Hibernate

Table of Contents

Hibernate is an Object Relation Mapping tool that maps:

Java classes to database tables
Java objects to rows

Due to this mapping, the user does not have to deal directly with SQL data types.
Such a mapping allows the programmer to interact with objects which are more natural to work with.

Hibernate example:


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

.
.
.

Person p = new Person ();
person.setAge (25);
person.setName ("John");


// Obtain hibernate session from factory
SessionFactory factory = new Configuration().configure().buildSessionFactory();
Session session = factory.openSession();

// Write using hibernate
session.beginTransaction();
session.save(p);
session.getTransaction().commit();


// Read using hibernate
Query q = session.createQuery ("from Person p where p.age = 25"); // note HQL's use of object notation.
itr = q.iterate();

while (itr.hasNext())
    System.out.println(itr.next());

Hibernate Mapping File

Hibernate needs a mapping file which maps each table to a class.

Example:

The following is a description of the commonly used tags and their attributes:

<id> - Primary key specification.
- name - Specifies the name of the id in class.
- column - The name of the database column for the id.
- <generator> - It is used to generate a unique id every time an object is persisted to the database. It also specifies how the id element will be created by using the following built in generators:
  - increment - Used most often, each time the generator needs a new id it performs a select on the database to determine the current largest id and increments it to the next value.
  - native - Picks the best strategy depending on the configured database.
  - assigned - Used if you need to assign the id yourself in your application. Set the id by using the set<identifier> method of the Java class. The application assumes responsibility on keeping the id unique.
<property> - This field is defined for each field in the Java class that needs to be persisted.
- name - Specifies class field’s name. The first character must be lowercase.
- column - Specifies the name of the column in the database.
<set> - The most common collection mapping used in Hibernate.
- <key column> - Specifies the foreign key column.
- <many-to-many> - Sub-element of set.

Note: The above method of specifying mapping files is slightly cumbersome.
Now-a-days, annotation based mechanism of specifying mappings is rapidly gathering popularity.
A working example with annotations will be covered in the later chapters.

Hibernate Configuration File

Hibernate also needs a configuration file in xml whose format is as follows:


<?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>

    <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
    <property name="hibernate.connection.url">jdbc:mysql://localhost:4000/yourdb-SID</property>
    
    <property name="hibernate.connection.username">admin</property>
    <property name="hibernate.connection.password">admin</property>
    
    <property name="hibernate.dialect">org.hibernate.dialect.Oracle9Dialect</property>
    
    <property name="show_sql">true</property>
    <property name="format_sql">true</property>
    
    <mapping resource="Person.hbm.xml" />
    <mapping resource="Student.hbm.xml" />
</session-factory>
</hibernate-configuration>

show_sql and format_sql are debug options.
They can be used to see formatted SQL generated by hibernate.

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