1. Chengyu Sun California State University, Los Angeles
CS5220 Advanced Topics in Web Programming Object-Relational Mapping with Hibernate and JPA (I)
Chengyu Sun
California State University, Los Angeles

2. The Object-Oriented Paradigm
The world consists of objects
So we use object-oriented languages to write applications
We want to store some of the application objects (a.k.a. persistent objects)
So we use a Object Database?

3. The Reality of DBMS Relational DBMS are still predominant
Best performance
Most reliable
Widest support
Bridge between OO applications and relational databases
CLI and embedded SQL
Object-Relational Mapping (ORM) tools

4. Call-Level Interface (CLI)
Application interacts with database through functions calls
String sql = "select name from items where id = 1";
Connection c = DriverManager.getConnection( url );
Statement stmt = c.createStatement();
ResultSet rs = stmt.executeQuery( sql );
if( rs.next() ) System.out.println( rs.getString(“name”) );

5. Embedded SQL SQL statements are embedded in host language String name;
#sql {select name into :name from items where id = 1};
System.out.println( name );

6. Employee – Application Object
public class Employee {
Integer id;
String name;
Employee supervisor; }

7. Employee – Database Table
create table employees (
id integer primary key,
name varchar(255),
supervisor_id integer references employees(id) );

8. From Database to Application
So how do we construct an Employee object based on the data from the database?
public class Employee {
Integer id;
String name;
Employee supervisor;
public Employee( Integer id ) {
// access database to get name and supervisor
… … }

9. Problems with CLI and Embedded SQL …
SQL statements are hard-coded in applications
public Employee( Integer id ) { …
PreparedStatment p;
p = connection.prepareStatment(
“select * from employees where id = ?” ); }

10. … Problems with CLI and Embedded SQL …
Tedious translation between application objects and database tables
public Employee( Integer id ) { …
ResultSet rs = p.executeQuery();
if( rs.next() ) {
name = rs.getString(“name”); }

11. … Problems with CLI and Embedded SQL
Application design has to work around the limitations of relational DBMS
public Employee( Integer id ) { …
ResultSet rs = p.executeQuery();
if( rs.next() ) {
supervisor = ?? }

12. The ORM Approach employee Application customer account ORM tool
PostgreSQL, MySQL, Oracle,
SQL Server …
Persistent Data Store

13. Hibernate and JPA Java Persistence API (JPA) Hibernate
Annotations for object-relational mapping
Data access API
An object-oriented query language JPQL
Hibernate
The most popular Java ORM library
An implementation of JPA

14. Hibernate Usage Hibernate without JPA Hibernate with JPA
API: SessionFactory, Session, Query, Transaction
More features
Hibernate with JPA
API: EntityManagerFactory, EntityManager, Query, Transaction
Better portability
Behaviors are better defined and documented

15. A Hibernate Example Java class Code to access the database
Employee.java
Code to access the database
EmployeeTest.java
JPA configuration file
persistence.xml
(Optional) Logging configuration file
log4j.properties

16. Persistent Class
A class whose objects need to be saved (i.e. persisted) in a database
Any Java model class can be a persistent class, though it is recommended that
Each persistent class has an identity field
Each persistent class implements the Serializable interface
Each persistent field has a pair of getter and setter, which don’t have to be public

17. O/R Mapping Annotations
Describe how Java classes are mapped to relational tables
@Entity
Persistent Java Class
@Id
Id field
@Basic (can be omitted)
Fields of simple types
@ManyToOne
@OneToMany
@ManyToMany
@OneToOne
Fields of class types

18. Association
Relationship between two entity classes (i.e. entity sets in ER model)
Types of relationships
Many-to-Many
Many-to-One / One-to-Many
One-to-One

19. Many-to-Many Relationship
Each entity in E1 can be related to many entities in E2
Each entity in E2 can be related to many entities in E1 E1 E2

20. Many-to-One Relationship
Each entity in E1 can be related to one entities in E2
Each entity in E2 can be related to many entities in E1 E1 E2

21. One-to-One Relationship
Each entity in E1 can be related to one entities in E2
Each entity in E2 can be related to one entities in E1 E1 E2

22. Relationship Type Examples
Employee and supervisor??
Classes and instructors??
Classes and students??

23. persistence.xml <persistence-unit> <properties>
name
<properties>
Database information
Provider-specific properties
No need to specify persistent classes

24. Access Persistent Objects
EntityManagerFactory
EntityManager
Query and TypedQuery
Transaction
A transaction is required for updates

25. Some EntityManager Methods
find( entityClass, primaryKey )
createQuery( query )
createQuery( query, resultClass )
persist( entity )
merge( entity )
getTransaction()

26. Persist() vs. Merge() Scenario Persist Merge
Object passed was never persisted
1. Object added to persistence context as new entity
2. New entity inserted into database at flush/commit
1. State copied to new entity.
2. New entity added to persistence context
3. New entity inserted into database at flush/commit
4. New entity returned
Object was previously persisted, but not loaded in this persistence context
1. EntityExistsException thrown (or a PersistenceException at flush/commit)
1. Existing entity loaded.
2. State copied from object to loaded entity
3. Loaded entity updated in database at flush/commit
4. Loaded entity returned
Object was previously persisted and already loaded in this persistence context
1. EntityExistsException thrown (or a PersistenceException at flush or commit time)
1. State from object copied to loaded entity
2. Loaded entity updated in database at flush/commit
3. Loaded entity returned

27. A Common Scenario That Needs Merge()
Load an object from database
Open EntityManager
Load object
Close EntityManager
Save the object in HTTP session
Get the object from HTTP session
Change some fields of the object
Save the object back to database
Save object
doGet()
doPost()

28. The Returned Value of Merge()
Employee e = new Employee();
e.setName( “Joe” );
entityManager.persist( e );
e.getId()  ??
Employee e = new Employee();
e.setName( “Joe” );
entityManager.merge( e );
e.getId()  ??

29. Java Persistence Query Language (JPQL)
A query language that looks like SQL, but for accessing objects
Automatically translated to DB-specific SQL statements
E.g. select e from Employee e where supervisor = :supervisor
See Chapter 4 of Java Persistence API, Version 2.1

30. Hibernate Query Language (HQL)
A superset of JPQL

31. Join in HQL … class User { Integer id; String username; … }
class Section {
Integer id;
User instructor; … }
users
sections id
username id
instructor_id 1
cysun 1 1 2
vcrespi 2 1 3 2

32. … Join in HQL …
Query: find all the sections taught by the user “cysun”.
SQL??
HQL??

33. … Join in HQL … class User { Integer id; String username; … }
class Section {
Integer id;
Set<User> instructors; … }
users
sections
instructors id
username id
section_id
instructor_id 1
cysun 1 1 1 2
vcrespi 2 2 1 3 2 2

34. … Join in HQL
Query: find all the sections for which “cysun” is one of the instructors
SQL??
HQL??
See SectionDaoImpl in CSNS2 for more HQL join examples

35. Benefits of ORM
Remove the mismatch between OO design in application and relational design in database
Simplify data access
Data is accessed as objects, i.e. no manual conversion between objects and rows/columns necessary
JPQL/HQL queries are usually simpler than SQL queries
Often times queries are automatically generated by the ORM tool, e.g. e.getSupervisor().getSupervisor().getName()
Improve DBMS independency
Object caching

36. How About NoSQL Databases
Key-Value Stores
Column Family Stores
Graph Databases
Document Databases
Object Databases??

37. “Document” Example { ‘first_name’: ‘John’, ‘last_name’: ‘Doe’,
‘age’: 20,
‘address’: {
‘street’: ‘123 Main’
‘city’: ‘Los Angeles’
‘state’: ‘CA’ }
A: It’s really an object!
B: Yes, but … …