Dinesh on Java

Hibernate

Saving Objects using Hibernate APIs

Introduction to Hibernate

An ORM tool.
Used in the data layer of application.
Implement JPA.
JPA (Java persistence api):- this is set of standards for and java persistence api.

The problem with JDBC

Mapping member variables to the column.
Mapping relationships.
Handling data types.
Example Boolean most of the databases do not have Boolean datatype.
Managing the state of object.

Saving with an object with hibernate.

JDBC database configuration >> Hibernate configuration .cfg.xml file.
The model object >> Annotations or Hibernate mapping file .hbm.xml files.
Service method to create the model object >> Use the hibernate.
Database design. >> No needed.
DAO method to save the object using sql queries >> No needed.

Book.java
Now, i am going to create a simple POJO class Book.java with some properties and their setters and getters.

package com.bloggers.model;
 
import java.util.Date;
 
public class Book
{
 /* for primary key in database table */
 private int bookId;
 private String bookName;
 private Date date;
 
 public int getBookId()
 {
  return bookId;
 }
 
 public void setBookId(int bookId)
 {
  this.bookId = bookId;
 }
 
 public String getBookName()
 {
  return bookName;
 }
 
 public void setBookName(String bookName)
 {
  this.bookName = bookName;
 }
 
 public Date getDate()
 {
  return date;
 }
 
 public void setDate(Date date)
 {
  this.date = date;
 }
 
  
}

Few point while creating a model object or JavaBeans (Book.java) class.

bookId property holds a unique identifier value for a book object. Provides an identifier property if you want to use the full feature set of Hibernate
Application need to distinguish objects by identifier.
Hibernate can access public, private, and protected accessor methods, as well as public, private and protected fields directly.
no-argument constructor is a requirement for all persistent classes. Hibernate create objects using Java Reflection.

Mapping file :-hbm file (Hibernate Mapping file)

Now second step is we have to create a hbm file with extension .hbm.xml. HBM file means Hibernate Mapping File. Mapping file gives all the information to hibernate about table columns etc which are mapped to object.
All persistent entity classes need a mapping to a table in the SQL database.
Basic structor of hbm file

<hibernate -mapping="-mapping">
  [...]
  </hibernate>

Note:

Hibernate not loads the DTD file from the web, it first look it up from the classpath of the application.
DTD file is included in hibernate-core.jar

Mapping file of Book.java class(Book.hbm.xml)

I will complete this step by step and try to explain all the elements.
1. Add a class element between two hibernate-mapping tags.

<?xml version="1.0"?>
   <!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
 
  <hibernate-mapping>
   <class name="com.bloggers.model.Book" >
     
   </class>
  </hibernate-mapping>

class=”com.bloggers.model.Book” .Specify the qualified class name in name attribute of class element.

2. Next, we need to tell Hibernate about the remaining entity class properties. By default, no properties of the class are considered persistent.

<?xml version="1.0"?>
   <!DOCTYPE hibernate-mapping PUBLIC
     "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
     "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
  
   <hibernate-mapping>
    <class name="com.bloggers.model.Book" >
     <id name="bookId"/>
     <property name="date" column="PUBLISH_DATE"/>
    </class>
   </hibernate-mapping>

name=”bookId” attribute of the property element tells Hibernate which getter and setter methods to use in our case hibernate will search for getBookId() , setBookId() etc methods.

Question : Why does the date property mapping include the column attribute, but the bookID does not?
Answer : Without the column attribute, Hibernate by default uses the property name as the column name. This works for bookId, however, date is a reserved keyword in most databases so you will need to map it to a different name.

Create a Hibernate Configuration File (hibernate.cfg.xml)

Hibernate required a configuration file for making the connection with database by default the file name is hibernate.cfg.xml

We can do hibernate configuration in 3 different ways and these are :-

we can use a simple hibernate.properties file
a more sophisticated hibernate.cfg.xml file
or even complete programmatic setup

I am going to use the 2 option means by using hibernate.cfg.xml file.

<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
  "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
  "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
 
<hibernate-configuration>
 
 <session-factory>
 
  <!-- Database connection settings -->
  <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
  <property name="connection.url">jdbc:mysql://localhost:3306/hibernatedb</property>
  <property name="connection.username">root</property>
  <property name="connection.password">root</property>
 
  <!-- JDBC connection pool (use the built-in) -->
  <property name="connection.pool_size">1</property>
 
  <!-- SQL dialect -->
  <property name="dialect">org.hibernate.dialect.MySQL5Dialect</property>
 
  <!-- Disable the second-level cache  -->
  <property name="cache.provider_class">org.hibernate.cache.NoCacheProvider</property>
 
  <!-- Echo all executed SQL to stdout -->
  <property name="show_sql">true</property>
 
  <!-- Drop and re-create the database schema on startup -->
  <property name="hbm2ddl.auto">create</property>
 
  <mapping resource="com/bloggers/model/Book.hbm.xml"/>
 
 </session-factory>
 
</hibernate-configuration>

SessionFactory is a global factory responsible for a particular database. If you have several databases, for easier startup you should use several configurations in several configuration files.
The first four property elements contain the necessary configuration for the JDBC connection.
The dialect property element specifies the particular SQL variant Hibernate generates.
In most cases, Hibernate is able to properly determine which dialect to use.
The hbm2ddl.auto option turns on automatic generation of database schema directly into the database. This can also be turned off by removing the configuration option, or redirected to a file with the help of the SchemaExport Ant task.
Finally, add the mapping file(s) for persistent classes to the configuration.

HibernateUtil.java
Utility class for getting the sessionFactory.

package com.bloggers.util;
 
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;
 
public class HibernateUtil
{
 private static final SessionFactory sessionFactory = buildSessionFactory();
 
 private static SessionFactory buildSessionFactory()
 {
  try
  {
   // Create the SessionFactory from hibernate.cfg.xml
   return new Configuration().configure().buildSessionFactory();
  }
  catch (Throwable ex)
  {
   // Make sure you log the exception, as it might be swallowed
   System.err.println("Initial SessionFactory creation failed." + ex);
   throw new ExceptionInInitializerError(ex);
  }
 }
 
 public static SessionFactory getSessionFactory()
 {
  return sessionFactory;
 }
}

Run the example
Create a simple class for running the example.

package com.bloggers;
 
import java.util.Date;
 
import org.hibernate.Session;
import org.hibernate.SessionFactory;
 
import com.bloggers.model.Book;
import com.bloggers.util.HibernateUtil;
 
public class HibernateTest
{
 
 public static void main(String[] args)
 {
  HibernateTest hibernateTest = new HibernateTest();
 
  /* creating a new book object */
  Book book = new Book();
  book.setBookId(1);
  book.setDate(new Date());
  book.setBookName("Hibernate in action");
 
  /* saving the book object */
  hibernateTest.saveBook(book);
 
 }
 
 public void saveBook(Book book)
 {
  SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
  Session session = sessionFactory.openSession();
  session.beginTransaction();
 
  /* saving the book object here */
  session.save(book);
  session.getTransaction().commit();
 }
 
}

Before running the HibernateTest class make sure you have created a schema named hibernatedb in MySQL database .

Question : Why schema named hibernatedb ?
Answer : Because in hibenate.cfg.xml

<property name=”connection.url”>jdbc:mysql://localhost:3306/hibernatedb</property>
we have specify hibernatedb. You are free to change whatever name you want. Create schema using mysql>create schema hibernate; command.

Question : Did you see the table it does not contains the column name bookName?
Answer : If yes then you can easily found that why we do not have bookName in table book and the answer is because we did not map the bookName property in our hbm file so map the property and run the main method again and you will observe that.