Intro To Jpa 1.0

Introduction to the Java Persistence API (JPA) 1.0

Edem Morny Genkey Africa Ltd http://edemmorny.wordpress.com

What are we discussing? ●

Justification and history of Java O/R Mapping

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JPA and current implementations

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The Persistence Unit

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Entity

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Mapping relationships

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The EntityManager

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JPA QL

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Cascades

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Etc

Justification and History of O/R Mapping

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Imagine trying to design the underlying model for a school. School – Student – Course etc. Imagine thinking in terms of classes and not in terms of how they map to a database structure. –

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Object/Relational Mapping – –

Hibernate, JDO, Recent convert : .Net Entity Framework

JPA 1.0 and Current Implementations ●

JPA 1.0 released with JEE 5 –

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Heavily influenced by Hibernate

Current JPA Persistence Providers are –

Hibernate(JBoss)

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Ibatis

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TopLink(Oracle)

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EclipseLink (IBM)

The Persistence Unit

Persistence provider

<provider>org.hibernate.ejb.HibernatePersistence

Entities

org.jaccra.jpaexample.School org.jaccra.jpaexample.Student <properties> <property name="hibernate.connection.username" value="edem"/> <property name="hibernate.connection.url" value="jdbc:mysql://localhost:3306/jaccra"/>

Db configuration details etc

Annotations

JPA Managed Entity JPA recomnended

@Entity public class School implements Serializable { private Long id; private String name; private String address; public String getId() … public void setId(Long id) ...

Follows JavaBean convention

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)

public String getName() ...

Mapping Relationships ●

Supports definition of RDBMS relationships and is expressed through annotations –

Single-valued :@OneToOne

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Multi-valued ● ●

@ManyToOne @ManyToMany –

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Supports both join tables and non-join table

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

Mapping Relationships - Annotations @Entity

public class Student implements Serializable { private School school; @ManyToOne @JoinColumn(nullable=false) public School getSchool() { return school; } ...

Mapping Relationships - Annotations @Entity

public class School implements Serializable { private List<Student> students; @OneToMany(mappedBy = "school") public List<Student> getStudents() { return students; } ...

The EntityManager ●

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Liason between your objects and the persistence layer. It enables us to in a safe way –

Save new objects (persist)

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Update existing objects (merge)

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Delete persistent object (remove)

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Find persistent objects (find)

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Query persistent object for single objects, Collections and scalar results.

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etc.

The EnityManager -Continued

To get an EM, you need to ask from the factory

EntityManagerFactory emf = Persistence.createEntityManagerFactory("jpaexamplePU"); EntityManager em = emf.createEntityManager(); EntityTransaction tx = em.getTransaction(); try{ School school = new School();

Start a transaction on the EM

school.setName("MEST"); em.persist(school); tx.commit();

Commit the transaction

}catch.... School school = em.find(School.class,Long(1)); System.out.println('The name of the school is: '+school.getName());

Find objects

Cascades ●

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Defines actions to be taken when changes are made to objects related to each other Cascade types –

ALL, MERGE,PERSIST,REMOVE,REFRESH

@OneToMany(mappedBy = "school", cascade = CascadeType.ALL) public List<Student> getStudents() {… ●

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

Query your model – JPA QL ●

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Enables us to express in object terms what we want to fetch from persistence. Allows the passing of parameters to such expressions. Provides optimisation of queries through preloading NamedQueries. Enables us to fetch an object, a Collection or scalar results as well as all types of joins.

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<School> schools = q.getResultList(); ●

Or a scalar result

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

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

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Advanced querying: Group by, order by etc,

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Etc

JPA – Other Features ●

Embeddable classes

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Inheritance and Polymorphism

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Override annotations with XML

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Native SQL invocation

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Locking : Optimistic and Persismistic

The Future - JPA 2.0 (JSR 317) ●

Currently at Proposed Final Draft stage

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Features proposed in specification –

Metamodel API (for spec implementors)

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Integration with Bean Validation (JSR 303)

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Criteria Query API

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Many many more annotations ●

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@Access, @OrderBy, @MapKeyClass, @Cacheable

And a whole bunch of improvements to JPA 1.0

Further references ●

Enterprise JavaBeans, v. 3.0. Java Persistence API.

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

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

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

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Java Persistence with Hibernate – Manning Publishers

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The End