Trouble Persisting Entities with Hibernate in Java: NullPointerException on Session Save

Question

I'm encountering an issue while trying to persist entities using Hibernate in my Java application. I have a straightforward setup with entity classes annotated with JPA annotations and a Hibernate session factory configured. However, when I attempt to save an entity using the session's save() method, I consistently encounter a NullPointerException. Here's a simplified version of my code

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

public class Main {
    private static SessionFactory factory;

    public static void main(String[] args) {
        try {
            factory = new Configuration().configure().buildSessionFactory();
        } catch (Throwable ex) {
            System.err.println("Failed to create session factory: " + ex);
            throw new ExceptionInInitializerError(ex);
        }

        Session session = factory.openSession();
        session.beginTransaction();

        // Create and persist a new entity
        MyEntity entity = new MyEntity();
        entity.setName("Test Entity");
        session.save(entity); // NullPointerException occurs here

        session.getTransaction().commit();
        session.close();
    }
}

And here's the entity class MyEntity

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

@Entity
public class MyEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
}

I've double-checked my Hibernate configuration and entity mappings, but I can't seem to figure out why this NullPointerException is happening. Any insights or suggestions would be greatly appreciated.

Answer 1

It seems like you're encountering a common issue with Hibernate session management. The NullPointerException you're facing typically occurs when trying to interact with a session that hasn't been properly initialized or has been closed prematurely.

In your case, the issue arises because you're attempting to use the session after it has been closed. When you call session.close() after committing the transaction, the session is effectively terminated, and any subsequent operations on it will result in a NullPointerException.

To fix this, you need to ensure that you perform all necessary operations within the scope of an open session. One way to achieve this is by moving the transactional block inside a try-with-resources statement, which automatically closes the session when done:

public static void main(String[] args) {
try (Session session = factory.openSession()) {
    session.beginTransaction();

    MyEntity entity = new MyEntity();
    entity.setName("Test Entity");
    session.save(entity);

    session.getTransaction().commit();
} catch (Exception ex) {

}

By encapsulating the session within the try-with-resources block, you ensure that it is properly closed when no longer needed, preventing any potential NullPointerExceptions. Make sure to handle any exceptions appropriately within the catch block.