An Introduction to Object/Relational Persistence and Hibernate

1. An Introduction to Object/Relational Persistence and Hibernate Yi Li 2009.11.20

2. The Book • Java Persistence with Hibernate • Gavin King, the founder of Hibernate open source project • Christian Bauer, core developer Gavin King

3. Outline • Understanding Object/Relational Persistence • Understanding Hibernate • Part I: Mapping • Part II: Processing • Designing the Persistence Layer

4. Understanding Object/Relational Persistence • Persistencein object-oriented applications • The problem • The solution • Introducing Hibernate

5. What is Object/Relational Persistence • The states of interconnected objects need to be stored to a relational database using SQL, and objects with the same state can be re-created at some point in the future

6. Why Object and Relational DB • Business Logic • Object-oriented concepts largely improves code reuse and maintainability • Business Data • Relational databases are flexible and robust approach to data management, due to the complete and consistent theoretical foundation of the relational data model

7. A Mismatch Problem • Object-oriented business domain model • class, object • composition, inheritance, polymorphism… • Relational persistent model • table, row, column • restriction, projection, join…

8. The Object/Relational Paradigm Mismatch Problem • The problem of… • Granularity • Subtypes • Identity • Associations • Data navigation

9. Granularity Mismatch • Class: several levels of granularity • Database: only 2 levels (table and column) User <<Table>> USER USERNAME ADDRESS_STREET ADDRESS_CITY ADDRESS_STATE ADDRESS_COUNTRY ADDRESS_ZIPCODE Address zipcode: String street: String city: String

10. Subtypes Mismatch • OO • Type inheritance • Polymorphism and polymorphic association • Relational DB • Table inheritance ? • Polymorphic query ? 1..* User BillingDetails CreditCard BankAccount

11. Identity Mismatch • Object • identity: a == b • equality: a.equals(b) • Database • identity: a.table_and_row == b.table_and_row a’ a b b’

12. Associations Mismatch • OO • one-to-one • one-to-many • many-to-many • Relational DB • foreign key (actually a many-to-one) Student School

13. Data Navigation Mismatch • OO • one by one: follow the pointers between objects • Relational DB • strive to minimize the number of requests to DB • sophisticated mechanisms for retrieving and updating data aUser.getBillingDetails().getAccountNumber(); select * from USERS u left outer join BILLING_DETAILS bd on bd.USER_ID = u.USER_ID where u.USER_ID = 3

14. Cost of the Mismatch Problem • In authors’ experience, 30% of Java application code is to handle the problems, and result doesn’t feel right • Bended and twisted business entities to match the SQL database schema, which often doesn’t follow OO principles very well

15. The solution • The 5 problems fall into 2 categories • Structural (static) • Behavioral (dynamic) • The solution is Object / Relational Mapping (ORM) • Using metadata to describe object/table mapping • Persistent object management, transaction and concurrency support • Automated and transparent

16. Possible Alternatives & Why Not • Why not serialization • a serialized network of interconnected objects can only be accessed as a whole • large datasets  • access / update a subset of objects  • high concurrency support 

17. Why not object-oriented database systems • data independence  • current deployment environments  • Why not XML persistence • data management  • object/hierarchical mismatch

18. Introducing Hibernate • Hibernate is a full ORM tool • Complete mapping support • Composition, inheritance, polymorphism • Fully Transparent • No persistence-specific base classes & interfaces needed in business layer • High Performance

19. Hibernate and the Standards • Java industry standards • Java Persistence API Specification (JPA) • Developers from the Hibernate team joined the specification expert group early • Hibernate is the recommended implementation for JPA

20. Understanding Hibernate • Mapping (Examples) • Inheritance • Associations • Polymorphism • Persistent Object Processing

21. Fundamental Concepts of Mapping • Fine-grained business model • More classes than tables • Surrogate primary key • Entity and value type <<Entity>> User Surrogate PK id: Long name: String <<Table>> USER ID <<PK>> NAME ADDRESS_STREET ADDRESS_CITY ADDRESS_ZIPCODE <<Value>> Address zipcode: String street: String city: String

22. Mapping Class Inheritance • Mapping strategies • Table per concrete class • Table per class hierarchy • Table per class

23. Table per Concrete Class 1..* BillingDetails User owner: String CreditCard BankAccount <<Table>> BANK_ACCOUNT <<Table>> CREDIT_CARD number: String expMonth: String expYear: String account: String bankname: String • Advantage • Simplest • Drawbacks • Poly-associations *  • Poly-query  • Schema evolution  BA_ID <<PK>> OWNER ACCOUNT BANKNAME CC_ID <<PK>> OWNER NUMBER EXP_MONTH EXP_YEAR *: Hibernate can implement this

24. Table per Class Hierarchy 1..* BillingDetails User owner: String CreditCard BankAccount <<Table>> BILLING_DETAILS number: String expMonth: String expYear: String account: String bankname: String • Advantage • Performance • Simplicity • Polymorphism support • Drawbacks • Loss of data integrity • Denormalized schema BD_ID <<PK>> BD_TYPE <<Discriminator>> OWNER CC_NUMBER CC_EXP_MONTH CC_EXP_YEAR BA_ACCOUNT BA_BANKNAME

25. Table per Class 1..* BillingDetails User owner: String CreditCard BankAccount <<Table>> BANK_ACCOUNT <<Table>> BILLING_DETAILS <<Table>> CREDIT_CARD number: String expMonth: String expYear: String account: String bankname: String • Advantage • Normalized schema • Data integrity • Polymorphism support • Drawbacks • Performance  BA_ID <<PK>> <<FK>> ACCOUNT BANKNAME CC_ID <<PK>> <<FK>> NUMBER EXP_MONTH EXP_YEAR BD_ID <<PK>> OWNER

26. Mapping 1-to-1 Association • Shared Primary Key Strategy <<Table>> USER <<Table>> USER <<Table>> CONTACT_INFO <<Table>> CONTACT_INFO USER_ID <<PK>> NAME AGE PASSWORD … USER_ID <<PK>> USER_CONTACT_ID <<FK>> <<UNIQUE>> NAME AGE PASSWORD … CI_ID <<PK>> <<FK>> EMAIL … CI_ID <<PK>> EMAIL … • Unique Foreign Key Strategy

27. Mapping One-to-many Associations with Join Tables 0..* 1 Item User <<Table>> ITEM <<Table>> USER <<Table>> ITEM_BUYER ITEM_ID <<PK>> NAME DESCRIPTION PRICE … USER_ID <<PK>> NAME … ITEM_ID <<PK>> <<FK>> <<UNIQUE>> USER_ID <<PK>> <<FK>> ITEM_BUYER

28. Mapping Many-to-many Associations with Join Tables 0..* 1..* Item Category <<Table>> ITEM <<Table>> CATEGORY <<Table>> CATEGORIZED_ITEM ITEM_ID <<PK>> NAME DESCRIPTION PRICE … CATEGORY_ID <<PK>> NAME … ITEM_ID <<PK>> <<FK>> CATEGORY_ID <<PK>> <<FK>> CATEGORIZED_ITEM

29. Other Features of Hibernate Mapping • Schema exporting • Automated support of polymorphic associations • Flexible type mapping system • Built-in types • Custom mapping types • Fully customizable SQL and stored proceduresallow developers to integrate legacy databases without changing business objects • Only the mapping metadata needs to be changed

30. Issues in Persistent Object Processing: At a Glance • 1. Transparent dirty checking • 2. Object identity == database identity • What if the application modifies two different instances that both represent the same row in the end of a transaction? • 3. Database transaction support

31. 1. UPDATE 3. COMMIT 2. SELECT 4. COMMIT Tx A Tx A • 4. Concurrent access control • Deal with the transaction isolation issues D1 D1 D1 D1 4. ROLLBACK 3. ROLLBACK Tx B Tx B 2. UPDATE 1. UPDATE Lost Update Dirty Read 1. SELECT 4. SELECT 1. SELECT 4. SELECT Tx A Tx A D1 D1 D1 D1 D2 D1 D2 3. COMMIT 3. COMMIT Tx B Tx B 2. UPDATE 2. INSERT Unrepeatable Read Phantom Read

32. 5. Sharing objects in different connections • 6. Transitive persistence • 7. Batch operations • 8. Data filtering and interception • 9. Optimizing data fetching and caching strategies • In the context of concurrency • 10. Object-based query language • ‘SQL’ in terms of object • 11. Optimizing query performance

33. The Last But Not the Least… • Hibernate is a fully transparent solution to object persistence • You can design and implement business entities and business logic as if there is no Hibernate at all

34. You Need a Persistence Layer • A typical layered architecture Presentation Layer Interceptors, Utility, and Helper Classes Business Layer Persistent Layer Provides abstraction and unified data access operations Database

35. Design Persistent Layer: the Generic DAO Pattern GenericDAO<T, ID> findById(ID id) findAll() findByExample(T exm) makePersistent(T entity) GenericDAOHibernateImpl ItemDAO<Item, Long> ItemDAOHibernateImpl getComments(Long id) UserDAO<Item, Long> UserDAOHibernateImpl Interfaces Concrete Classes

36. Using Data Access Objects in Business Logic Long itemId = …; DAOFactory factory = DAOFactory.getFactory(); ItemDAOitemDAO = factory.getItemDAO(); Item item = itemDAO.findById(itemId); List comments = itemDAO.getComments(itemId); … return new HibernateFactory(true); config.xml • <dao-factory> • <class = “com.xxx.dao.HibernateFactory”> • <paramname=“option1”>true</param> • </dao-factory>