JPA - Java Persistance API. we want to take java objects data and store it in DB (tables).

JPA provides a way of mapping from POJO to the database tables. we can use SQL statements/ Criteria API, JPQL to query the database.

Important Annotations - @Entity, @Table(name = "tablename"), @Id @GenerateValue, @Column, @OneToOne, @OneToMany, @ManyToMany.

Useful Notes:

1. JPA is an interface/specification - Hibernate(ORM tool) is an implementation of interface.
2. EntityManager is used to manage all the Entities. The EntityManager API is used to access a database in a particular unit of work. It is used to create and remove persistent entity instances, to find entities by their primary key identity, and to query over all entities.
3. There are many useful methods that we can make use of in EntityManger like persist etc.. More info click here..
4. Transaction management can be managed by @Transactional can be used on class level or method level.
5. add spring.jpa.show-sql= true, spring.h2.console.enabled=true
6. url for h2 - http://localhost:8080/h2-console, use db url jdbc:h2:mem:testdb.
7. Lets look whats happening in background - springboot auto configration is doing all the work. as we are using in memory database h2. spring boot initializes the schema. HibernateJpaAutoConfiguration , DataSourceAutoConfiguration, EntityManager, TransactionManager is auto configured.

SPRING DATA JPA provides JpaRespository<EntityClassName, Long> and lot of important methods.