Topic : JPA Kaster Nurmukan

Overview of JPA EntityManager

O/R Mapping Primary entity annotations EntityManager Entity Relationships Practice

Database Table POJO Forward Engineering Reverse Engineering Java Persistence API

Java Persistence API (JPA) –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

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

Java classes – Employee.java JPA configuration file – persistence.xml Code to access the persistent objects – EmployeeTest.java (Optional) Logging configuration files – log4j.properties

Plain Java classes (POJOs); however, 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

@Entity. Mark class as an Define primary Define composite Define table name for entity Define column Ignored by persistence framework. Autopopulate column with sequence generator.

Maintains a cache of instances within a transactional context (persistence context) We can acquire EntityManager instance using :  Dependency Injection  EntityManagerFactory  JNDI Lookup Operations : persist(), merge(), remove(), find(), createNamedQuery(), createQuery() Persistence unit is declared in persistence.xml

@OneToOne. One to One is represented by a single- value entity reference at one or both ends of the This annotation is added to a Collection relationship Indicating that is an entity is part of a This annotation is assigned to a Collection relationship field to indicate the target entity also has a Collection of the source entity type. Lazy vs Eager Binding Cascade (ALL, PERSIST, MERGE, REMOVE, REFRESH)

v110912Java Persistence: EntityManager12 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

v110912Java Persistence: EntityManager13 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

v110912Java Persistence: EntityManager14 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);

v110912Java Persistence: EntityManager15 Example Author POJO public 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 = 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.

v110912Java Persistence: EntityManager16 Creating Entity in Database Author author = new Author(); //primary key will be gen author.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 created author:id=50, fn=dr, ln=seuss, subject=children, pdate=Fri Sep 15 11:54:15 EDT 2006

v110912Java Persistence: EntityManager17 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

v110912Java Persistence: EntityManager18 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

19 Persistence Context Examples Transaction-scoped (inside server EntityManager 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 begins Author author = em.find(Author.class, authorId); tx.commit(); //tx 1 ends, but author remains managed... tx.begin(); //tx 2 begins author.setType(type); tx.commit(); //tx 2 ends, and author is still managed until close

20 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)

v110912Java Persistence: EntityManager21 Example Component Layout META-INF/ +---persistence.xml ejava + ---examples +---… +---DAOException.class +---AuthorDAO.class +---jpa | +---JPAAuthorDAO.class | +---JPADAOBase.class +--domain +---Author.class

22 Example persistence.xml <persistence xmlns=" xmlns:xsi=" xsi:schemaLocation=" version="1.0"> java:/ejavaDS <property name="hibernate.show_sql" value="true"/ 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

v110912Java Persistence: EntityManager23 Another Example persistence.xml <persistence xmlns=" xmlns:xsi=" xsi:schemaLocation=" version="1.0"> org.hibernate.ejb.HibernatePersistence <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"/> </persistence

24 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 es or 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

25 Java SE Steps Startup – Get EntityManagerFactory Runtime – Create EntityManager – Start Transaction – Interact with Entity Manager – Commit Transaction – Close EntityManager Shutdown – Close EntityManagerFactory

26 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

v110912Java Persistence: EntityManager27 (optional!)Potential Utiltity Class package 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 factories = new HashMap (); 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(); }

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