About Java and JRuby Development
JEE, Spring, Guice
Hibernate, Java Persistence (JPA)
and various Web Frameworks

What is Hibernate tutorial?

This
tutorial explains what Hibernate is and will show some examples why
it is useful to use Hibernate.

Generals

Author:

Sebastian
Hennebrueder

http://www.laliluna.de/tutorials.html

Date:

February,
25th 2005

PDF Version des Tutorials:

http://www.laliluna.de/assets/tutorials/what-is-hibernate-tutorial-en.pdf

Main features

Hibernate
is a solution for object relational mapping and a persistence
management solution or persistent layer. This is probably not
understandable for anybody learning Hibernate.

What
you can imagine is probably that you have your application with some
functions (business logic) and you want to save data in a database.
When you use Java all the business logic normally works with objects
of different class types. Your database tables are not at all
objects.

Hibernate
provides a solution to map database tables to a class. It copies the
database data to a class. In the other direction it supports to save
objects to the database. In this process the object is transformed to
one or more tables.

Saving
data to a storage is called persistence. And the copying of tables to
objects and vice versa is called object relational mapping.

Why using Object Relational Mapping?

Better system architecture

When
you include all functionality of your application and the access to
the database within your dialogs, you will have some severe
disadvantages.

It
is really difficult to reuse code. You will repeat code at many
places. If you change anything it is quite hard to find out all
places where you have to add changes.

When
you separate your dialogs from your logic and the logic from the
persistence mechanism you can more easily apply changes to one part
without influencing the other parts.

Reduce time for standard DB actions

Most
queries in database development are simple ?insert, update, delete?
statements. There is no need to develop all these tedious statements.
Hibernate helps you save time.

Loading classes from the database
looks like

Query query = session.createQuery("select b from Bug as b");<br>      for (Iterator iter = query.iterate(); iter.hasNext();) {<br>        bugs.add((Bug) iter.next());<br>      }<br>    return bugs;

saving a class ?bug? to the database looks like


session.update(bug);

Advanced features difficult to develop yourself


Caching
solutions, transactions and other features Hibernate provides are not
so easy to implement. It is actually non sense to develop something
which allready exist.


Using
Hibernate everything is there. You just have to use it.


How Hibernate works


What is nice and in my opinion an
advantage over entity beans, hibernate starts from simple java
classes (Pojo = Plain Old Java Objects).


To use hibernate you will create a
simple java class:


public class Bug
    implements Serializable
{
    private int hashValue = 0;
    private java.lang.Integer id;
    private java.lang.String title;

    public Bug()
    {
    }
    public Bug(java.lang.Integer fid)
    {
        this.setId(fid);
    }
    public java.lang.Integer getId() {
      return id;
    }
    public void setId(java.lang.Integer id) {
      this.id = id;
    }
    public java.lang.String getTitle() {
      return title;
    }
    public void setTitle(java.lang.String title) {
      this.title = title;
    }
………….




Than you create a mapping file. The
mapping file explains Hibernate which field in the class is mapped to
which field in the database.

&lt;hibernate-mapping package="de.laliluna.fehlerbehebung"&gt;<br>    &lt;class name="Bug" table="bug"&gt;<br>        &lt;id name="id" column="fid" type="java.lang.Integer"&gt;<br>            &lt;generator class="sequence"&gt;<br>              &lt;param name="sequence"&gt;public.bug_fid_seq&lt;/param&gt;<br>            &lt;/generator&gt;<br>        &lt;/id&gt;<br>        &lt;property name="title" column="ftitle" type="java.lang.String" /&gt;<br>    &lt;/class&gt;<br>&lt;/hibernate-mapping&gt;



And you start using your hibernate
class.


Example to create a new entry in the
database:


      Bug bug = new Bug();
      bug.setTitle(“Title”);
      bug.setTuserFk(tuser);
	Transaction tx = session.beginTransaction();
      session.save(bug);
      tx.commit();


The creation of the description file
can be done with tools like MyEclipse. MyEclipse provides
functionality to create classes and description files directly from
database tables.


Hibernate includes tools to

Development
speed is very high using hibernate.

That’
s all.