Java Persistence API

What is Second Level Caching in JPA?

• Shares entries across transactions.
• Second level cache is available between multiple sessions.
• It is available at factory level.
• We use the provider like ehcache
• Example
• https://github.com/gauravmatta/springmvc/blob/acaa221780634c79e5c8273465ea0f7a474b1955/bean-lifecycle-web/src/main/java/com/springimplant/SecondLevelCacheExample.java

What is the difference between hibernate and JPA?

• JPA defines the specification. It is an API.
• Provides following definitions
• How we define the entities
• How do we define primary key?
• Who manages the entities?
• How we map attributes
• JPA stands for Java persistence API
• The JPA specification was developed by Java as a standard way to map objects to relational databases.
• JPA just provides specification It does not provide the implementation.
• So we cannot use JPA By itself, we need to use a JPA provider That implements that JPA framework Like eclipselink, toplink and hibernate.
• The advantage of using a framework that implements the JPA specification Is that you can easily switch the framework.
• So if we are using top link, we can easily change our framework to eclipse link or hibernate without having to change code.
• If we use JPA methods In our code, we will be easily able to switch to some other JPA provider Like eclipse link or top link.
• Spring Data JPA Is an abstraction layer on top of JPA To reduce the amount of boiler plate code required to implement data access object(DAO) layer.
• Spring data JPA Uses hibernate as a default JPA provider.
• Spring data JPA cannot work without a JPA provider.
• A JPA repository abstracts our database operations.
• Hibernate is an implementation of JPA

• Using hibernate directly would result in a lock into hibernate.
• There are other JPA implementation available like top link, et cetera.
• When we use hibernate in our code, we can use the proprietary methods Off hibernate, or we can use the JPA methods implemented by hibernate.
• Hibernate is a Java based ORM Tool that provides a framework for mapping application domain objects to Relational database tables, and vice versa.
• Hibernate is a JPA provider(JPA specification implementation) Spring data JPA is not a JPA provider. It simply hides the Java persistent API.(And the JPA provider) Behind its repository Abstraction.

What is the difference between @Entity and @Table annotations in JPA?

• We can use the name attribute of the @Entity annotation to provide an explicit entity name to match with the database table name. For our example we could have used @Entity(name = “tbl_address”) in our Address class if our table name was tbl_address.
• @Entity is needed for annotating a POJO class as an entity
• We can use a @Table (defined in the javax.persistence package) annotation just below the @Entity annotation and use its name attribute to specify the table name explicitly
• @Table is optional

Q What are the difference values of spring.jpa.hibernate.ddl-auto?

Ans 

• Create
• Drops and Creates the tables whenever we start the server
• Create-Drop
• Drops all the tables as soon as we stop the server.
• Recreates them fresh when we start the server
• update
• Only updates the database when we start the server.
• insert
• validate