Java Data Persistence Using Hibernate Jack Gardner October 2004

2 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

3 What is Hibernate? An object/relational mapping (ORM) implementation Open source Development started late 2001

4 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

5 Create Example // create a new object Widget w = new Widget(); w.setName(“WidgetA”); w.setValue(10); // persist it session.save(w);

6 Read Examples // get a known widget Widget w = (Widget) session.load(Widget.class, “WidgetA”); // get all widgets List widgets = session.find(“from Widget”);

7 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;} }

8 Database Table Persistable classes have an associated table

9 Mapping Class to Table widget.hbm.xml

10 Configuration import net.sf.hibernate.cfg.Configuration; import net.sf.hibernate.SessionFactory; // build configuration based on properties Configuration config = new Configuration(); // add persistable classes to configuration config.addClass(Widget.class); // build a session factory based on configuration SessionFactory sessionFactory = config.buildSessionFactory();

11 Configuration Properties hibernate.properties hibernate.dialect=net.sf.hibernate.dialect.Oracle9Dialect hibernate.connection.driver_class=oracle.jdbc.driver.OracleDriver hibernate.connection.username= hibernate.connection.password=

12 Session Lightweight and inexpensive to create/destroy Not threadsafe – each thread needs its own Obtained from SessionFactory instance Session session = sessionFactory.openSession(); // perform persistence operations session.close();

13 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 operations tx.commit();

14 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

15 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 API Criteria c = session.createCriteria(Widget.class); c.add(Expression.gt(“value”, someValue); List widgets = c.list(); SQL Query q = session.createSQLQuery(“select {w.*} from WIDGET {w} where VALUE > :value”, “w”, Widget.class); q.setParameter(“value”, someValue); List widgets = q.list();

16 Associations Supports all association types one-to-one one-to-many many-to-many Inherently unidirectional Supports bidirectional

17 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  Member public class Grouppublic class Member{ private String name; private String name; … private Group group; } … }

18 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

19 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

20 One-to-Many Table Structure

21 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  Member public class Grouppublic class Member{ private String name; private String name; … private Collection groups; } … }

22 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

23 Many-to-Many Table Structure Underlying table structure not affected by directionality Implemented using join table

24 Many-to-Many Table Structure

25 References Bauer, Christian and Gaven King, Hibernate in Action, Manning, Hibernate Reference Documentation, Version 2.1.6, Hibernate, 2004.