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Overview• Earlier versions of EJB Specification defined

the persistence layer– javax.ejb.EntityBean

• Java EE 5 moved persistence to its own specification– Java Persistence API (JPA) version 1.0– javax.persistence

• ease of use API above JDBC• Provides

– Object/Relational Mapping (ORM) Engine– Query Language (SQL-Like, based on former EJB-QL)

• Java EE 6 uses JPA 2.0

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javax.persistence.EntityManager

• Replaces much of the EJB 2.x “Home” functionality

• Handles O/R Mapping of Entities to the database• Provides APIs

– inserting objects into database– getting objects from database– synchronizing objects with database– querying database

• Provides caching• Coordinates with transactional services (JTA)• Tightly integrated with Java EE and EJB, but not

limited to that environment

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javax.persistence.EntityManager

• Replaces much of the EJB 2.x “Home” functionality

• Handles O/R Mapping of Entities to the database• Provides APIs

– inserting objects into database– getting objects from database– synchronizing objects with database– querying database

• Provides caching• Coordinates with transactional services (JTA)• Tightly integrated with Java EE and EJB, but not

limited to that environment

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Entities • (formerly and sometimes still called Entity

Beans)• are now Plain Old Java Objects (POJOs)

– nothing special happens when calling new

Author author = new Author();

• are not persistent until associated with an EntityManager

em.persist(author);

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Example Author POJO Entity@javax.persistence.Entitypublic class Author { private long id; private long version=0; private String firstName; private String lastName; private String subject; private Date publishDate; public Author() {} public Author(long id) { this.id = id; } @Id @GeneratedValue public long getId() { return id;} private void setId(long id) { this.id = id; } public String getFirstName() { return firstName; } public void setFirstName(String firstName) { this.firstName = firstName; } ... }

Warning: Using GeneratedValue without specifying a specific strategy should only be used when you have no intention of controlling how the provider manages primary keys for this object type. This would be rare.

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Creating Entity in DatabaseAuthor author = new Author(); //primary key will be genauthor.setFirstName("dr");author.setLastName("seuss");author.setSubject("children");author.setPublishDate(new Date());

log_.info("creating author:" + author);em.persist(author);log_.info("created author:" + author);

//output-creating author:id=0, fn=dr, ln=seuss, subject=children,

pdate=Fri Sep 15 11:54:15 EDT 2006-created author:id=50, fn=dr, ln=seuss, subject=children,

pdate=Fri Sep 15 11:54:15 EDT 2006

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Managed and Unmanaged Entities

• Unmanaged state (detached)– instance not associated with an EntityManager– state changes are not tracked– can be serialized to client and returned to be

synchronized with database– nothing equivalent to this state in EJB 2.1 entity beans

• Managed state (attached)– instance associated with an EntityManager– state changes are tracked within a Persistence Context– EJB 2.1 entity beans were always managed

• client interfaced with data through a proxy or state transferred through a Data Transfer Object

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Persistence Context• A set of attached entity instances managed

by an EntityManager• All entities become detached once closed• Two types

– Transaction-scoped Persistence Contexts• begin/end at transaction boundaries• only made available through container managed

persistence contexts– Extended Persistence Contexts

• live beyond any single transaction• allow longer-lived interactions with database without

lengthy transactions tying up database resources

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Persistence Context Examples• Transaction-scoped (inside server container)

@PersistenceContext(unitName=”jpaDemo”)EntityManager em;

@TransactionAttribute(REQUIRED)public void update(long authorId, String type) {

Author author = em.find(Author.class, authorId);author.setType(type);

}

• Extended (inside or outside server container)EntityManager em = Persistence.

createEntityManagerFactory(“jpaDemo”).createEntityManager();tx.begin(); //tx 1 beginsAuthor author = em.find(Author.class, authorId);tx.commit(); //tx 1 ends, but author remains managed...tx.begin(); //tx 2 beginsauthor.setType(type);tx.commit(); //tx 2 ends, and author is still managed until

close

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Persistence Unit• A set of classes that are mapped to the

database• defined in META-INF/persistence.xml • must have an identity

– “” is a valid identity• Classes

– may be named in persistence.xml file– may be automatically scanned for in the classpath

• orm.xml– optionally provided to augment, provide, or replace

class persistence metadata– (more on orm.xml in Core ORM topic)

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Example Component Layout META-INF/ +---persistence.xmlejava + ---examples +---… +---DAOException.class +---AuthorDAO.class +---jpa | +---JPAAuthorDAO.class | +---JPADAOBase.class +--domain +---Author.class

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Example persistence.xml<?xml version="1.0" encoding="UTF-8"?><persistence xmlns="http://java.sun.com/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd" version="1.0">

<persistence-unit name="jpaDemo">

<jta-data-source>java:/ejavaDS</jta-data-source>

<properties> <property name="hibernate.hbm2ddl.auto" value="create"/> <property name="hibernate.show_sql" value="true"/ </properties> </persistence-unit> </persistence>

referenced by name

• global JNDI name by which provider references resource(will be used when deployed within server)• may use properties element in Java SE environments that lack JNDI

• vendor-specific way to configure persistence provider

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Another Example persistence.xml

<?xml version="1.0" encoding="UTF-8"?><persistence xmlns="http://java.sun.com/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd" version="1.0">

<persistence-unit name="jpaDemo">

<provider>org.hibernate.ejb.HibernatePersistence</provider>

<properties> <property name="hibernate.cache.provider_class" value="net.sf.ehcache.hibernate.Provider"/> <property name="hibernate.dialect" value="org.hibernate.dialect.HSQLDialect"/> <property name="hibernate.connection.url" value="jdbc:hsqldb:hsql://localhost:9001"/> <property name="hibernate.connection.driver_class" value="org.hsqldb.jdbcDriver"/> <property name="hibernate.connection.password" value=""/> <property name="hibernate.connection.username" value="sa"/> <property name="hibernate.show_sql" value="false"/> <property name="hibernate.hbm2ddl.auto" value="create"/> </properties> </persistence-unit>

</persistence

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persistence.xml elements• name – identity to reference Persistence Unit• provider – fully qualified name of javax.persistence.PersistenceProvider

– not needed if provider found in classpath acceptable• mapping-file – resource path to optional mapping file

– can be used to specify <class>es or specify/override @Annotation details• jta-data-source

– vendor-specific reference to data source using JTA transactions• non-jta-data-source

– vendor-specific reference to data source using RESOURCE_LOCAL transactions

• jar-file– optional/additional jar file to scan for classes

• class– specifies entity classes not automatically scanned by provider

• exclude-unlisted-classes– if set, provider will not automatically scan archive for entity classes

• properties– may be used to provide vendor-specific properties to configure persistence

providers

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Java SE Steps• Startup

– Get EntityManagerFactory• Runtime

– Create EntityManager– Start Transaction– Interact with Entity Manager– Commit Transaction– Close EntityManager

• Shutdown– Close EntityManagerFactory

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updating entities• Updates to managed entities automatically

get propagated to database according to flush policy public Author update(Author author) { Author dbAuthor = em.find(Author.class,author.getId()); dbAuthor.setFirstName(author.getFirstName()); dbAuthor.setLastName(author.getLastName()); dbAuthor.setSubject(author.getSubject()); dbAuthor.setPublishDate(author.getPublishDate()); return dbAuthor;}

– Note that if author passed in was already managed...• the changes have already been queued• the dbAuthor returned from the find() will be the same object as author• the sets are unnecessarily changing the values of the Author to their current values

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(optional!)Potential Utiltity Classpackage ejava.examples.dao.jpa;

import java.util.HashMap;import java.util.Map;import javax.persistence.EntityManagerFactory;import javax.persistence.Persistence;

public class JPAUtil { private static final Map<String, EntityManagerFactory> factories = new HashMap<String, EntityManagerFactory>(); public static EntityManagerFactory getEntityManagerFactory(String puName) { EntityManagerFactory emf = factories.get(puName); if (emf == null) { synchronized(factories) { emf = factories.get(puName); if (emf == null) { emf = Persistence.createEntityManagerFactory(puName); factories.put(puName, emf); } } } return emf; } public static void close() { synchronized(factories) { for(String puName : factories.keySet()) { factories.get(puName).close(); } factories.clear(); } }}