
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
 
import roseindia.tutorial.hibernate.Contact;
 
public class BookInsert {
 
    /**
     * @param args
     */
    public static void main(String[] args) {
Session session = null;
 
try{
// This step will read hibernate.cfg.xml and prepare hibernate for use
SessionFactory sessionFactory = new Configuration().configure().buildSessionFactory();
session =sessionFactory.openSession();
Transaction tx = session.beginTransaction();
//Create new instance of Book and set values in it by reading them from form object
 
System.out.println("Inserting Record");
Book book = new Book();
List <String> lessions = new LinkedList<String>();
lessions.add("chp.1");
lessions.add("chp.2");
lessions.add("index");
Date date = new Date();

book.setChapters(lessions);
book.setId((long)1);
book.setIsbn("#$#$%#%$%$");
book.setName("Hibernate tutorial");
book.setPrice(100);
book.setPublishDate(date);
session.save(book);
tx.commit();
System.out.println("Done");

}catch(Exception e){
System.out.println(e.getMessage());
}finally{
// Actual contact insertion will happen at this step
session.flush();
session.close();
 
}
    }
 
}
  

The following is the content of my mapping file:

  
<class name="Book" table="BOOK">
<id name="id" type="long" column="ID">
<generator class="native"/>
</id>
<property name="isbn" type="string">
<column name="ISBN" length="50" not-null="true" unique="true" />
</property>
<property name="name" type="string">
<column name="BOOK_NAME" length="100" not-null="true" />
</property>
<property name="publishDate" type="date">
 <column name="PUBLISH_DATE" />
</property>
<property name="price" type="int">
 <column name="PRICE" />
</property>
<bag name="chapters" table="BOOK_CHAPTER">
 <key column="BOOK_ID" />
 <element column="CHAPTER" type="string" length="100" />
</bag>
 </class>  

When run the second program, i am getting the following output:
log4j:WARN No appenders could be found for logger (org.hibernate.cfg.Environment).
log4j:WARN Please initialize the log4j system properly.
null
Exception in thread "main" java.lang.NullPointerException
at roseindia.tutorial.hibernatecollection.BookInsert.main(BookInsert.java:52)

The line where the NullPointerException took place corresponds to:

session.flush();

Kindly let me know where i could have gone wrong.

[Paul Sturrock]

So you are trying to flush a session that is null. So I would assume that something has gone wrong in these two lines:

// This step will read hibernate.cfg.xml and prepare hibernate for use
SessionFactory sessionFactory = new Configuration().configure().buildSessionFactory();
session =sessionFactory.openSession();

What other exception messages do you have in your log? Has the SessionFactory been properly configured? Is the DataSource valid and can you connect to the database.
[ September 11, 2008: Message edited by: Paul Sturrock ]

[Cameron Wallace McKenzie]

As far as finding tutorials on collection mapping, might I suggest my own?

I've got some very simple and straight forward examples on how to map one to one, one-to-many and many-to-many associations. Many people have found these tutorials very helpful.

Mapping Collections with Hibernate

-Cameron McKenzie