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Introduction to the Java Persistence

API (JPA) 1.0

Edem Morny

Genkey Africa Ltd

http://edemmorny.wordpress.com

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What are we discussing?

Justification and history of Java O/R Mapping JPA and current implementations

The Persistence Unit

Entity

Mapping relationships

The EntityManager

JPA QL

Cascades

Etc

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Justification and History of O/RMapping

Imagine trying to design the underlying model for aschool.

School

Student Course etc.

Imagine thinking in terms of classes and not in terms ofhow they map to a database structure.

Object/Relational Mapping

Hibernate, JDO,

Recent convert : .Net Entit Framework

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JPA 1.0 and Current Implementations

JPA 1.0 released with JEE 5

Heavily influenced by Hibernate

Current JPA Persistence Providers are

Hibernate(JBoss)

Ibatis

TopLink(Oracle)

EclipseLink (IBM)

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The Persistence Unit

org.hibernate.ejb.HibernatePersistence

org.jaccra.jpaexample.School

org.jaccra.jpaexample.Student

Persistence

provider

Entities

Db

configuration

details etc

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JPA Managed Entity

@Entity

public class School implements Serializable {

private Long id;private String name;

private String address;

public String getId()

public void setId(Long id) ...

Annotations

JPA

recomnended

Follows

JavaBean

convention

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JPA Managed Entity - Annotations

@Entity@Table(name=skool)

public class School implements Serializable {

private Long id;

private String name;

@Id@GeneratedValue(strategy = GenerationType.AUTO)

public String getId()

@Column(name=first_name,nullable=false,length=20)

ublic Strin etName ...

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Mapping Relationships

Supports definition of RDBMS relationships and isexpressed through annotations

Single-valued :@OneToOne

Multi-valued @ManyToOne

@ManyToMany

Supports both join tables and non-join table

Expressed through any Collection i.e. Set or List, aswell as using a Map.

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Mapping Relationships - Annotations@Entity

public class Student implements Serializable {

private School school;

@ManyToOne

@JoinColumn(nullable=false)

public School getSchool() {return school;

}

...

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Mapping Relationships - Annotations

@Entity

public class School implements Serializable {

private List students;

@OneToMany(mappedBy = "school")

public List getStudents() {

return students;}

...

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The EntityManager

Liason between your objects and the persistencelayer.

It enables us to in a safe way

Save new objects (persist)

Update existing objects (merge)

Delete persistent object (remove)

Find persistent objects (find) Query persistent object for single objects,

Collections and scalar results.

etc.

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The EnityManager -Continued

EntityManagerFactory emf =Persistence.createEntityManagerFactory("jpaexamplePU");

EntityManager em = emf.createEntityManager();

EntityTransaction tx = em.getTransaction();

try{

School school = new School();school.setName("MEST");

em.persist(school);

tx.commit();

}catch....

School school = em.find(School.class,Long(1));

System.out.println('The name of the school is: '+school.getName());

To get an

EM, you

need to ask

from the

factory

Start a

transaction

on the EM

Commit the

transaction

Find objects

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Cascades

Defines actions to be taken when changes are madeto objects related to each other

Cascade types

ALL, MERGE,PERSIST,REMOVE,REFRESH

@OneToMany(mappedBy = "school", cascade =CascadeType.ALL)

public List getStudents() {

In the above cascade, if any school object is deleted,all it's orphaned students will be deleted.

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Query your model JPA QL

Enables us to express in object terms what we wantto fetch from persistence.

Allows the passing of parameters to suchexpressions.

Provides optimisation of queries through preloadingNamedQueries.

Enables us to fetch an object, a Collection or scalar

results as well as all types of joins.

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JPA QL - Continued Simple query to select a school by name 'MEST'

Query q = em.createQuery("Select s from School s where s.name = 'MEST'") Same query with parameters

Query q = em.createQuery("Select s from School s where s.name =:name").setParameter("name", "MEST");

Expecting only one result?School school = (School) q.getSingleResult();

Or a collection?

List schools = q.getResultList();

Or a scalar result

Long count = (Long) em.createQuery("Select count(s) from Schools).getSingleResult();

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JPA QL - Continued

Projection Returns a list containing Object[]

Select s from School s , st from Student st

Joins

Select s from School s join s.students st where st.name='Francis';

Subselects

Advanced querying: Group by, order by etc,

Etc

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JPA Other Features

Embeddable classes

Inheritance and Polymorphism

Override annotations with XML

Native SQL invocation

Locking : Optimistic and Persismistic

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The Future - JPA 2.0 (JSR 317)

Currently at Proposed Final Draft stage Features proposed in specification

Metamodel API (for spec implementors)

Integration with Bean Validation (JSR 303)

Criteria Query API

Many many more annotations

@Access, @OrderBy, @MapKeyClass, @Cacheable

And a whole bunch of improvements to JPA 1.0

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Further references

Enterprise JavaBeans, v. 3.0. Java Persistence API.

JSR-307: JPA 2. http://jcp.org/en/jsr/detail?id=307

JSR-303: Bean Validation.http://jcp.org/en/jsr/detail?id=303

JDBC 4.0 Specification. http://java.sun.com/products/jdbc.

Java Persistence with Hibernate Manning Publishers

http://edemmorny.wordpress.com

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