Hibernate Basics

1. Hibernate Basics

2. Basics • What is persistence? • Persistence in Object Oriented Applications? • Paradigm Mismatch. • The Problem of Granularity. • The Problem of Subtypes. • The Problem of Identity. • Problem relating to Association. • The Problem of Object graph Navigation.

3. The problem of Granularity • User.java – name, userName, address, billingDetails etc • BillingDetails .java – accountNumber, accountName, accountType,user

4. Problem of Subtype • SQL – no support for inheritance. • Polymorphic association • Polymorphic query

5. Problem of Identity • Object identity • Object Equality • Database identity • UserName as key. Surrogate keys. • Problem relating to Associations • Association in OO world – object reference • Association in Relational Databases – foreign key • Difference -Directional • Example- User have Set of BillingDetails • Example – In SQL by table joins and projection.

6. The problem of Object graph navigation- walking the object graph- Ex. user.getBillingDetails().getAccountNumber()- Relational database – joinSolutions of mismatch Paradigm:- Layered architecture- Hand coding a persistence layer with SQL/JDBC- Serialization- EJB- OO databases system – interact with database via intermediate SQL.

7. Other options : ORM- ORM - Advantages-No SQL -Productivity -Maintainability - Performance - Vendor independent

8. “Hello World” with Hibernate Message.java package hello; public class Message { private Long id; private String text; private Message nextMessage; private Message() {} public Message(String text) { this.text = text; } // getter and setter

9. Sample hibernate.cfg.xml configuration file ?xml version='1.0'encoding='utf-8'?> <!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD//EN" "http://hibernate.sourceforge.net/hibernate-configuration-2.0.dtd"> <hibernate-configuration> <session-factory name="java:/hibernate/HibernateFactory"> <property name="show_sql">true</property> <property name="connection.datasource"> java:/comp/env/jdbc/AuctionDB </property> <property name="dialect"> net.sf.hibernate.dialect.PostgreSQLDialect </property> <property name="transaction.manager_lookup_class"> net.sf.hibernate.transaction.JBossTransactionManagerLookup </property> <mapping resource="auction/Item.hbm.xml"/> </session-factory> </hibernate-configuration>

10. Hibernate mapping file. <?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>

11. Saving a new message in database. Session session = getSessionFactory().openSession(); Transaction tx = session.beginTransaction(); Message message = new Message("Hello World"); session.save(message); tx.commit(); session.close(); Generated SQL: -insert into MESSAGES (MESSAGE_ID, MESSAGE_TEXT, NEXT_MESSAGE_ID)‏ values (1, 'Hello World', null)‏

12. Updating a record Session session = getSessionFactory().openSession(); Transaction tx = session.beginTransaction(); // 1 is the generated id of the first message Message message = (Message) session.load( Message.class, new Long(1) ); message.setText("Greetings Earthling"); Message nextMessage = new Message("Take me to your leader (please)"); message.setNextMessage( nextMessage ); tx.commit(); session.close(); automatic dirty checking cascade all transactional write behind

13. This code calls three SQL statements inside the same transaction: select m.MESSAGE_ID, m.MESSAGE_TEXT, m.NEXT_MESSAGE_ID from MESSAGES m where m.MESSAGE_ID = 1 insert into MESSAGES (MESSAGE_ID, MESSAGE_TEXT, NEXT_MESSAGE_ID)‏ values (2, 'Take me to your leader (please)', null)‏ update MESSAGES set MESSAGE_TEXT = 'Greetings Earthling', NEXT_MESSAGE_ID = 2 where MESSAGE_ID = 1

14. Core interfaces • Five core interfaces • Session Interface • Primary, lightweight, not thread safe,cache or collection. • SessionFactory Interface • Create sessions, single,shared by many application thread. • Configuration Interface • To configure and bootstrap the hibernate. • Specify location of mapping files and create sessionFactory.

15. Transaction interface • Optional • Abstract application code from underlying transaction • Query and Criteria interface • Allow to perform queries • Queries written in HQL or native SQL • Bind query parameter, limit result and execute query • Criteria interface similar to Query, allow to execute OO criteria queries. • Callback interface • Lifecycle and validate interface.

16. Implementing the domain model • Domain model should concern only with modeling of business domain. • Advantage – reuse, ease to unit test. • Leakage of concerns - • Transparent and Automated persistence • Example - Message.java

17. HQL and Query by criteria (QBC)‏ • OO dialect of relation SQL language • It is not DML • HQL support – restriction,projection, order, pagination,subqueries, aggregate function etc. • Query by Criteria • Example: Criteria object • Criteria criteria = session.createCriteria(User.class); • criteria.add( Expression.like("firstname", "Max") ); • List result = criteria.list(); • Criterion object tree,less readable,validate at compile time,extendable

18. Fetching Strategies • Different styles of fetching: • Immediate fetching • linked objects fetched immediate together with parent • lazy fetching • linked object fetched when link is navigated • eager (outer join) fetching • linked objects fetched immediate together with parent • select-clause contains outer join-clause • batch fetching • not strictly a fetching strategy • used to improve performance of lazy fetching

19. Fetching Strategies continue.... • Different styles of fetching: • Lazy Fetching = true • <set name=”bids” lazy=”true” inverse=”true” > • - n+1 trip to database • Batch fetching: • <set name=”bids” lazy=”true” inverse=”true” batch-size=”10”> • - n/10 + 1 trip to database • - hibernate prefetch the next 10 collection when the first collection is accessed • Eager fetching • <set name=”bids” lazy=”true” inverse=”true” outer-join=”true”> • - not good, reduce concurrency- read lock • - retrieving unnecessary data • - solution : run time eager fetching mode

20. Hierarchy Mapping • Hierarchical relations between entities not supported in database • Three alternatives: • Table per concrete class (concrete table inheritance)‏ • table per class hierarchy (single table inheritance)‏ • table per subclass (class table inheritance)‏

21. Table per subtype • create pk-fk relationships in database • lots of joins to compose object • SQL can not enforce consistency of model

22. Table per class hierarchy • used with few subclasses with few attributes • gives a lot of null values in table • violates normalisation rules • easy refactoring • discriminator

23. Table per concrete class • No special mapping needed • Create one mapping per class • used when super class is abstract • entity integrity can not be enforced by the database • each change to super class -> change of all subclass tables

24. Hierarchy mapping – general comments • You can not mix strategies within one hierarchy • You can mix strategies in your application • Choose a hierarchy mapping strategy • No polymorphic queries or associations needed • table-per-class strategy • Polymorphic queries or associations needed • not to many subclasses and not to many attributes in subclasses • table-per-class-hierarchy • many subclasses or many attributes in subclasses • table-per-subclass