inf5750---lecture-2.-c---hibernate-intro.pdf
- 1. Object Relational Mapping (ORM) and Hibernate INF5750/9750 - Lecture 2 (Part III)
- 2. Problem area ● When working with object-oriented systems, there’s a mismatch between the object model and the relational database ● How do we map one to the other? public class Student { private String name; private String address; private Set<Course> courses; private Set<Degree> degrees; } Java object with properties and associations Relational database with tables and columns
- 3. Problem area ● How to map associations between objects? ○ References are directional, foreign keys not ○ Foreign keys can’t represent many-to-many associations Student Degree N N public class Student { private Collection<Degree> degrees; ... DEGREE degree_id type name STUDENT student_id name address degree_id Java Relational database
- 4. Need for ORM ● Write SQL conversion methods by hand using JDBC ○ Tedious and requires lots of code ○ Extremely error-prone ○ Non-standard SQL ties the application to specific databases ○ Vulnerable to changes in the object model ○ Difficult to represent associations between objects Student Course Degree public void addStudent( Student student ) { String sql = ”INSERT INTO student ( name, address ) VALUES ( ’” + student.getName() + ”’, ’” + student.getAddress() + ”’ )”; // Initiate a Connection, create a Statement, and execute the query }
- 5. Approaches to ORM ● Use Java serialization – write application state to a file ○ Can only be accessed as a whole ○ Not possible to access single objects ● Object oriented database systems ○ No complete query language implementation exists ○ Lacks necessary features
- 6. The preferred solution ● Use a Object-Relational Mapping System (eg. Hibernate) ● Provides a simple API for storing and retrieving Java objects directly to and from the database ● Non-intrusive: No need to follow specific rules or design patterns ● Transparent: Your object model is unaware Student Course Degree ORM / Hibernate Magic happens here! (Domain model) (Relational database)
- 7. ORM and Architecture Presentation Layer Service/Business Layer Persistence Layer ORM / Hibernate (Database) Domain Model ● Middleware that manages persistence ● Provides an abstraction layer between the domain model and the database
- 8. Example app: The EventManager Java objects Hibernate mapping files Hibernate configuration Hibernate API
- 9. Java objects (POJO) public class Event { private int id; private String title; private Date date; private Set<Person> persons = new HashSet<Person>(); public Event() { } public int getId() { return id; } private void setId( int id ) { this.id = id; } public String getTitle() { return title; } public void setTitle( String title ) { this.title = title; } // Getter and setter for date and persons } Declare accessors and mutators for persistent fields (optional) ● Properties need not be declared public - Hibernate can persist a property with a default, protected or private get / set pair. No-argument constructor (required) Identifier property (optional) Prefer non-final classes (optional)
- 10. Example app: The EventManager Java objects Hibernate mapping files Hibernate configuration Hibernate API
- 11. Hibernate mapping files ● Tells Hibernate which tables and columns to use to load and store objects DTD <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> <hibernate-mapping> <class name=”no.uio.inf5750.Event” table=”events”> <id name="id” column=”event_id”> <generator class="native"/> </id> <property name=”title” not-null=”true” unique=”true”/> <property name=”date” type=”date” column=”event_date”/> <set name=”persons” table=”event_persons”> <key column=”event_id”/> <many-to-many column=”person_id” class=”no.uio.inf5750.example.model.Person”/> </set> </class> </hibernate-mapping> Class element Identifier mapping & generation Property mapping Filename: Event.hbm.xml Unidirectional many-to-many association mapping
- 12. Property mapping ... <property name=”title” not-null=”true” unique=”true”/> <property name=”date” type=”Date” column=”event_date”/> ... The name property refers to the get/set-methods Property name used as default if no column is specified Types are Hibernate mapping types. Hibernate will guess if no type is specified. Title must be not null and unique
- 13. Association mapping ... <set name=”persons” table=”event_persons”> <key column=”event_id”/> <many-to-many column=”person_id” class=”no.uio.inf5750.example.model.Person”/> </set> ... The name property refers to the get/set-methods Many-to-many associations require a link table Column name for ”this” side of association Column name for ”other” side of association Reference to the associated class
- 14. Hibernate mapping types ● Hibernate will translate Java types to SQL / database types for the properties of your mapped classes Java type Hibernate type SQL type java.lang.String string VARCHAR java.util.Date date, time DATE, TIME java.lang.Integer, int integer INT java.lang.Class class varchar java.io.Serializable serializable BLOB, BINARY
- 15. Example app: The EventManager Java objects Hibernate mapping files Hibernate configuration Hibernate API
- 16. Hibernate configuration ● Also referred to hibernate properties ● Each database has a dialect ○ hibernate.dialect = org.hibernate.dialect.H2Dialect ● Must also specify: ○ JDBC driver class ○ Connection URL ○ Username ○ Password ● More later in the lecture...
- 17. Example app: The EventManager Java objects Hibernate mapping files Hibernate configuration Hibernate API
- 18. The SessionFactory interface ● When all mappings have been parsed by the org. hibernate.cfg.Configuration, the application must obtain a factory to get org.hibernate.Session ● SessionFactory provides org.hibernate.Session instances to the application ● Shared among application threads ● Most important method is getCurrentSession OR let Spring inject it for you SessionFactory sessionFactory = cfg.buildSessionFactory(); <bean id="sessionFactory" class="org.springframework.orm.hibernate3.LocalSessionFactoryBean"> <property name="dataSource" ref="dataSource"/> <property name="mappingResources"> …. </bean
- 19. The Session interface ● Obtained from a SessionFactory ● Main runtime interface between a Java application and Hibernate ● Responsible for storing and retrieving objects ● Think of it as a collection of loaded objects related to a single unit of work Session session = sessionFactory.getCurrentSession();
- 20. Instance states ● An object instance state is related to the persistence context ● The persistence context = a Hibernate Session instance ● Three types of instance states: ○ Transient ■ The instance is not associated with any persistence context ○ Persistent ■ The instance is associated with a persistence context ○ Detached ■ The instance was associated with a persistence context which has been closed – currently not associated
- 21. The Session interface Event event = new Event( ”title”, new Date() ); Integer id = (Integer) session.save( event ); Make a transient object persistent Event event = (Event) session.load( Event.class, id ); Load an object – if matching row exists Load an object – if unsure about matching row Event event = (Event) session.get( Event.class, id ); Delete an object – make it transient again session.delete( event );
- 22. The Session interface session.update( event ); Update an object – if its detached Synchronize database with persistence context session.flush(); // Happens auto. at transaction.commit() session.saveOrUpdate( event ); Update or save an object – if you’re unsure about the state
- 23. The Criteria interface ● You need a query when you don’t know the identifiers of the objects you are looking for ● Criteria used for programmatic query creation Criteria criteria = session.createCriteria( Event.class ); List events = criteria.list(); Retrieve all instances of Event Criteria criteria = session.createCriteria( Event.class ); criteria.add( Restrictions.eq( ”title”, ”Rolling Stones” ) );; criteria.add( Restrictions.gt( ”date”, new Date() ) ); criteria.setMaxResults( 10 ); List events = criteria.list(); Narrow the result set
- 24. Transactions ● Transaction: A set of database operations which must be executed in entirety or not at all ● Should end either with a commit or a rollback ● All communication with a database has to occur inside a transaction! Transaction begins Operation A: INSERT INTO... Operation B: INSERT INTO... Transaction commit (SUCCESS) (ERROR) Transaction rollback
- 25. Hibernate in real-life apps ● Spring used for SessionFactory management ○ Spring has excellent ORM integration ○ Custom SessionFactory management is “boilerplate-code” ● Spring used for Transaction management ○ Custom tx management is error prone ○ Support for declarative tx management with annotations ○ Consistent programming model across JTA, JDBC, Hibernate ● Annotate transactional methods / class with @Transactional ● Hibernate’s own connection pooling is basic. Hibernate allows external JDBC pooling through library called C3P0
- 27. Spring-Hibernate configuration <tx:annotation-driven transaction-manager="transactionManager"/> <bean id="transactionManager" class="org.springframework.orm.hibernate3.HibernateTransactionManager"> <property name="sessionFactory" ref="sessionFactory"/> <property name="dataSource" ref="dataSource"/> </bean> <bean id="sessionFactory" class="org.springframework.orm.hibernate3.LocalSessionFactoryBean"> <property name="dataSource" ref="dataSource"/> <property name="mappingResources"> <list> <value>hibernate/Event.hbm.xml</value> <value>hibernate/Person.hbm.xml</value> </list> </property> <property name="hibernateProperties"> <props> <prop key="hibernate.dialect">org.hibernate.dialect.H2Dialect</prop> <prop key="hibernate.hbm2ddl.auto">create-drop</prop> </props> </property> </bean> <bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource" destroy-method="close"> <property name="driverClass" value="org.h2.Driver"/> <property name="jdbcUrl" value="jdbc:h2:mem:eventmanager;DB_CLOSE_ON_EXIT=FALSE"/> <property name="user" value="sa"/> <property name="password" value=""/> </bean> Enables annotations Can be injected like any bean Points to mapping files Hibernate properties ● H2 database (fast inmemory DB) ● Automatically create tables Connection properties
- 28. Example: The EventManager Java objects Hibernate mapping files Hibernate configuration Hibernate API
- 29. Advantages of ORM ● Productivity ○ Eliminates lots of repetitive code – focus on business logic ○ Database schema is generated automatically ● Maintainability ○ Fewer lines of code – easier to understand ○ Easier to manage change in the object model ● Database vendor independence ○ The underlying database is abstracted away ○ Can be configured outside the application ● Performance ○ Lazy loading – associations are fetched when needed ○ Caching
- 30. Resources ● Spring documentation chapter 13 http://static.springsource.org/spring/docs/3.2.x/spring- framework-reference/html/orm.html ● Hibernate reference documentation ●www.hibernate.org -> Documentation -> Reference