Hibernate First Level Cache with Example

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In this post, we will understand the Hibernate first-level cache with an example.

Important points

First level cache retrieval example

In this example, we are fetching student object from the database using a hibernate session. we will retrieve it multiple times and will observe the SQL logs to see the differences.
package net.javaguides.hibernate.tutorial;

import org.hibernate.Session;
import org.hibernate.Transaction;

import net.javaguides.hibernate.tutorial.config.HibernateJavaConfig;
import net.javaguides.hibernate.tutorial.entity.Student;

public class FirstLevelCacheDemo {

    public static void main(String[] args) {

        Transaction transaction = null;
        try (Session session = HibernateJavaConfig.getSessionfactory().openSession()) {

            // start the transaction
            transaction = session.beginTransaction();

            // get the student entity using id
            Student student1 = session.load(Student.class, new Long(1));

            System.out.println(student1.getFirstName());
            System.out.println(student1.getLastName());
            System.out.println(student1.getEmail());

            // load student entity by id
            Student student2 = session.load(Student.class, new Long(1));
            System.out.println(student2.getFirstName());
            System.out.println(student2.getLastName());
            System.out.println(student2.getEmail());

            // commit transaction
            transaction.commit();

        } catch (Exception e) {
            if (transaction != null) {
                transaction.rollback();
            }
        }
    }
}
Output:
Hibernate: select student0_.id as id1_0_0_, student0_.email as email2_0_0_, student0_.first_name as first_na3_0_0_, student0_.last_name as last_nam4_0_0_ from student student0_ where student0_.id=?
Prabhas
Fadatare
prabhas@gmail.com
Prabhas
Fadatare
prabhas@gmail.com
As you can see the second “session.load()” statement does not execute the select query again and loads the student entity directly.

First level cache retrieval example with a new session

With the new session, an entity is fetched from the database again irrespective of whether it is already present in any other session in the application.
package net.javaguides.hibernate.tutorial;

import java.io.Serializable;

import org.hibernate.Session;
import org.hibernate.Transaction;

import net.javaguides.hibernate.tutorial.config.HibernateJavaConfig;
import net.javaguides.hibernate.tutorial.entity.Student;

public class FirstLevelCacheDemo1 {

    public static void main(String[] args) {

        Transaction transaction = null;
        try (Session session = HibernateJavaConfig.getSessionfactory().openSession(); Session session1 = HibernateJavaConfig.getSessionfactory().openSession()) {

            // start the transaction
            transaction = session.beginTransaction();

            // get the student entity using id
            Student student1 = session.load(Student.class, new Long(1));

            System.out.println(student1.getFirstName());
            System.out.println(student1.getLastName());
            System.out.println(student1.getEmail());

            // load student entity by id
            Student student2 = session.load(Student.class, new Long(1));
            System.out.println(student2.getFirstName());
            System.out.println(student2.getLastName());
            System.out.println(student2.getEmail());

            // load student entity by id
            Student student3 = session1.load(Student.class, new Long(1));
            System.out.println(student3.getFirstName());
            System.out.println(student3.getLastName());
            System.out.println(student3.getEmail());

            // commit transaction
            transaction.commit();

        } catch (Exception e) {
            if (transaction != null) {
                transaction.rollback();
            }
        }
    }
}
Output:
Hibernate: select student0_.id as id1_0_0_, student0_.email as email2_0_0_, student0_.first_name as first_na3_0_0_, student0_.last_name as last_nam4_0_0_ from student student0_ where student0_.id=?
Prabhas
Fadatare
prabhas@gmail.com
Prabhas
Fadatare
prabhas@gmail.com
Hibernate: select student0_.id as id1_0_0_, student0_.email as email2_0_0_, student0_.first_name as first_na3_0_0_, student0_.last_name as last_nam4_0_0_ from student student0_ where student0_.id=?
Prabhas
Fadatare
prabhas@gmail.com
You can see that even if the student entity was stored in the “session” object, still another database query was executed when we use another session object “session1”.

Removing cache objects from first level cache example

Though we can not disable the first-level cache in hibernate, we can certainly remove some of the objects from it when needed. This is done using two methods :
  • Session.evict()
  • Session.clear() 
Here Session.evict() is used to remove a particular object from the cache associated with a session, and a clear() method is used to remove all cached objects associated with a session. So they are essentially like remove one and remove all.
package net.javaguides.hibernate.tutorial;

import org.hibernate.Session;
import org.hibernate.Transaction;

import net.javaguides.hibernate.tutorial.config.HibernateJavaConfig;
import net.javaguides.hibernate.tutorial.entity.Student;

public class RemoveFirstCacheDemo {

    public static void main(String[] args) {

        Transaction transaction = null;
        try (Session session = HibernateJavaConfig.getSessionfactory().openSession()) {

            // start the transaction
            transaction = session.beginTransaction();

            // get the student entity using id
            Student student1 = session.load(Student.class, new Long(1));

            System.out.println(student1.getFirstName());
            System.out.println(student1.getLastName());
            System.out.println(student1.getEmail());

            // load student entity by id
            Student student2 = session.load(Student.class, new Long(1));
            System.out.println(student2.getFirstName());
            System.out.println(student2.getLastName());
            System.out.println(student2.getEmail());

            //session.evict(student2);
            session.clear();
            // load student entity by id
            Student student3 = session.load(Student.class, new Long(1));
            System.out.println(student3.getFirstName());
            System.out.println(student3.getLastName());
            System.out.println(student3.getEmail());

            // commit transaction
            transaction.commit();

        } catch (Exception e) {
            if (transaction != null) {
                transaction.rollback();
            }
        }
    }
}
Clearly, Session.evict() method removed the student object from the cache so that it was fetched again from the database.

Conclusion

In this article, we understood how the Hibernate first-level cache works with an example. Learn the complete hibernate framework at https://www.javaguides.net/p/hibernate-tutorial.htm

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