Causes and Solutions for LazyInitializationException in Hibernate

        In the previous post, we discussed the differences between the update() and merge() methods in Hibernate. In this post, we will discuss the causes of LazyInitializationException in Hibernate and explore the different approaches to resolve it.

       A LazyInitializationException occurs when the Hibernate Session has been closed and the entity becomes detached. If you then attempt to access a lazily loaded property or associated object of the detached entity, Hibernate throws the following exception:

org.hibernate.LazyInitializationException: could not initialize proxy - no Session

In other words, Hibernate does not support lazy initialization for detached entities. Accessing a lazily loaded association outside the context of an active Hibernate Session results in a LazyInitializationException because Hibernate can no longer retrieve the required data from the database.

The following example demonstrates this scenario:

Employee.java,

@Entity
@Table (name="employee")
public class Employee {
 
     @Id
     @Column(name="id")
     private int id;
 
     @Column(name="emp_name")
     private String name;
 
     @OneToMany(cascade=CascadeType.ALL, FetchType.LAZY) 
     @JoinColumn(name="address_id")
     private Set<Address> address;
       
        // setters and getters
}

         In the above example, Employee and Address entity have the One to Many relationship. The default fetch type of One to Many relationship is LAZY.

Address.java,

@Entity
@Table (name="address")
public class Address {
 
     @Id
     @Column(name="id")
     private int id;
 
     @Column(name="address")
     private String address;
       
        // setters and getters
}

The Hibernate class to get the details from entity as follows,

HibernateMain.java,

package com.adnblog;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import com.adnblog.Employee;

public class HibernateMain {

      public static void main(String[] args) {

            SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
            Session session = sessionFactory.openSession();
            Employee emp = session.get(Employee.class, 1); 

            session.close();

            emp.getId();                     // will throw LazyInitializationException                     
            sessionFactory.close();
      }
}

The above code throw the error:--
Exception in thread "main" org.hibernate.LazyInitializationException: could not initialize proxy - no Session


Solution:--

 Use FetchType.EAGER for the address association (mapped by the address_id foreign key) in the Employee entity. With FetchType.EAGER, Hibernate loads the associated Address entity along with the Employee entity when session.get() is called. As a result, even after the Hibernate Session is closed, you can access the Address details without encountering a LazyInitializationException.

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