hibernate.md

Hibernate

• Object Relational Mapping (ORM)
  • Map Java classes to Relational Database tables
• Abstracts away JDBC
• No more sql (which is database specific)
• Hibernate generates the correct sql syntax based on the Hibernate Dialect

Hibernate Exceptions

• HibernateException is the base Throwable type for Hibernate
• It extends RuntimeException
• Hibernate wraps all JDBC exceptions (SQLException)

Configuration

• Main configuration in hibernate.cfg.xml
• Mapping configuration either in xml or annotations

hibernate.cfg.xml

• username
• password
• url
• driver
• dialect
• where the mapping configuration is
• and much more...

example

<hibernate-configuration>
    <session-factory>

      <property name="hibernate.connection.username">taylor</property>
      <property name="hibernate.connection.password">p4ssw0rd</property>
      <property name="hibernate.connection.url">jdbc:oracle:thin:@<public-dns-name>:1521:orcl</property>
	    <property name="hibernate.connection.driver_class">oracle.jdbc.driver.OracleDriver</property>
      <property name="hibernate.dialect">org.hibernate.dialect.Oracle10gDialect</property>

      <property name="hibernate.show_sql">true</property>

      <mapping class="com.example.domain.FlashCard"/>    

    </session-factory>
</hibernate-configuration>

Mapping Configuration

Assume we are mapping a java class FlashCard to a database table FLASH_CARD

XML config

• We obviously need the java class and the database table
• The mapping configuration is done in a separate xml file called a Hibernate Mapping File (HBM)
• So for class FlashCard, we would have an hbm called FlashCard.hbm.xml

Annotation config

• We can do the mapping right inside the FlashCard class
• Hibernate follows a standard called Java Persistence API (JPA) and both have their own annotations
• By convention, we use JPA annotations

example

package com.example.domain;

import javax.persistence.*;

@Entity
@Table(name="flash_card")
public class FlashCard {

	@Id
	@Column(name="fc_id")
	private Integer id;

	@Column(name="fc_question")
	private String question;

	@Column(name="fc_answer")
	private String answer;


  // no arg constructor

  // getters and setters

}

Important Interfaces

• Configuration
  • this is a class
• SessionFactory
• Session
• Transaction
• Query
• Criteria

Configuration

• Used to create a SessionFactory
• Specify location of hibernate.cfg.xml

SessionFactory sessionFactory = new Configuration().configure("hibernate.cfg.xml").buildSessionFactory();

SessionFactory

• Used to create Session objects
• Only need one

Session

• Not to be confused with HttpSession
• Represents your session with a database
• methods
  • save
  • get
  • update
  • delete
  • beginTransaction
  • createQuery
  • createCriteria

Session session = sessionFactory.openSession();

//stuff

session.close();

Transaction

• In Hibernate, autoCommit is false by default
  • In JDBC, it was true by default
• This means, you must begin and commit a transaction if you modify anything

Transaction tx = session.beginTransaction();

//stuff

tx.commit();

Query

• Can do more complicated CRUD than the Session methods
• Use Hibernate Query Language (HQL)
  • It's like SQL, but database agnostic

retrieve

Query query = session.createQuery("from BankUser where username = :usernameVar");
query.setString("usernameVar", username);
List<BankUser> bankUsers = query.list();

modify

String hql = "update BankUser set password=:pwdVar where username=:usrVar";

Query query = session.createQuery(hql);
query.setString("usrVar", username);
query.setString("pwdVar", password);

int numAffected = query.executeUpdate();

Criteria

• Can only retrieve from database
• Cannot modify anything
• It's nice because you can create complicated queries

Criteria criteria = session.createCriteria(BankUser.class);

// add as many Restrictions as you want

List<BankUser> bankUsers = criteria.list();