Hibernate ORM

• Persistence 
• The state of an object can be saved to  a data store, and recreated at a later point in time. It can be persisted to a file or a db.
• ORM
• Object Relational mapping
• It refers to the technique of mapping the representation of data from Java Objects to Relational Database (and vice versa).
• Hibernate
• Object Relational mapping solution for Java
• Framework for mapping Object model to Relational databases
• It maps Java data types to Database specific data types
• Generates sql queries automatically and hence, reduces the development time
• Database independent - can switch to different DB by editing single configuration file
• Communication happens between Java -> Hibernate -> DB, and vice versa
• Setup Steps
• 1. Download hybernate orm zip from http://hibernate.org/
• 2. Create lib directory in the project, copy jars from downloaded zip lib folders -> required, jpa, java8 directories to project lib folder in eclipse
• 3. Copy jdbc driver into project lib folder
• 4. Add jar to class path
    project properties -> build path -> libraries -> add jars
• Hibernate configuration
• 1. Add hibernate configuration file to src directory hibernate.cfg.xml
  <!DOCTYPE hibernate-configuration PUBLIC
          "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
          "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
  
  <hibernate-configuration>
      <session-factory>
          <!-- JDBC Database connection settings -->
          <property name="connection.driver_class">oracle.jdbc.driver.OracleDriver</property>
          <property name="connection.url">jdbc:oracle:thin:@(DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=SLC03RYC.us.oracle.com)(PORT=1521)) (CONNECT_DATA=(SERVER=DEDICATED)(SERVICE_NAME=xe)))</property>
          <property name="connection.username">vdesu</property>
          <property name="connection.password">vdesu</property>
  
          <!-- JDBC connection pool settings ... using built-in test pool -->
          <property name="connection.pool_size">1</property>
  
          <!-- Select our SQL dialect -->
          <property name="dialect">org.hibernate.dialect.OracleDialect</property>
  
          <!-- Echo the SQL to stdout -->
          <property name="show_sql">true</property>
  
          <!-- Set the current session context -->
          <property name="current_session_context_class">thread</property>
  
      </session-factory>
  </hibernate-configuration>
• Annotate java class(mapping class to db table).  Entity class - Java class that is mapped to a database table
• Option 1: XML config file (legacy)
• Option 2: Java annotations (modern, preferred)
• @Entity
  @Table(name="student")
•     @Id
      @Column(name="id")
•     @Column(name="first_name")
•  Develop java code to perform database operations
• package com.luv2code.hibernate.demo;
  
  import org.hibernate.Session;
  import org.hibernate.SessionFactory;
  import org.hibernate.cfg.Configuration;
  
  import com.luv2code.hibernate.demo.entity.Student;
  
  public class CreateStudentDemo {
  
      public static void main(String[] args) {
         
          SessionFactory factory = new Configuration()
                                  .configure("hibernate.cfg.xml")
                                  .addAnnotatedClass(Student.class)
                                  .buildSessionFactory();
         
          Session session = factory.getCurrentSession();
  
        try{
          Student tempStudent = new Student("venkat","desu","venkat.desu@gmail.com");
           session.beginTransaction();
          session.save(tempStudent);
          session.getTransaction().commit();
          System.out.println("done");
      } finally {
          factory.close();
      }
         
  
      }
  
  }
• Execute the program.
• Retrieve a java object with hibernate
• Student mystudent=session.get(Student.class, <ID value>);
• Retrieve all Students using Hibernate Query Language (HQL)
• List<Student> theStudents = session.createQuery("from Student").list();
• List<Student> theStudents = session.createQuery("from Student s where s.lastName='desu'").list();
•  List<Student> theStudents = session.createQuery("from Student s where s.lastName='desu' OR s.firstName='venkat'").list();
• List<Student> theStudents = session.createQuery("from Student s where s.lastName like 'desu%'").list(); 
• Update Object
• Student mystudent=session.get(Student.class, <ID value>);
  mystudent.setFirstName("asdf");
• Bulk Update
•  session.createQuery("update Student set lastName='desu'").executeUpdate();
• Delete Object
• Student mystudent=session.get(Student.class, <ID value>);
  session.delete(mystudent);
• Bulk Delete
•  session.createQuery("delete from Student where id=0").executeUpdate();

Hibernate Query Language:

• HQL is an Object Oriented Query Language
• Unlike SQL, HQL uses/operates on Classes instead of Tables
• Advantages
• Database independent - Easy migration from one DB to another
• Easy for Java programmers - Quick to learn and implement
• Creating Tables with Primary and foreign key

Hibernate Object States & LifeCycle:

• Transient 
• It is just a POJO Object eg: Student Object
• Persistent -> Transient Object have association with table. Starts once Transaction is started. The state ends with Commit()
• session.save()
• session.persist()
• session.saveOrUpdate() 
• session.load()
• session.get()
• session.byId()
• session.byNaturalId()
• Detached - starts with one of the following methods
• session.evict() - evicts from session
• session.clear()
• session.close() 
• We can move from detached to Persistent using update(), merge(), saveOrUpdate()
• It not used then it will be removed by garbage collector
• Removed
• session.delete() 
• It will be available for garbage collection and cannot be used

JPA vs Hibernate(Entity Manger vs Session)