Top Hibernate Interview Questions and Answers

Hibernate is an open-source, object-relational mapping (ORM) framework that simplifies database access in Java applications by mapping Java objects to database tables. 

Here are top Hibernate interview questions,

1. What is Hibernate and why is it used?

Hibernate is an open-source, object-relational mapping (ORM) framework that simplifies database access in Java applications by mapping Java objects to database tables. It provides a way to store, retrieve, and manage Java objects in a relational database.

It simplifies database access by providing a layer of abstraction that hides the complexity of SQL and JDBC.

 

2. Explain the key features of Hibernate.

Hibernate features:

- Object-Relational Mapping (ORM)

- Automatic Table Generation

- Transparent Persistence

- Caching

- Lazy Loading

- HQL (Hibernate Query Language)

- Second-level Cache

 

3. What is the SessionFactory in Hibernate?

`SessionFactory` is a factory class that creates `Session` instances in Hibernate. It's a thread-safe and immutable cache of compiled mappings for a single database. It is costly to create, so typically, only one instance is created per application.

 

4. Explain the Session in Hibernate.

`Session` is a short-lived, lightweight object that represents a single-threaded unit of work within Hibernate. It provides methods to perform CRUD (Create, Read, Update, Delete) operations on the objects mapped to the database.

 

5. What is the difference between save(), persist(), and saveOrUpdate() methods in Hibernate?

- `save()` and `persist()` methods are used to save an object to the database. `save()` returns the generated identifier, while `persist()` doesn't.

- `saveOrUpdate()` updates an existing object or saves a new one if it doesn't exist.

 

6. Explain the Hibernate caching mechanisms.

Hibernate provides two levels of caching:

- First-level Cache: Session-level cache for entities, managed automatically by Hibernate within a session.

- Second-level Cache: Application-level cache shared across sessions, enhancing performance by caching queries and entities.

 

7. What is lazy loading in Hibernate?

Lazy loading is a technique where the associated objects of a class are not loaded unless they are explicitly accessed. It helps improve performance by reducing unnecessary database access.

 

8. Explain the different mapping types in Hibernate.

Hibernate supports various mapping types, including:

- Primitive types

- Collection types (lists, sets, maps, arrays)

- Component types

- Associations (one-to-one, many-to-one, many-to-many)

 

9. What is the difference between FetchType.LAZY and FetchType.EAGER in Hibernate?

- `FetchType.LAZY` loads the associated objects when they are needed (on-demand), improving performance.

- `FetchType.EAGER` loads the associated objects immediately with the main object, potentially causing performance issues.

 

10. What are the different ways to perform mapping in Hibernate?

Hibernate supports mapping through:

- XML configuration files

- Annotation-based mapping

- Java-based mapping (using Java Persistence API annotations)

 

11. Explain the difference between transient, persistent, and detached objects in Hibernate.

- Transient objects are not associated with any Hibernate session.

- Persistent objects are associated with a Hibernate session and managed by it.

- Detached objects were once associated with a session but are no longer, although they retain their state.

 

12. What is a detached object in Hibernate?

A detached object is an object that was once associated with a Hibernate session but has been disconnected from the session. It is no longer managed by Hibernate, but its state can be re-attached to another session.

 

13. What is HQL (Hibernate Query Language)?

HQL is an object-oriented query language in Hibernate used to perform database operations on entities. It is similar to SQL but operates on the objects and properties of Java classes.

 

14. Explain the concept of transactions in Hibernate.

Transactions in Hibernate are units of work that are atomic and consistent. A transaction comprises multiple database operations that either succeed as a whole or fail entirely. Hibernate uses the `Transaction` interface to manage transactions.

 

15. What is a named query in Hibernate?

A named query is a pre-defined query that is declared in the Hibernate mapping files or annotated with `@NamedQuery` in the entity class. It allows you to reuse queries by a name instead of writing them in the code.

 

16. Explain the concept of cascading in Hibernate.

Cascading in Hibernate allows operations (e.g., save, delete) to be cascaded from a parent entity to its associated entities. For example, if a parent is saved, its associated entities can be automatically saved as well.

 

17. What is a composite key in Hibernate?

A composite key, also known as a composite primary key, is a key composed of multiple columns in a database table. In Hibernate, you can map a composite key using `@Embeddable` and `@EmbeddedId` annotations.

 

18. What is a Hibernate interceptor?

A Hibernate interceptor allows you to intercept and modify events (e.g., save, update, delete) in Hibernate. You can use it to customize the behavior of Hibernate before or after certain operations.

 

19. Explain the concept of dirty checking in Hibernate.

Dirty checking is a feature in Hibernate that automatically tracks changes made to the persistent objects and synchronizes these changes with the database during the transaction commit. It helps reduce manual update calls.

 

20. What are the differences between Hibernate and JDBC?

- Hibernate is an ORM framework, while JDBC is a low-level API for interacting with databases.

- Hibernate abstracts database-related tasks and provides higher-level object-oriented APIs, while JDBC involves writing SQL queries and handling result sets directly.

Above are few top Hibernate interview questions. Remember to prepare and expand on these answers.

Good luck with your interview! 👍