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Hibernate 3.0, the latest Open Source persistence technology at the heart of J2EE EJB 3.0 is available for download from Hibernet.org.The Hibernate 3.0 core is 68,549 lines of Java code together with 27,948 lines of unit tests, all freely available under

Introduction to Hibernate 3.0

What is Hibernate?
Hibernate 3.0, the latest Open Source persistence technology at the heart of J2EE EJB 3.0 is available for download from Hibernet.org.The Hibernate 3.0 core is 68,549 lines of Java code together with 27,948 lines of unit tests, all freely available under the LGPL, and has been in development for well over a year. Hibernate maps the Java classes to the database tables. It also provides the data query and retrieval facilities that significantly reduces the development time. Hibernate is not the best solutions for data centric applications that only uses the stored-procedures to implement the business logic in database. It is most useful with object-oriented domain modes and business logic in the Java-based middle-tier. Hibernate allows transparent persistence that enables the applications to switch any database. Hibernate can be used in Java Swing applications, Java Servlet-based applications, or J2EE applications using EJB session beans.

Features of Hibernate

Hibernate 3.0 provides three full-featured query facilities: Hibernate Query Language, the newly enhanced Hibernate Criteria Query API, and enhanced support for queries expressed in the native SQL dialect of the database.
Filters for working with temporal (historical), regional or permissioned data.
Enhanced Criteria query API: with full support for projection/aggregation and subselects.
Runtime performance monitoring: via JMX or local Java API, including a second-level cache browser.
Eclipse support, including a suite of Eclipse plug-ins for working with Hibernate 3.0, including mapping editor, interactive query prototyping, schema reverse engineering tool.
Hibernate is Free under LGPL: Hibernate can be used to develop/package and distribute the applications for free.
Hibernate is Scalable: Hibernate is very performant and due to its dual-layer architecture can be used in the clustered environments.
Less Development Time: Hibernate reduces the development timings as it supports inheritance, polymorphism, composition and the Java Collection framework.
Automatic Key Generation: Hibernate supports the automatic generation of primary key for your.
JDK 1.5 Enhancements: The new JDK has been released as a preview earlier this year and we expect a slow migration to the new 1.5 platform throughout 2004. While Hibernate3 still runs perfectly with JDK 1.2, Hibernate3 will make use of some new JDK features. JSR 175 annotations, for example, are a perfect fit for Hibernate metadata and we will embrace them aggressively. We will also support Java generics, which basically boils down to allowing type safe collections.
EJB3-style persistence operations: EJB3 defines the create() and merge() operations, which are slightly different to Hibernate's saveOrUpdate() and saveOrUpdateCopy() operations. Hibernate3 will support all four operations as methods of the Session interface.
Hibernate XML binding enables data to be represented as XML and POJOs interchangeably.
The EJB3 draft specification support for POJO persistence and annotations.
Hibernate Architecture

In this lesson you will learn the architecture of Hibernate. The following diagram describes the high level architecture of hibernate:

The above diagram shows that Hibernate is using the database and configuration data to provide persistence services (and persistent objects) to the application.
To use Hibernate, it is required to create Java classes that represents the table in the database and then map the instance variable in the class with the columns in the database. Then Hibernate can be used to perform operations on the database like select, insert, update and delete the records in the table. Hibernate automatically creates the query to perform these operations.
Hibernate architecture has three main components:
- Connection Management
  Hibernate Connection management service provide efficient management of the database connections. Database connection is the most expensive part of interacting with the database as it requires a lot of resources of open and close the database connection.
- Transaction management:
  Transaction management service provide the ability to the user to execute more than one database statements at a time.
- Object relational mapping:
  Object relational mapping is technique of mapping the data representation from an object model to a relational data model. This part of the hibernate is used to select, insert, update and delete the records form the underlying table. When we pass an object to a Session.save() method, Hibernate reads the state of the variables of that object and executes the necessary query.
Hibernate is very good tool as far as object relational mapping is concern, but in terms of connection management and transaction management, it is lacking in performance and capabilities. So usually hibernate is being used with other connection management and transaction management tools. For example apache DBCP is used for connection pooling with the Hibernate.
Hibernate provides a lot of flexibility in use. It is called "Lite" architecture when we only uses the object relational mapping component. While in "Full Cream" architecture all the three component Object Relational mapping, Connection Management and Transaction Management) are used.

Writing First Hibernate Code

In this section I will show you how to create a simple program to insert record in MySQL database. You can run this program from Eclipse or from command prompt as well. I am assuming that you are familiar with MySQL and Eclipse environment.

Configuring Hibernate
In this application Hibernate provided connection pooling and transaction management is used for simplicity. Hibernate uses the hibernate.cfg.xml to create the connection pool and setup required environment.

Here is the code:

<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<hibernate-configuration>
<session-factory>
<property name="hibernate.connection.driver_class">

com.mysql.jdbc.Driver</property>
<property name="hibernate.connection.url">

jdbc:mysql://localhost/hibernatetutorial</property>
<property name="hibernate.connection.username">root</property>
<property name="hibernate.connection.password"></property>
<property name="hibernate.connection.pool_size">10</property>
<property name="show_sql">true</property>
<property name="dialect">org.hibernate.dialect.MySQLDialect</property>
<property name="hibernate.hbm2ddl.auto">update</property>

<mapping resource="contact.hbm.xml"/>
</session-factory>
</hibernate-configuration>

In the above configuration file we specified to use the "hibernatetutorial"which is running on localhost and the user of the database is root with no password. The dialect property is org.hibernate.dialect.MySQLDialectwhich tells the Hibernate that we are using MySQL Database. Hibernate supports many database. With the use of the Hibernate (Object/Relational Mapping and Transparent Object Persistence for Java and SQL Databases), we can use the following databases dialect type property:

DB2 - org.hibernate.dialect.DB2Dialect
HypersonicSQL - org.hibernate.dialect.HSQLDialect
Informix - org.hibernate.dialect.InformixDialect
Ingres - org.hibernate.dialect.IngresDialect
Interbase - org.hibernate.dialect.InterbaseDialect
Pointbase - org.hibernate.dialect.PointbaseDialect
PostgreSQL - org.hibernate.dialect.PostgreSQLDialect
Mckoi SQL - org.hibernate.dialect.MckoiDialect
Microsoft SQL Server - org.hibernate.dialect.SQLServerDialect
MySQL - org.hibernate.dialect.MySQLDialect
Oracle (any version) - org.hibernate.dialect.OracleDialect
Oracle 9 - org.hibernate.dialect.Oracle9Dialect
Progress - org.hibernate.dialect.ProgressDialect
FrontBase - org.hibernate.dialect.FrontbaseDialect
SAP DB - org.hibernate.dialect.SAPDBDialect
Sybase - org.hibernate.dialect.SybaseDialect
Sybase Anywhere - org.hibernate.dialect.SybaseAnywhereDialect

The <mapping resource="contact.hbm.xml"/> property is the mapping for our contact table.

Writing First Persistence Class
Hibernate uses the Plain Old Java Objects (POJOs) classes to map to the database table. We can configure the variables to map to the database column. Here is the code for Contact.java:

package roseindia.tutorial.hibernate;

/**
* @author Deepak Kumar
*
* Java Class to map to the datbase Contact Table
*/
public class Contact {
  private String firstName;
  private String lastName;
  private String email;
  private long id;

  /**
* @return Email
*/
  public String getEmail() {
  return email;
  }

  /**
* @return First Name
*/
  public String getFirstName() {
  return firstName;
  }

  /**
* @return Last name
*/
  public String getLastName() {
  return lastName;
  }

  /**
* @param string Sets the Email
*/
  public void setEmail(String string) {
  email = string;
  }

  /**
* @param string Sets the First Name
*/
  public void setFirstName(String string) {
  firstName = string;
  }

  /**
* @param string sets the Last Name
*/
  public void setLastName(String string) {
  lastName = string;
  }

  /**
* @return ID Returns ID
*/
  public long getId() {
  return id;
  }

  /**
* @param l Sets the ID
*/
  public void setId(long l) {
  id = l;
  }

}

Mapping the Contact Object to the Database Contact table
The file contact.hbm.xml is used to map Contact Object to the Contact table in the database. Here is the code for contact.hbm.xml:

<?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="roseindia.tutorial.hibernate.Contact" table="CONTACT">
   <id name="id" type="long" column="ID" >
   <generator class="assigned"/>
</id>

<property name="firstName">
   <column name="FIRSTNAME" />
</property>
<property name="lastName">
<column name="LASTNAME"/>
</property>
<property name="email">
<column name="EMAIL"/>
</property>
</class>
</hibernate-mapping>

Setting Up MySQL Database
In the configuration file(hibernate.cfg.xml) we have specified to use hibernatetutorialdatabase running on localhost. So, create the databse ("hibernatetutorial") on the MySQL server running on localhost.

Developing Code to Test Hibernate example
Now we are ready to write a program to insert the data into database. We should first understand about the Hibernate's Session. Hibernate Session is the main runtime interface between a Java application and Hibernate. First we are required to get the Hibernate Session.SessionFactory allows application to create the Hibernate Sesssion by reading the configuration from hibernate.cfg.xml file. Then the save method on session object is used to save the contact information to the database:

session.save(contact)

Here is the code of FirstExample.java

package roseindia.tutorial.hibernate;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

/**
* @author Deepak Kumar
*
* http://www.roseindia.net
* Hibernate example to inset data into Contact table
*/
public class FirstExample {
  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();
//Create new instance of Contact and set

values in it by reading them from form object
System.out.println("Inserting Record");
  Contact contact = new Contact();
  contact.setId(3);
  contact.setFirstName("Deepak");
  contact.setLastName("Kumar");
  contact.setEmail("deepak_38@yahoo.com");
  session.save(contact);
  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();

  }

  }
}

In the next section I will show how to run and test the program.

Understanding Hibernate O/R Mapping

In the last example we created contact.hbm.xml to map Contact Object to the Contact table in the database. Now let's understand the each component of the mapping file.

To recall here is the content of contact.hbm.xml:

Hibernate mapping documents are simple xml documents. Here are important elements of the mapping file:.

<hibernate-mapping> element
The first or root element of hibernate mapping document is <hibernate-mapping> element. Between the <hibernate-mapping> tag class element(s) are present.
<class> element
The <Class> element maps the class object with corresponding entity in the database. It also tells what table in the database has to access and what column in that table it should use. Within one <hibernate-mapping> element, several <class> mappings are possible.
<id> element
The <id> element in unique identifier to identify and object. In fact <id> element map with the primary key of the table. In our code :
<id name="id" type="long" column="ID" >
primary key maps to the ID field of the table CONTACT. The attributes of the id element are:

name: The property name used by the persistent class.
column: The column used to store the primary key value.
type: The Java data type used.
unsaved-value: This is the value used to determine if a class has been made persistent. If the value of the id attribute is null, then it means that this object has not been persisted.

<generator> element
The <generator> method is used to generate the primary key for the new record. Here is some of the commonly used generators :

* Increment - This is used to generate primary keys of type long, short or int that are unique only. It should not be used in the clustered deployment environment.

* Sequence - Hibernate can also use the sequences to generate the primary key. It can be used with DB2, PostgreSQL, Oracle, SAP DB databases.

* Assigned - Assigned method is used when application code generates the primary key.
<property> element
The property elements define standard Java attributes and their mapping into database schema. The property element supports the column child element to specify additional properties, such as the index name on a column or a specific column type.

Understanding Hibernate <generator> element

In this lesson you will learn about hibernate <generator> method in detail. Hibernate generator element generates the primary key for new record. There are many options provided by the generator method to be used in different situations.

The <generator> element

This is the optional element under <id> element. The <generator> element is used to specify the class name to be used to generate the primary key for new record while saving a new record. The <param> element is used to pass the parameter (s) to the class. Here is the example of generator element from our first application:
<generator class="assigned"/>
In this case <generator> element do not generate the primary key and it is required to set the primary key value before calling save() method.

Using Hibernate to generate id incrementally
Posted on: January 18, 2008 at 12:00 AM

As we have seen in the last section that the increment class generates identifiers of type long, short or int that are unique only when no other process is inserting data into the same table.

Using Hibernate <generator> to generate id incrementally

As we have seen in the last section that the increment class generates identifiers of type long, short or int that are unique only when no other process is inserting data into the same table. In this lesson I will show you how to write running program to demonstrate it. You should not use this method to generate the primary key in case of clustured environment.

In this we will create a new table in database, add mappings in the contact.hbm.xml file, develop the POJO class (Book.java), write the program to test it out.

Create Table in the mysql database:
User the following sql statement to create a new table in the database.
CREATE TABLE `book` (
`id` int(11) NOT NULL default '0',
`bookname` varchar(50) default NULL,
PRIMARY KEY (`id`)
) TYPE=MyISAM

Developing POJO Class (Book.java)
Book.java is our POJO class which is to be persisted to the database table "book".

/**
* @author Deepak Kumar
*
* http://www.roseindia.net
* Java Class to map to the database Book table
*/
package roseindia.tutorial.hibernate;

public class Book {
  private long lngBookId;
  private String strBookName;

  /**
* @return Returns the lngBookId.
*/
  public long getLngBookId() {
  return lngBookId;
  }
  /**
* @param lngBookId The lngBookId to set.
*/
  public void setLngBookId(long lngBookId) {
  this.lngBookId = lngBookId;
  }
  /**
* @return Returns the strBookName.
*/
  public String getStrBookName() {
  return strBookName;
  }
  /**
* @param strBookName The strBookName to set.
*/
  public void setStrBookName(String strBookName) {
  this.strBookName = strBookName;
  }
}

Adding Mapping entries to contact.hbm.xml
Add the following mapping code into the contact.hbm.xml file

Note that we have used increment for the generator class. *After adding the entries to the xml file copy it to the bin directory of your hibernate eclipse project(this step is required if you are using eclipse).

Write the client program and test it out
Here is the code of our client program to test the application.

/**
* @author Deepak Kumar
*
* http://www.roseindia.net
* Example to show the increment
class of hibernate
generator element to
* automatically generate
the primay key
*/
package roseindia.tutorial.hibernate;

//Hibernate Imports
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

public class IdIncrementExample {
  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();

  org.hibernate.Transaction tx =
session.beginTransaction();

  //Create new instance
of Contact and set values in
it by reading them from form object
System.out.println("
Inserting Book object
into database..");
  Book book = new Book();
  book.setStrBookName("Hibernate
Tutorial");
  session.save(book);
  System.out.println("Book object
persisted to the database.");
  tx.commit();
  session.flush();
  session.close();
  }catch(Exception e){
  System.out.println(e.getMessage());
  }finally{
  }

  }
}

To test the program Select Run->Run As -> Java Application from the eclipse menu bar. This will create a new record into the book table.

Hibernate Update Query

In this tutorial we will show how to update a row with new information by retrieving data from the underlying database using the hibernate. Lets first write a java class to update a row to the database.

Create a java class:
Here is the code of our java file (UpdateExample.java), where we will update a field name "InsuranceName"with a value="Jivan Dhara" from a row of the insurance table.

Here is the code of delete query: UpdateExample .java

package roseindia.tutorial.hibernate;

import java.util.Date;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;

public class UpdateExample {
  /**
* @param args
*/
  public static void main(String[] args) {
  // TODO Auto-generated method stub
  Session sess = null;
  try {
  SessionFactory fact = new Configuration()
.configure().buildSessionFactory();
  sess = fact.openSession();
  Transaction tr = sess.beginTransaction();
  Insurance ins = (Insurance)sess.get
(Insurance.class, new Long(1));
  ins.setInsuranceName("Jivan Dhara");
  ins.setInvestementAmount(20000);
  ins.setInvestementDate(new Date());
  sess.update(ins);
  tr.commit();
  sess.close();
  System.out.println("Update successfully!");
  }
  catch(Exception e){
  System.out.println(e.getMessage());
  }
  }
}

Hibernate Delete Query

In this lesson we will show how to delete rows from the underlying database using the hibernate. Lets first write a java class to delete a row from the database.

Create a java class:
Here is the code of our java file (DeleteHQLExample.java), which we will delete a row from the insurance table using the query "delete from Insurance insurance where id = 2"

Here is the code of delete query: DeleteHQLExample.java

package roseindia.tutorial.hibernate;

import org.hibernate.Query;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;

public class DeleteHQLExample {
  /**
* @author vinod Kumar
*
* http://www.roseindia.net Hibernate
Criteria Query Example
*
*/
  public static void main(String[] args) {
  // TODO Auto-generated method stub
  Session sess = null;
  try {
  SessionFactory fact = new
Configuration().configure().buildSessionFactory();
  sess = fact.openSession();
  String hql = "delete from
Insurance insurance where id = 2";
  Query query = sess.createQuery(hql);
  int row = query.executeUpdate();
  if (row == 0){
  System.out.println("Doesn'
t deleted any row!");
  }
  else{
  System.out.println("Deleted
Row: " + row);
  }
  sess.close();
  }
  catch(Exception e){
  System.out.println(e.getMessage());
  }
  }
}

Hibernate Query Language

Hibernate Query Language or HQL for short is extremely powerful query language. HQL is much like SQL and are case-insensitive, except for the names of the Java Classes and properties. Hibernate Query Language is used to execute queries against database. Hibernate automatically generates the sql query and execute it against underlying database if HQL is used in the application. HQL is based on the relational object models and makes the SQL object oriented. Hibernate Query Language uses Classes and properties instead of tables and columns. Hibernate Query Language is extremely powerful and it supports Polymorphism, Associations, Much less verbose than SQL.

There are other options that can be used while using Hibernate. These are Query By Criteria (QBC)and Query BY Example (QBE) using Criteria API and the Native SQLqueries. In this lesson we will understand HQL in detail.

Why to use HQL?

Full support for relational operations: HQL allows representing SQL queries in the form of objects. Hibernate Query Language uses Classes and properties instead of tables and columns.
Return result as Object: The HQL queries return the query result(s) in the form of object(s), which is easy to use. This elemenates the need of creating the object and populate the data from result set.
Polymorphic Queries: HQL fully supports polymorphic queries. Polymorphic queries results the query results along with all the child objects if any.
Easy to Learn: Hibernate Queries are easy to learn and it can be easily implemented in the applications.
Support for Advance features: HQL contains many advance features such as pagination, fetch join with dynamic profiling, Inner/outer/full joins, Cartesian products. It also supports Projection, Aggregation (max, avg) and grouping, Ordering, Sub queries and SQL function calls.
Database independent: Queries written in HQL are database independent (If database supports the underlying feature).

Understanding HQL Syntax
Any Hibernate Query Language may consist of following elements:

Clauses
Aggregate functions
Subqueries

Clauses in the HQL are:

from
select
where
order by
group by

Aggregate functions are:

avg(...), sum(...), min(...), max(...)
count(*)
count(...), count(distinct ...), count(all...)

Subqueries
Subqueries are nothing but its a query within another query. Hibernate supports Subqueries if the underlying database supports it.

Preparing table for HQL Examples

In this lesson we will create insurance table and populate it with the data. We will use insurance table for rest of the HQL tutorial.

To create the insurance table and insert the sample data, run the following sql query:

/*Table structure for table `insurance` */

drop table if exists `insurance`;

CREATE TABLE `insurance` (

`ID` int(11) NOT NULL default '0',

`insurance_name` varchar(50) default NULL,

`invested_amount` int(11) default NULL,

`investement_date` datetime default NULL,

PRIMARY KEY (`ID`)

) TYPE=MyISAM;

/*Data for the table `insurance` */