Unable to evaluate the expression method threw ‘org.hibernate.lazyinitializationexception’ exception.

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Unable to Evaluate the Expression Method Threw ‘org.hibernate.LazyInitializationException’ Exception

The org.hibernate.LazyInitializationException is a common exception in Hibernate when accessing a lazy-loaded entity or collection outside the session. This exception typically occurs when trying to access uninitialized properties of an entity that was loaded lazily.

Let’s understand this exception with an example. Suppose we have two entities: Order and Customer. The Order entity contains a reference to the Customer entity, which is a lazy-loaded association: 

    
  public class Order {
      // other properties
      
      @ManyToOne(fetch = FetchType.LAZY) // lazy-loaded association
      private Customer customer;
      
      // getters and setters
  }

  public class Customer {
      // other properties
      
      // getters and setters
  }

Now, let’s say we retrieve an Order entity from the database: 

    
  Order order = entityManager.find(Order.class, orderId);
  
  // Accessing the customer outside the session
  Customer customer = order.getCustomer();

In the above code, when we call order.getCustomer(), Hibernate tries to fetch the Customer entity from the database. However, since the association is lazy-loaded, Hibernate raises a LazyInitializationException because the session is closed and it cannot retrieve the lazily-loaded entity.

To fix this issue, you have a few options depending on your requirements:

  1. Eager Loading: Change the fetch type to FetchType.EAGER in the @ManyToOne annotation. This will fetch the customer entity immediately along with the order. However, be cautious as it may cause unnecessary fetching of related entities.
  2. Open a New Session: If you want to access the lazily-loaded entity, you can open a new Hibernate session and retrieve the entity within that session. This can be done using getCurrentSession() method (if using Hibernate’s built-in session management) or opening a new session using the session factory.
  3. Use DTO or Projection: Instead of working with the entities directly outside the session, you can create Data Transfer Objects (DTOs) or projections that contain only the required data. This way, you can fetch the necessary data from the database within the session and work with them afterwards.

These are just a few possible solutions to handle org.hibernate.LazyInitializationException. The most appropriate solution would depend on your specific use case and performance considerations.

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