Java Persistence API (JPA)

Provides ORM facility for managing relational data in Java applications

• Entity (equals to a table in a relational DB)
• Must have a public/ protected no-arg constructor
• Must not be declared final
• Entity instance (equals to a table row in a relational DB)
• Entity Manager Interface 

The EntityManager API creates and removes persistent entity instances, finds entities by the entity’s primary key, and allows queries to be run on entities.

A sample relational mapping is as follows

@OneToOne
@JoinColumns({
    @JoinColumn(name="PARTNUMBER",
        referencedColumnName="PARTNUMBER"),
    @JoinColumn(name="PARTREVISION",
        referencedColumnName="REVISION")
})
public Part getPart() {
    return part;
}