A Session is used to get a physical connection with a database. The Session object is lightweight and designed to be instantiated each time an interaction is needed with the database. Persistent objects are saved and retrieved through a Session object.
The session objects should not be kept open for a long time because they are not usually thread safe and they should be created and destroyed them as needed. The main function of the Session is to offer, create, read, and delete operations for instances of mapped entity classes.
Instances may exist in one of the following three states at a given point in time −
• transient − A new instance of a persistent class, which is not associated with a Session and has no representation in the database and no identifier value is considered transient by Hibernate.
• persistent − You can make a transient instance persistent by associating it with a Session. A persistent instance has a representation in the database, an identifier value and is associated with a Session.
• detached − Once we close the Hibernate Session, the persistent instance will become a detached instance.
A Session instance is serializable if its persistent classes are serializable. A typical transaction should use the following idiom −
Session session = factory.openSession();
Transaction tx = null;

try {
tx = session.beginTransaction();
// do some work
...
tx.commit();
}

catch (Exception e) {
if (tx!=null) tx.rollback();
e.printStackTrace(); 
} finally {
session.close();
}

If the Session throws an exception, the transaction must be rolled back and the session must be discarded.

Session Interface Methods

There are number of methods provided by the Session interface, but I’m going to list down a few important methods only, which we will use in this tutorial. You can check Hibernate documentation for a complete list of methods associated with Session and SessionFactory.

Sr.No.	Session Methods & Description
1	Transaction beginTransaction() Begin a unit of work and return the associated Transaction object.
2	void cancelQuery() Cancel the execution of the current query.
3	void clear() Completely clear the session.
4	Connection close() End the session by releasing the JDBC connection and cleaning up.
5	Criteria createCriteria(Class persistentClass) Create a new Criteria instance, for the given entity class, or a superclass of an entity class.
6	Criteria createCriteria(String entityName) Create a new Criteria instance, for the given entity name.
7	Serializable getIdentifier(Object object) Return the identifier value of the given entity as associated with this session.
8	Query createFilter(Object collection, String queryString) Create a new instance of Query for the given collection and filter string.
9	Query createQuery(String queryString) Create a new instance of Query for the given HQL query string.
10	SQLQuery createSQLQuery(String queryString) Create a new instance of SQLQuery for the given SQL query string.
11	void delete(Object object) Remove a persistent instance from the datastore.
12	void delete(String entityName, Object object) Remove a persistent instance from the datastore.
13	Session get(String entityName, Serializable id) Return the persistent instance of the given named entity with the given identifier, or null if there is no such persistent instance.
14	SessionFactory getSessionFactory() Get the session factory which created this session.
15	void refresh(Object object) Re-read the state of the given instance from the underlying database.
16	Transaction getTransaction() Get the Transaction instance associated with this session.
17	boolean isConnected() Check if the session is currently connected.
18	boolean isDirty() Does this session contain any changes which must be synchronized with the database?
19	boolean isOpen() Check if the session is still open.
20	Serializable save(Object object) Persist the given transient instance, first assigning a generated identifier.
21	void saveOrUpdate(Object object) Either save(Object) or update(Object) the given instance.
22	void update(Object object) Update the persistent instance with the identifier of the given detached instance.
23	void update(String entityName, Object object) Update the persistent instance with the identifier of the given detached instance.

Hibernate – Persistent Class

The entire concept of Hibernate is to take the values from Java class attributes and persist them to a database table. A mapping document helps Hibernate in determining how to pull the values from the classes and map them with table and associated fields.
Java classes whose objects or instances will be stored in database tables are called persistent classes in Hibernate. Hibernate works best if these classes follow some simple rules, also known as the Plain Old Java Object (POJO) programming model.
There are following main rules of persistent classes, however, none of these rules are hard requirements −
• All Java classes that will be persisted need a default constructor.
• All classes should contain an ID in order to allow easy identification of your objects within Hibernate and the database. This property maps to the primary key column of a database table.
• All attributes that will be persisted should be declared private and have getXXX and setXXX methods defined in the JavaBean style.
• A central feature of Hibernate, proxies, depends upon the persistent class being either non-final, or the implementation of an interface that declares all public methods.
• All classes that do not extend or implement some specialized classes and interfaces required by the EJB framework.
The POJO name is used to emphasize that a given object is an ordinary Java Object, not a special object, and in particular not an Enterprise JavaBean.

Simple POJO Example

Based on the few rules mentioned above, we can define a POJO class as follows −

public class Employee {
private int id;
private String firstName; 
private String lastName; 
private int salary;

public Employee() {}
public Employee(String fname, String lname, int salary) {
this.firstName = fname;
this.lastName = lname;
this.salary = salary;
}

public int getId() {
return id;
}

public void setId( int id ) {
this.id = id;
}

public String getFirstName() {
return firstName;
}

public void setFirstName( String first_name ) {
this.firstName = first_name;
}

public String getLastName() {
return lastName;
}

public void setLastName( String last_name ) {
this.lastName = last_name;
}

public int getSalary() {
return salary;
}

public void setSalary( int salary ) {
this.salary = salary;
}
}

Hibernate – Examples

Let us now take an example to understand how we can use Hibernate to provide Java persistence in a standalone application. We will go through the different steps involved in creating a Java Application using Hibernate technology.

Create POJO Classes

The first step in creating an application is to build the Java POJO class or classes, depending on the application that will be persisted to the database. Let us consider our Employee class with getXXX and setXXX methods to make it JavaBeans compliant class.
A POJO (Plain Old Java Object) is a Java object that doesn’t extend or implement some specialized classes and interfaces respectively required by the EJB framework. All normal Java objects are POJO.
When you design a class to be persisted by Hibernate, it is important to provide JavaBeans compliant code as well as one attribute, which would work as index like id attribute in the Employee class.

public class Employee {
private int id;
private String firstName; 
private String lastName; 
private int salary;

public Employee() {}
public Employee(String fname, String lname, int salary) {
this.firstName = fname;
this.lastName = lname;
this.salary = salary;
}

public int getId() {
return id;
}

public void setId( int id ) {
this.id = id;
}

public String getFirstName() {
return firstName;
}

public void setFirstName( String first_name ) {
this.firstName = first_name;
}

public String getLastName() {
return lastName;
}

public void setLastName( String last_name ) {
this.lastName = last_name;
}

public int getSalary() {
return salary;
}

public void setSalary( int salary ) {
this.salary = salary;
}
}

Create Database Tables

Second step would be creating tables in your database. There would be one table corresponding to each object, you are willing to provide persistence. Consider above objects need to be stored and retrieved into the following RDBMS table −

create table EMPLOYEE (
id INT NOT NULL auto_increment,
first_name VARCHAR(20) default NULL,
last_name VARCHAR(20) default NULL,
salary INT default NULL,
PRIMARY KEY (id)
);

Create Mapping Configuration File

This step is to create a mapping file that instructs Hibernate how to map the defined class or classes to the database tables.

<?xml version = "1.0" encoding = "utf-8"?>
<!DOCTYPE hibernate-mapping PUBLIC 
"-//Hibernate/Hibernate Mapping DTD//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping>
<class name = "Employee" table = "EMPLOYEE">

<meta attribute = "class-description">
This class contains the employee detail. 
</meta>

<id name = "id" type = "int" column = "id">
<generator class="native"/>
</id>

<property name = "firstName" column = "first_name" type = "string"/>
<property name = "lastName" column = "last_name" type = "string"/>
<property name = "salary" column = "salary" type = "int"/>

</class>
</hibernate-mapping>

You should save the mapping document in a file with the format <classname>.hbm.xml. We saved our mapping document in the file Employee.hbm.xml. Let us see little detail about the mapping document −
• The mapping document is an XML document having <hibernate-mapping> as the root element which contains all the <class> elements.
• The <class> elements are used to define specific mappings from a Java classes to the database tables. The Java class name is specified using the name attribute of the class element and the database table name is specified using the table attribute.
• The <meta> element is optional element and can be used to create the class description.
• The <id> element maps the unique ID attribute in class to the primary key of the database table. The name attribute of the id element refers to the property in the class and the column attribute refers to the column in the database table. The type attribute holds the hibernate mapping type, this mapping types will convert from Java to SQL data type.
• The <generator> element within the id element is used to generate the primary key values automatically. The class attribute of the generator element is set to native to let hibernate pick up either identity, sequence or hilo algorithm to create primary key depending upon the capabilities of the underlying database.
• The <property> element is used to map a Java class property to a column in the database table. The name attribute of the element refers to the property in the class and the column attribute refers to the column in the database table. The type attribute holds the hibernate mapping type, this mapping types will convert from Java to SQL data type.
There are other attributes and elements available, which will be used in a mapping document and I would try to cover as many as possible while discussing other Hibernate related topics.

Create Application Class

Finally, we will create our application class with the main() method to run the application. We will use this application to save few Employee’s records and then we will apply CRUD operations on those records.
import java.util.List;
import java.util.Date;
import java.util.Iterator;

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

public class ManageEmployee {
private static SessionFactory factory; 
public static void main(String[] args) {

try {
factory = new Configuration().configure().buildSessionFactory();
} catch (Throwable ex) { 
System.err.println("Failed to create sessionFactory object." + ex);
throw new ExceptionInInitializerError(ex); 
}

ManageEmployee ME = new ManageEmployee();

/* Add few employee records in database */
Integer empID1 = ME.addEmployee("Zara", "Ali", 1000);
Integer empID2 = ME.addEmployee("Daisy", "Das", 5000);
Integer empID3 = ME.addEmployee("John", "Paul", 10000);

/* List down all the employees */
ME.listEmployees();

/* Update employee's records */
ME.updateEmployee(empID1, 5000);

/* Delete an employee from the database */
ME.deleteEmployee(empID2);

/* List down new list of the employees */
ME.listEmployees();
}

/* Method to CREATE an employee in the database */
public Integer addEmployee(String fname, String lname, int salary){
Session session = factory.openSession();
Transaction tx = null;
Integer employeeID = null;

try {
tx = session.beginTransaction();
Employee employee = new Employee(fname, lname, salary);
employeeID = (Integer) session.save(employee); 
tx.commit();
} catch (HibernateException e) {
if (tx!=null) tx.rollback();
e.printStackTrace(); 
} finally {
session.close(); 
}
return employeeID;
}

/* Method to READ all the employees */
public void listEmployees( ){
Session session = factory.openSession();
Transaction tx = null;

try {
tx = session.beginTransaction();
List employees = session.createQuery("FROM Employee").list(); 
for (Iterator iterator = employees.iterator(); iterator.hasNext();){
Employee employee = (Employee) iterator.next(); 
System.out.print("First Name: " + employee.getFirstName()); 
System.out.print(" Last Name: " + employee.getLastName()); 
System.out.println(" Salary: " + employee.getSalary()); 
}
tx.commit();
} catch (HibernateException e) {
if (tx!=null) tx.rollback();
e.printStackTrace(); 
} finally {
session.close(); 
}
}

/* Method to UPDATE salary for an employee */
public void updateEmployee(Integer EmployeeID, int salary ){
Session session = factory.openSession();
Transaction tx = null;

try {
tx = session.beginTransaction();
Employee employee = (Employee)session.get(Employee.class, EmployeeID); 
employee.setSalary( salary );
session.update(employee); 
tx.commit();
} catch (HibernateException e) {
if (tx!=null) tx.rollback();
e.printStackTrace(); 
} finally {
session.close(); 
}
}

/* Method to DELETE an employee from the records */
public void deleteEmployee(Integer EmployeeID){
Session session = factory.openSession();
Transaction tx = null;

try {
tx = session.beginTransaction();
Employee employee = (Employee)session.get(Employee.class, EmployeeID); 
session.delete(employee); 
tx.commit();
} catch (HibernateException e) {
if (tx!=null) tx.rollback();
e.printStackTrace(); 
} finally {
session.close(); 
}
}
}

Compilation and Execution

Here are the steps to compile and run the above mentioned application. Make sure, you have set PATH and CLASSPATH appropriately before proceeding for the compilation and execution.
• Create hibernate.cfg.xml configuration file as explained in configuration chapter.
• Create Employee.hbm.xml mapping file as shown above.
• Create Employee.java source file as shown above and compile it.
• Create ManageEmployee.java source file as shown above and compile it.
• Execute ManageEmployee binary to run the program.
You would get the following result, and records would be created in the EMPLOYEE table.
$java ManageEmployee
…….VARIOUS LOG MESSAGES WILL DISPLAY HERE……..

First Name: Zara Last Name: Ali Salary: 1000
First Name: Daisy Last Name: Das Salary: 5000
First Name: John Last Name: Paul Salary: 10000
First Name: Zara Last Name: Ali Salary: 5000
First Name: John Last Name: Paul Salary: 10000
If you check your EMPLOYEE table, it should have the following records −
mysql> select * from EMPLOYEE;
+—-+————+———–+——–+
| id | first_name | last_name | salary |
+—-+————+———–+——–+
| 29 | Zara | Ali | 5000 |
| 31 | John | Paul | 10000 |
+—-+————+———–+——–+
2 rows in set (0.00 sec

mysql>

So, this brings us to the end of blog. This Tecklearn ‘Using Sessions and POJO Classes in Hibernate’ blog helps you with commonly asked questions if you are looking out for a job in Java Programming. If you wish to learn Hibernate and build a career Java Programming domain, then check out our interactive, Java and JEE Training, that comes with 24*7 support to guide you throughout your learning period. Please find the link for course details:

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Java Database Connectivity (JDBC)

• Introduction to SQL: Connect, Insert, Update, Delete, Select
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• Batch Processing Transaction
• Management: Commit and Rollback

Java Enterprise Edition – Servlets

• Introduction to J2EE
• Client Server architecture
• URL, Port Number, Request, Response
• Need for servlets
• Servlet fundamentals
• Setting up a web project in Eclipse
• Configuring and running the web app with servlets
• GET and POST request in web application with demo
• Servlet lifecycle
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• Fundamentals of Java Server Page
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Hibernate

• Introduction to Hibernate
• Introduction to ORM
• ORM features
• Hibernate as an ORM framework
• Hibernate features
• Setting up a project with Hibernate framework
• Basic APIs needed to do CRUD operations with Hibernate
• Hibernate Architecture

POJO (Plain Old Java Object)

• POJO (Plain Old Java Object)
• Persistent Objects
• Lifecycle of Persistent Object

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