2.1 “Hello World” with Hibernate

"Hibernate applications define persistent classes that are 'mapped' to database tables... Our persistent class can be used in any execution context at all---no special container is needed." Hibernate depends on its Session object for its "context":

Session session = getSessionFactory().openSession();
Transaction tx = session.beginTransaction();
Message message = new Message("Hello World");
session.save(message);
tx.commit();
session.close();

The Message is defined by:

package hello;
public class Message
{
    /*
        There is no need to set the id property explicitly.
        The XML configuration file associates
        the <generator /> element
        with this "identifier property".
    */
    private Long id;

    /*
        The XML configuration file "maps" the private text
        member to the MESSAGE_TEXT column in the database
        with the <property /> element.
    */
    private String text;

    /*
        this reference to another Message object is used by
        the XML configuration in the <many-to-one /> element.
    */
    private Message nextMessage;

    //Hibernate requires this private constructor:
    private Message() {}

    public Message(String text) { this.text = text; }

    public Long getId() { return id; }

    /*
        Note that setId() is private because
        of Hibernate (see private id above):
    */
    private void setId(Long id) { this.id = id; }

    public String getText() { return this.text; }
    public void setText(String text) { this.text = text; }

    /*
        The members below are useful because of Hibernate,
        its "cascading save" feature.
    */
    public Message getNextMessage() { return this.nextMessage; }
    public void setNextMessage(Message this.nextMessage)
    {
        this.nextMessage = nextMessage;
    }
}

Hibernate recognizes the Message class because of this XML configuration file or "XML mapping document":

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">

<hibernate-mapping>
<class name="hello.Message" table="MESSAGES">

<id name="id" column="MESSAGE_ID">
    <generator class="increment"/>
</id>

<property name="text" column="MESSAGE_TEXT"/>

<many-to-one
    name="nextMessage"
    cascade="all"
    column="NEXT_MESSAGE_ID"/>

</class>
</hibernate-mapping>