Object Relation Mapping(ORM) and Hibernate

1. Overview

Object Relation Mapping(ORM) provides a simple API for storing and retrieving Java objects directly to and from the database.

2. Features

• Non-intrusive: No need to follow specific rules or design patterns
• Transparent: Your object model is unaware

3. Advantages of ORM

3.1 Productivity

Eliminates lots of repetitive code – focus on business logic – Database schema is generated automatically

3.2 Maintainability

Fewer lines of code – easier to understand – Easier to manage change in the object model

3.3 Performance:

• Lazy loading – associations are fetched when needed
• Caching

3.4 Database vendor independence

The underlying database is abstracted away – Can be configured outside the application

4. ORM and Architecture

 

5. EventManager

5.1 Java Objects (POJO)

5.2 Hibernate mapping files

Tells Hibernate which tables and columns to use to load and store objects

5.2.1 Property mapping

5.2.2 Association mapping

5.2.3 Hibernate mapping types

Hibernate will translate Java types to SQL / database types for the properties of your mapped classes

Java Type	Hibernate Type	SQL Type
java.lang.String	string	VARCHAR
java.util.Date	date, time	DATE, TIME
java.lang.Integer, int	integer	INT
java.lang.Class	class	varchar
java.io.Serializable	serializable	BLOB, BINARY

5.3 Hibernate configuration

• Each database has a dialect – hibernate.dialect = org.hibernate.dialect.H2Dialect
• Must also specify: JDBC driver class, Connection URL, Username, Password

6. The SessionFactory interface

• Provide Session instances to the application
• Shared among application threads
• most important method is getCurrentSession

7. The Session Interface

• Obtained from a SessionFactory
• Main runtime interface between a Java application and Hibernate
• Responsible for storing and retrieving objects
• Think of it as a collection of loaded objects related to a single unit of work

Session session = sessionFactory.getCurrentSession();

8. Instance states

An object instance state is related to the persistence context.

The persistence context = a Hibernate Session instance

Three types of instance states:

8.1 Transient

The instance is not associated with any persistence context –

8.2 Persistent

The instance is associated with a persistence context –

8.3 Detached

The instance was associated with a persistence context which has been closed – currently not associated

9. Reference

https://kingbbode.tistory.com/27

https://www.uio.no/studier/emner/matnat/ifi/INF5750/h12/undervisningsmateriale/object-relational-mapping-and-hibernate.pdf