JPA-Java Persistence API Basics

What is JPA?

										
											JPA stands for Java Persistence API. It is a Java API specification which describe the management of relational data.
									

										What's the need for JPA?

										    Object-relational technologies aims at mapping the relational data in database to Object-oriented model in Java. Various object-relational technologies like Hibernate, Toplink, EJB, JDO are in existence. JPA was introduces as part of Java EE 5 platform and it provides a standardized way of dealing with data persistence.

										JPA implementations?

										    As mentioned earlier, JPA is an API and various implementations like Hibernate, OpenJPA, Toplink, batooJPA, EclipseLink JPA.

										Building Blocks of JPA:

										Entity:

										    Entity is the domain object which is used for persistence. Usually, an entity represents a table. Each instance corresponds 

										to a row. In order for a POJO to be an entity class

												Should be annotated with @Entity

												Should have default or no arg constructor

												Should implement Serializable if the entity will be used as a detached object

												Persistence variables should not be public.

											Entity Properties:

												All properties of entity are considered persistent except static and transient properties

										
														 Annotation
													
														 Example
													
														 @Id
													
														 @Column
													
														@Column(updatable = false, name = "flight_name", nullable = false, length=50)
													
														 @Transient
													
														 @Basic
													
														 @Lob
													
														 @Version
													
														 @Embedded
													
														 @Embeddable
													
														 @EmbeddedId
													
														 @Inheritance
													
														 @OneToOne
													
														 @ManyToOne
													
														 @OneToMany
													
														 @ManyToOne
													
														 @ManyToMany
													
														 @NamedQuery

										Querying Entities

										    There are two ways for querying entities

												JPQL - Java Persistence Query Language

												Criteria API 

											persistence.xml

										    Similar to hibernate.cfg.xml file in hibernate, JPA used its own configuration file named persistence.xml. The provider is required to locate all JPA config files by classpath lookup of the META-INF/persistence.xml resource name.

										persistence.xml files should provide a unique name for each persistence unit. This name is how applications reference the configuration while obtaining an javax.persistence.EntityManagerFactory reference. 