Java Data Persistence Using Hibernate

1. Java Data Persistence Using Hibernate Jack Gardner October 2004

2. 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 Overview

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 { privateString name; privateint value; public Widget() {} publicString getName() {return name;} publicvoid setName(String s) {name = s;} publicint getValue() {return value;} publicvoid setValue(int i) {value = i;} }

8. Database Table • Persistable classes have an associated table

9. Mapping Class to Table • widget.hbm.xml <hibernate-mapping> <classname=“mypackage.Widget”table=“WIDGET”> <idname=“name”column=“NAME”> <generatorclass=“assigned”/> </id> <propertyname=“value”column=“VALUE”/> </class> </hibernate-mapping>

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.url=jdbc:oracle:thin:@oracle.cis.ksu.edu:1521:ORACLE hibernate.connection.username=<myusername> hibernate.connection.password=<mypassword>

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 { { privateString name; privateString name; private Collection members; … … } } • Option 2: Group  Member public class Group public class Member { { privateString name; privateString name; … private Group group; } … }

18. One-to-Many Example: Bidirectional • Group  Member public class Group public class Member { { privateString name; privateString 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 { { privateString name; privateString name; private Collection members; … … } } • Option 2: Group  Member public class Group public class Member { { privateString name; privateString name; … private Collection groups; } … }

22. Many-to-Many Example: Bidirectional • Group  Member public class Group public class Member { { privateString name; privateString 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, 2005. • Hibernate Reference Documentation, Version 2.1.6, Hibernate, 2004.