Hibernate Tutorial for Beginers
1. Introduction Hibernate is an open source java based library used to work with relational databases. Hibernate is an Object/Relational Mapping (ORM) tool for Java environments. The term Object/Relational Mapping (ORM) refers to the technique of mapping a data representation from an object model to a relational data model with a SQL-based schema. It is a very powerful ORM solution built on top of JDBC (Java Database Connectivity) API.
Hibernate not only takes care of the mapping from Java classes to database tables (and from Java data types to SQL data types), but also provides data query and retrieval facilities. It can also significantly reduce development time otherwise spent with manual data handling in SQL and JDBC.
Hibernate can be also configured to use a database connection pooling mechanism to improve the performance for database operation. In this connections are checked out from database and maintained in a pool and after every database operation is returned back to pool. These connections can be returned back to database after a certain period if idleness so other applications, if any would not go out of database connections. By default, hibernate uses an inbuilt connection poll, but for production based environment it is not suggested. Hibernate can be configured for Apache DBCP or C3P0, a more famous and reliable connection pooling APIs.
Hibernate makes use of persistent objects called POJOs (Plain Old Java Objects) along with XML mapping documents for persisting objects into database. POJOs are a simple Java Object has getter and setter methods for an attribute. These objects works as a data carrier called as Data Transfer Object (DTO).They are used to carry data between different layers. The data is binded to method of these classes.
2. Hibernate Architecture
Hibernate High level architecture (2.1)
Below are some definitions of the objects depicted in the 2.1 diagram:
- SessionFactory (org.hibernate.SessionFactory)
- Session (org.hibernate.Session)
- Persistent Objects and Collections
- Transient and Detached Objects and Collections
- Transaction (org.hibernate.Transaction)
- ConnectionProvider (org.hibernate.connection.ConnectionProvider)
- TransactionFactory (org.hibernate.TransactionFactory)
- Extension Interfaces
Instance States :
An instance of persistent class can be in three different states; these states are defined in relation to a persistence context. The Hibernate Session object is the persistence context. The three different states are as follows:
- Transient
- Persistent
- Detached
Transaction Management :
Transaction Management service provides the ability to the user to execute more than one database statement at a time. To start a new transaction session.beginTransaction() method should be invoked on a session object.
If transaction is successfully executed it should be committed to persist the data in database by invoking transaction.commit() method.
In case of any exception or failure, the transaction must be rolled back using transaction.rollback() to synchronize the database state, otherwise database will throw an exception as current transaction is aborted.
3. Building a Simple Application Using Hibernate To build a simple application using hibernate, first we need to create a java project in eclipse or IDE in which you feel comfortable. After creation of a java project, add these below libraries to the project’s build path. You can download the latest version of hibernate from www.hibernate.org, if you have not already got one. The version used in this tutorial is 3.0. You may find it easier to download the package containing all the dependencies so that you know that all the necessary jar files are present. Some of these libraries are used by hibernate at run time.
Now we need to create a User Details table in database to store user’s detail and a database dialect in hibernate configuration file to tell hibernate to use database queries as per the database.
In this tutorial we are using PostgreSQL8.3 so org.hibernate.dialect.PostgreSQLDialect is used as database dialect in hibernate configuration file. If you are using database other than PostgreSQL you can refer the below table to find the appropriate dialects. In hibernate 3.3 only these below databases are supported by hibernate.
RDBMS
Dialect
DB2
org.hibernate.dialect.DB2Dialect
DB2 AS/400
org.hibernate.dialect.DB2400Dialect
DB2 OS390
org.hibernate.dialect.DB2390Dialect
PostgreSQL
org.hibernate.dialect.PostgreSQLDialect
MySQL
org.hibernate.dialect.MySQLDialect
MySQL with InnoDB
org.hibernate.dialect.MySQLInnoDBDialect
MySQL with MyISAM
org.hibernate.dialect.MySQLMyISAMDialect
Oracle (any version)
org.hibernate.dialect.OracleDialect
Oracle 9i
org.hibernate.dialect.Oracle9iDialect
Oracle 10g
org.hibernate.dialect.Oracle10gDialect
Sybase
org.hibernate.dialect.SybaseDialect
Sybase Anywhere
org.hibernate.dialect.SybaseAnywhereDialect
Microsoft SQL Server
org.hibernate.dialect.SQLServerDialect
SAP DB
org.hibernate.dialect.SAPDBDialect
Informix
org.hibernate.dialect.InformixDialect
HypersonicSQL
org.hibernate.dialect.HSQLDialect
Ingres
org.hibernate.dialect.IngresDialect
Progress
org.hibernate.dialect.ProgressDialect
Mckoi SQL
org.hibernate.dialect.MckoiDialect
Interbase
org.hibernate.dialect.InterbaseDialect
Pointbase
org.hibernate.dialect.PointbaseDialect
FrontBase
org.hibernate.dialect.FrontbaseDialect
Firebird
org.hibernate.dialect.FirebirdDialect
Courtesy : http://docs.jboss.org/hibernate/core/3.3/reference/en/html/session-configuration.html#configuration-optional-dialects
Below is the SQL script for creating a User Details table in database. This below SQL script is according to PostGresSQL8.3.1, so if you are using a database other than PostgreSQL you need to write a similar one as per your database.
User_Details.sql : We will use the below SQL query to create the table in the database.
CREATE TABLE user_details (
user_id integer NOT NULL,
user_name character varying(20),
user_password character varying(20),
CONSTRAINT "USER_pkey" PRIMARY KEY (user_id)
)
4. Application Configuration – hibernate.cfg.xml The hibernate.cfg.xml is a configuration file that is used for defining the database parameters such as database username, password, connection URL, database driver class, connection provider, and connection pool size. This also includes hibernate mapping files (hbm), a xml based hibernate mapping file that have mapping of database column to java attributes.
Below is a sample hibernate configuration file.
hibernate.cfg.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
<session-factory>
<property name="hibernate.connection.url">jdbc:postgresql://127.0.0.1:5432/qc</property>
<property name="hibernate.connection.username">postgres</property>
<property name="hibernate.connection.password">postgres</property>
<property name="hibernate.connection.pool_size">10</property>
<property name="hibernate.connection.driver_class">org.postgresql.Driver</property>
<property name="hibernate.dialect">org.hibernate.dialect.PostgreSQLDialect</property>
<property name="current_session_context_class">thread</property>
<property name="transaction.factory_class">org.hibernate.transaction.JDBCTransactionFactory</property>
<property name="show_sql">true</property>
<mapping resource="com/hibernatetest/demo/User.hbm.xml" />
</session-factory>
</hibernate-configuration>
These configurations are used by hibernate to build session factory i.e. creation of database connections.
- hibernate.connection.url : This is the database Connection URL signifies the database URL e.g. jdbc:postgresql://127.0.0.1:5432/qc
- hibernate.connection.username : database username e.g. postgres
- hibernate.connection.password : database password e.g. postgres
- hibernate.connection.pool_size : database connection pool size. It signifies that this number of connections will be checked out by application at the time of creation of session factory.
- hibernate.connection.driver_class : Database Driver class e.g. for PostgreSQL it is org.postgresql.Driver
- hibernate.dialect : Dialect to tell hibernate to do syntax conversion for database queries based on the database. e.g. org.hibernate.dialect.PostgreSQLDialect
- current_session_context_class : context in which current session need to be maintained. e.g. thread
- transaction.factory_class : transaction factory e.g. org.hibernate.transaction.JDBCTransactionFactory>
- show_sql : Should Hibernate show SQL for every database operation. Very good for debugging purpose. In production environment it should be tuned off. e.g. true/false
- mapping resource="com/hibernatetest/demo/User.hbm.xml" : Mapping of hbm files.
<?xml version="1.0" encoding="UTF-8"?>
<!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.hibernatetest.demo.User" table="user_details" schema="public" optimistic-lock="version">
<id name="userId" "java.lang.Integer">
<column name="user_id" />
<generator "increment" />
</id>
<property name="userName" "string">
<column name="user_name" length="20" />
</property>
<property name="userPassword" "string">
<column name="user_password" length="20" />
</property>
</class>
</hibernate-mapping>
Below is the description for above xml file.
1. Class : name of the class (pojo) with package.
Table : name of the database table
Schema : name of the database schema.
2. Optimistic-lock : It is a kind of locking mechanism but in real it does not lock any row or column in database. It signifies a kind of database row state checker, that tell if the row that is getting modified by current transaction is already updated by another transaction after its read from database. Lets we take an example to get it understand in more detail.
1. Let say a user want to book an air ticket. On booking portal he found that only one ticket is available. He starts booking the ticket. In this duration an another user also came on the same web portal and he also start booking the ticket for same flight, in this case only one user would be able to book the ticket for booking as for one it would fail. Actually this issue started as when the first user is booking the ticket, but still system is allowing to book ticket for another user when only one ticket is left. This is called the dirty read.
2. To overcome this kind of situation, hibernate use a versioning check so it can know that this record is already updated when other users is also trying to updating the same record at the same time.
3. For this a field named as version is maintained in table. When user updates a record, it checks the version value in database with the getting updated record is same or not. If it is same, then it will allow updating the database and increment the version counter otherwise it will throw a stale record exception. As if in the duration of this process, some other user also read and try to update the same record, the version counter would be different than the database one.
Generator : This is used to update the primary key. This tells hibernate to use which strategy to update the primary key.
Mainly these below strategies are used to update the primary key.
1. assigned : It signifies a user itself provide the value for primary key. This is not suggested to use in a cluster based application.
2. increment : hibernate will increment the primary key automatically.
3. sequence : A sequence defined in db can be used to auto increment the primary key.
Property : attribute of pojo class, this is mapped to database column name.
HibernateUtil.java : It is a java class that returns a session factory object that is used for getting the session(connection) object.
package com.hibernatetest.demo;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;
/**
* @author rjain6
*/
public class HibernateUtil
{
private static SessionFactory sessionfactory = null;
public static SessionFactory getSessionFactory()
{
Configuration cfg = new Configuration();
sessionfactory = cfg.configure("com/hibernatetest/demo/hibernate.cfg.xml").buildSessionFactory();
return sessionfactory;
}
}
5. Creating the POJO(Plain Old Java Objects) Now for persisting data to the database, hibernate requires a Java bean (POJO) object to be passed. For this we are writing a User’s pojo object having all getter and setter methods of attributes. These attributes will be work as carrier of data that will be stored in database. As these POJO Objects will travel across the network, these objects should implement the Serializable interface.
package com.hibernatetest.demo;
import java.io.Serializable;
/**
* This class is a POJO that works as a carrier of the
* data and will be stored in database by hibernate.
* @author rjain6
*/
public class User implements Serializable
{
private int userId;
private String userName;
private String userPassword;
public int getUserId()
{
return userId;
}
public void setUserId(int userId)
{
this.userId = userId;
}
public String getUserName()
{
return userName;
}
public String getUserPassword()
{
return userPassword;
}
public void setUserName(String userName)
{
this.userName = userName;
}
public void setUserPassword(String userPassword)
{
this.userPassword = userPassword;
}
}
6. Creating the DAO (Data Access Objects) Since this is a very simple application, here only one DAO (Data Access object) is involved. This Dao class has methods to perform all the basic CRUD (Create, Read, Update, and Delete) functionalities.
package com.hibernatetest.demo;
import java.io.Serializable;
import java.util.List;
import org.hibernate.HibernateException;
import org.hibernate.Query;
import org.hibernate.Session;
import org.hibernate.Transaction;
/**
* This is DAO class for performing database related functionalities.
* @author rjain6
*/
public class UserDAO
{
/**
* Creating a user in the database
*/
public int createRecord(User user)
{
Transaction tx = null;
Session session = HibernateUtil.getSessionFactory().openSession();
tx = session.beginTransaction();
Serializable s = session.save(user);
tx.commit();
return ((Integer)s).intValue();
}
/**
* Updating a user’s details in the database
*/
public boolean updateRecord(User user)
{
boolean isSuccess = false;
Transaction tx = null;
Session session = HibernateUtil.getSessionFactory().openSession();
try
{
tx = session.beginTransaction();
session.update(user);
tx.commit();
isSuccess = true;
} catch (HibernateException e)
{
isSuccess = false;
}
return isSuccess;
}
/**
* Deleting the user from the database
*/
public boolean deleteRecord(User user)
{
boolean isSuccess = false;
Transaction tx = null;
Session session = HibernateUtil.getSessionFactory().openSession();
try
{
tx = session.beginTransaction();
session.delete(user);
tx.commit();
isSuccess = true;
} catch (HibernateException e)
{
isSuccess = false;
}
return isSuccess;
}
/**
* Retrieving All users’ details from the database
*/
public List retrieveAllRecord()
{
Transaction tx = null;
Session session = HibernateUtil.getSessionFactory().openSession();
List ls = null;
tx = session.beginTransaction();
Query q = session.createQuery("from User");
ls = q.list();
return ls;
}
/**
* Retrieving All users’ names from the database
*/
public List retrieveAllUserName()
{
Transaction tx = null;
Session session = HibernateUtil.getSessionFactory().openSession();
List ls = null;
tx = session.beginTransaction();
//Native Query
Query q = session.createSQLQuery("select user_name from User_Details");
ls = q.list();
return ls;
}
}
7. Testing the DAO functionality using JUnit Test The final stage in this simple tutorial is to create Junit test class to check the Session factory object and functionality of the data layer.
The contents of the class are shown below.
HibernateUtilTest.java : It is Test class for testing the working of session factory object. If Session factory object is not null, that means hibernate is able to get the database connection from DB.
package com.hibernatetest.demo;
import org.junit.Test;
/**
* This is a test class for chcecking is sessionfactory(database connection) is loaded by hibernate properly.
* @author rjain6
*/
public class HibernateUtilTest
{
/**
* Test method for {@link com.hibernatetest.demo.HibernateUtil#getSessionFactory()}.
*/
@Test
public void testGetSessionFactory()
{
System.out.println("session factory:"
+ HibernateUtil.getSessionFactory());
}
}
UserDAOTest.java :
package com.hibernatetest.demo;
import java.util.Collection;
import java.util.List;
import org.junit.Test;
/**
* This is test class having test methods to check the functionality of UserDAO class .
* @author rjain6
*/
public class UserDAOTest
{
@Test
public void createRecordTest()
{
UserDAO userDAO = new UserDAO();
User user = new User();
user.setUserName("demo");
user.setUserPassword("demo");
int recordId = userDAO.createRecord(user);
System.out.println("recordId:" + recordId);
}
@Test
public void updateRecordTest()
{
UserDAO userDAO = new UserDAO();
User user = new User();
user.setUserId(2);
user.setUserName("demo123");
user.setUserPassword("demo123");
boolean status = userDAO.updateRecord(user);
System.out.println("status:" + status);
}
@Test
public void deleteRecordTest()
{
UserDAO userDAO = new UserDAO();
User user = new User();
user.setUserId(3);
boolean status = userDAO.deleteRecord(user);
System.out.println("status:" + status);
}
@Test
public void retrieveRecordTest()
{
UserDAO userDAO = new UserDAO();
List list = userDAO.retrieveAllRecord();
if (isNullSafe(list))
{
for (int i = 0;i < list.size();i++)
{
System.out.println("UserName:" +((User)list.get(i)).getUserName());
System.out.println("UserPassord:" + ((User)list.get(i)).getUserPassword());
}
}
}
@Test
public void retrieveAllUserNameTest()
{
UserDAO userDAO = new UserDAO();
List ls = userDAO.retrieveAllUserName();
if (isNullSafe(ls))
{
for (int i = 0;i < ls.size();i++)
{
System.out.println("UserName:" + ls.get(i));
}
}
}
/**
* @param ls
* @return
*/
private boolean isNullSafe(Collection col)
{
if (col != null && col.size() > 0)
return true;
else
return false;
}
}.
8. Conclusion Hibernate is an ORM tool that is used to map the database structures to java objects at run time. Using a persistence framework like Hibernate allows developers to focus on writing business logic code instead of writing an accurate and good persistence layer which include writing the SQL Queries, JDBC Code , connection management etc.
9. References 1. https://www.hibernate.org/