jpa

Topics related to jpa:

Getting started with jpa

JPA is the Java Persistence API, a specification handling the mapping of Java objects and their relationships to a relational database. This is called an object-relational mapper (ORM). It is an alternative for (or supplement to) the more low-level JDBC. It is most useful when pursuing a Java-oriented approach and when complex object graphs need to be persisted.

JPA in itself is not an implementation. You will need a persistence provider for that (see examples). Current implementations of the latest JPA 2.1 standard are EclipseLink (also the reference implementation for JPA 2.1, which means "proof that the spec can be implemented"); Hibernate, and DataNucleus.

Metadata

The mapping between Java objects and database tables is defined via persistence metadata. The JPA provider will use the persistence metadata information to perform the correct database operations. JPA typically defines the metadata via annotations in the Java class.

Object-Relational Entity Architecture

The entity architecture is composed of:

entities
persistence units
persistence contexts
entity manager factories
entity managers

Basic mapping

There always needs to be a default constructor, that is, the parameterless one. In the basic example, there was no constructor specified, so Java added one; but if you add a constructor with arguments, be sure to add the parameterless constructor, too.

Table per concrete class inheritance strategy

Vehicle, TransportationVehicle and PassengerVehicle are abstract classes and they will not have separate table in database.
Truck, Car and Bike are concrete classes so they will be mapped to corresponding tables. These tables should include all the fields for classes annotated with @MappedSuperClass because they don’t have corresponding tables in database.
So, Truck table will have columns to store fields inherited from TransportationVehicle and Vehicle.
Similarly, Car and Bike will have columns to store fields inherited from PassengerVehicle and Vehicle.

Full example can be found here

Single Table Inheritance Strategy

The advantage of single table strategy is it does not require complex joins for retrieval and insertion of entities, but on the other hand it wastes database space as many columns need to be nullable and there isn’t any data for them.

Complete example and article can be found here

Relations between entities

Relations Between Entities Basics

A foreign key can be one or more columns that reference a unique key, usually the primary key, in another table.

A foreign key and the primary parent key it references must have the same number and type of fields.

Foreign keys represents relationships from a column or columns in one table to a column or columns in another table.

Joined Inheritance strategy

One to One mapping

One to Many relationship

Many To One Mapping

Many to Many Mapping

@TableGenerator and @GeneratedValue are used for automatic ID creation using jpa table generator.
@ManyToMany annotation specifies the relationship between Employee and Project entities.
@JoinTable specifies the name of the table to use as join table jpa many to many mapping between Employee and Project using name = “employee_project”. This is done because there is no way to determine the ownership of a jpa many to many mapping as the database tables do not contain foreign keys to reference to other table.
@JoinColumn specifies the name of column that will refer to the Entity to be considered as owner of the association while @inverseJoinColumn specifies the name of inverse side of relationship. (You can choose any side to be considered as owner. Just make sure those sides in relationship). In our case we have chosen Employee as the owner so @JoinColumn refers to idemployee column in join table employee_project and @InverseJoinColumn refers to idproject which is inverse side of jpa many to many mapping.
@ManyToMany annotation in Project entity shows inverse relationship hence it uses mappedBy=projects to refer to the field in Employee entity.

Full example can be referred here