Java Data Persistence Using Hibernate

Jack Gardner

October 2004

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Overview

What is Hibernate What is ORM Create Example Read Examples Persistable Classes Database Table Mapping Class to Table Configuration Configuration Properties

Session Transaction Object Lifecycle Query Options Associations References

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What is Hibernate?

An object/relational mapping (ORM) implementation

Open source Development started late 2001

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What is Object/Relational Mapping?

Provides a transparent bridge between objects and database tables

Allows source code to work with objects and their attributes vs. tables and columns

Eliminates need for most/all SQL Eliminates use of query result sets

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Create Example

// create a new objectWidget w = new Widget();w.setName(“WidgetA”);w.setValue(10);

// persist itsession.save(w);

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Read Examples

// get a known widgetWidget w = (Widget) session.load(Widget.class,

“WidgetA”);

// get all widgetsList widgets = session.find(“from Widget”);

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Persistable Classes

Classes are simply JavaBeans

public class Widget{

private String name;private int value;

public Widget() {}public String getName() {return name;}public void setName(String s) {name = s;}public int getValue() {return value;}public void setValue(int i) {value = i;}

}

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Database Table

Persistable classes have an associated table

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Mapping Class to Table

widget.hbm.xml

<hibernate-mapping><class name=“mypackage.Widget” table=“WIDGET”>

<id name=“name” column=“NAME”> <generator class=“assigned”/></id><property name=“value” column=“VALUE”/>

</class></hibernate-mapping>

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Configuration

import net.sf.hibernate.cfg.Configuration;import net.sf.hibernate.SessionFactory;

// build configuration based on propertiesConfiguration config = new Configuration();

// add persistable classes to configurationconfig.addClass(Widget.class);

// build a session factory based on configurationSessionFactory sessionFactory =

config.buildSessionFactory();

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Configuration Properties

hibernate.propertieshibernate.dialect=net.sf.hibernate.dialect.Oracle9Dialecthibernate.connection.driver_class=oracle.jdbc.driver.OracleDriverhibernate.connection.url=jdbc:oracle:thin:@oracle.cis.ksu.edu:1521:ORACLEhibernate.connection.username=<myusername>hibernate.connection.password=<mypassword>

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Session

Lightweight and inexpensive to create/destroy Not threadsafe – each thread needs its own Obtained from SessionFactory instance

Session session = sessionFactory.openSession();// perform persistence operationssession.close();

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Transaction

Set of operations that are all committed or all rolled back

Obtained from Session instance Can perform multiple transactions within

session

Transaction tx = session.beginTransaction();// perform persistence operationstx.commit();

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Object Lifecycle

Transient Newly created object

Persistent New object has been “saved” Previously saved object has been “read”

Detached Session closed Persistent object serialized Can be reattached later to become persistent

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Query Options

Hibernate Query Language (HQL)Query q = session.createQuery(“from Widget w where w.value > :value”);q.setParameter(“value”, someValue);List widgets = q.list();

Criteria APICriteria c = session.createCriteria(Widget.class);c.add(Expression.gt(“value”, someValue);List widgets = c.list();

SQLQuery q = session.createSQLQuery(“select {w.*} from WIDGET {w} where VALUE > :value”, “w”, Widget.class);q.setParameter(“value”, someValue);List widgets = q.list();

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Associations

Supports all association types one-to-one one-to-many many-to-many

Inherently unidirectional Supports bidirectional

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One-to-Many Example: Unidirectional Option 1: Group Member

public class Group public class Member{ { private String name; private String name; private Collection members; … … }}

Option 2: Group Memberpublic class Group public class Member{ { private String name; private String name; … private Group group;} …

}

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One-to-Many Example: Bidirectional Group Member

public class Group public class Member{ { private String name; private String name; private Collection members; private Group group; … …} }

Application responsible for maintaining each end of association

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One-to-Many Table Structure

Underlying table structure not affected by directionality

Standard implementation Foreign key in many-side table

Alternate implementation Use join table, with no-duplicate constraint on

many-side foreign key

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One-to-Many Table Structure

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Many-to-Many Example: Unidirectional Option 1: Group Member

public class Group public class Member{ { private String name; private String name; private Collection members; … … }}

Option 2: Group Memberpublic class Group public class Member{ { private String name; private String name; … private Collection groups;} …

}

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Many-to-Many Example: Bidirectional Group Member

public class Group public class Member{ {

private String name; private String name; private Collection members; private Collection

groups; … …

} }

Application responsible for maintaining each end of association

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Many-to-Many Table Structure

Underlying table structure not affected by directionality

Implemented using join table

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Many-to-Many Table Structure

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References

Bauer, Christian and Gaven King, Hibernate in Action, Manning, 2005.

Hibernate Reference Documentation, Version 2.1.6, Hibernate, 2004.