59 Hibernate Interview Questions: Ace Your Next Job!

What is Hibernate, and Why Should We Care?

Hibernate ORM, also known as just "Hibernate," is a lightweight, open-source object-relational mapping tool for the Java programming language. It provides a framework for mapping object-oriented domain models to relational databases.

Hibernate makes it easier to create objects independent of database software and develop persistence logic. In simple, non-technical terms, It is a free tool that facilitates interactions between your application and the database.

You can find out more about Hibernate and other popular Java applications here.

Let us have a quick look and the top hibernate interview questions asked and then will move on to the descriptive hibernate interview questions for freshers and experienced candidates.

Top Frequently Asked Hibernate Interview Questions

Here are the top ten hibernate interview questions that are most frequently asked. 

1. Why is Hibernate better than Java Database Connectivity (JDBC)?
2. What is a Session in Hibernate? 
3. List and describe the Hibernate framework’s essential interfaces.
4. What can you tell about the Hibernate Configuration File? 
5. What is an Entity in Hibernate?
6. Explain the concept of ORM (Object-Relational Mapping).
7. What’s the difference between Session and SessionFactory?
8. What is HQL?
9. Differentiate between get() and load() in Hibernate session. 
10. What is lazy loading?

Basic Level Hibernate Interview Questions

Let's start with the fundamentals of these hibernate interview questions. These first eight Hibernate interview questions for freshers that cover simple, entry-level basics. 

1. What is an ORM tool?

An Object Relational Mapping (ORM) tool simplifies data creation, manipulation, and access by internally using Java API to interact with databases. It's a technique for mapping objects stored in a database.

2. What does "lightweight" mean?

In the context of computers, "lightweight" describes an app, computer program, or device that doesn't use many system resources due to its small memory footprint (RAM) and low CPU usage.

3. What are the advantages of Hibernate?

Here's a list of Hibernate's many advantages:

• It's fast
• It's lightweight and open-source
• It reduces code length, removing boilerplate code and freeing up developers for other tasks
• It strengthens the object-level relationship
• It facilitates the generation of independent database queries
• It provides resources for creating tables automatically
• Integrating with other Java Enterprise Edition (EE) frameworks is easy.

4. Why is Hibernate better than Java Database Connectivity (JDBC)?

Hibernate outclasses JDBC because:

• Hibernate code is cleaner and more readable thanks to the elimination of boilerplate code, something found in JDBC
• Unlike JDBC API, Hibernate supports associations, collections, and inheritances
• HQL (Hibernate Query Language) is closer to Java and is more object-oriented
• Developers don't need to write code to store and load data into the database
• Hibernate enables faster application development

5. What is "persistence"?

In Java, persistence describes data and objects that last beyond the process used to create them.

6. Name some databases that Hibernate supports.

Hibernate supports databases like:

• DB2/NT
• FrontBase
• HSQL Database Engine
• Informix Dynamic Server
• Microsoft SQL Server Database
• MySQL
• Oracle
• PostgreSQL
• SQL Server
• Sybase

7. What is HQL?

HQL stands for Hibernate Query Language, a powerful object-oriented language independent of the database. It's like SQL, except it uses objects instead of table names. HQL is a simple, efficient, and flexible query language used to perform various operations on a relational database without complex database queries.

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8. Name the four ORM levels in Hibernate.

Hibernate's 4 ORM levels are:

• Full Object Mapping
• Light Object Mapping
• Medium Object Mapping
• Pure Relational

9. What is a Session in Hibernate? 

A Session in Hibernate is a lightweight, non-thread-safe object representing a single unit of work with the database. It is used to perform CRUD operations on persistent objects. 

10. What is a SessionFactory? 

A SessionFactory in Hibernate is a heavyweight and thread-safe object used to create and manage multiple sessions across an application. It is also configuring and managing the underlying connection pooling and caching mechanisms. 

11. What do you think about the "session being a thread-safe object" statement? 

The statement is incorrect. A Session in Hibernate is a lightweight, non-thread-safe object that should not be shared across multiple threads. 

12. What is the difference between first-level cache and second-level cache? 

The first-level cache is associated with a Session and stores the currently loaded objects in memory. The second-level cache is associated with a Session Factory and holds objects across multiple sessions, thereby reducing the number of database queries needed. 

13. What can you tell about the Hibernate Configuration File? 

The Hibernate Configuration File (hibernate.cfg.xml) is an XML file used to configure the basic settings of Hibernate, such as the database URL, username, password, and dialect. It also contains the mapping information of the persistent classes and the resources required for connection pooling. 

14. How do you create an immutable class in hibernate? 

To create an immutable class in Hibernate, you should mark all class properties as 'final' and provide only getter methods for them without any setters. Additionally, you can use the 'mutable' attribute of the 'property' or 'component' element in the Hibernate mapping file to make a property or component immutable. 

15. Can you explain the concept behind Hibernate Inheritance Mapping? 

Hibernate Inheritance Mapping represents the inheritance relationships between classes in a relational database. It allows the developer to map a single table to multiple classes using table per class hierarchy, table per subclass, and table per concrete class. 

16. Is hibernate prone to SQL injection attacks?

Hibernate, by itself, is not prone to SQL injection attacks. However, the application may be vulnerable to SQL injection attacks if user input is concatenated with the HQL or Criteria queries.

17. What is an Entity Hibernate?

In Hibernate, an entity is a very light, natural, persistent domain object that models a relational database table. Every instance of an entity represents one database table row, and the class usually holds properties mapping to columns in the table. In Hibernate, entities model real things in the world and their interrelation within a Java application.

The Hibernate entity may be defined by using the annotation '@Entity' in a Java class. Most of the time, the class will also contain a primary key field annotated with '@Id.' This signals Hibernate to treat that class as an entity—instances of this class can persist in the database.

Intermediate Level Hibernate Interview Questions

Let's push the difficulty level up a few notches with this set of eight moderately challenging Hibernate interview questions for 5 years experience or more.

18. Name Hibernate's five collection types used in one-to-many relationship mappings.

The five collection types are:

• Array
• Bag
• List
• Map
• Set

19. What is "dirty checking"?

The dirty checking feature helps developers and users avoid time-consuming write actions, reducing database write times. Dirty checking changes or updates only the fields that require action while keeping the rest untouched and unchanged.

20. What is Hibernate's default cache service?

Hibernate's default cache service is EHCache, though the framework additionally supports OSCache, SWARMCache, and TreeCache.

21. What is Light Object Mapping?

Light Object Mapping is one of the more valuable levels of ORM quality. This approach uses specific design patterns to hide the syntax from business logic. All entities are represented as classes and mapped manually. The Light Object Mapping approach works well with applications that have fewer entities and applications that use metadata-driven data models.

22. List and describe the Hibernate framework's essential interfaces.

Hibernate's necessary interfaces are:

• SessionFactory (org.hibernate.SessionFactory). SessionFactory is an immutable thread-safe cache of compiled mappings for a single database. After users initialize SessionFactory once, they can cache and reuse it. SessionFactory is designed to return the session objects for database operations.
• Session (org. hibernate. Session). A session is a single-threaded, short-lived object representing a dialogue between the persistent store and the application. It is the interface that exists between the Hibernate framework and the Java application code, providing methods for CRUD operations. A session should be opened only when required and closed as soon as the user finishes.
• Transaction (org.hibernate.transaction). The transaction is a single-threaded, short-lived object that the application uses to specify atomic units of work.

23. What is lazy loading?

Lazy loading is a technique in which objects are loaded as needed instead of an entire page, for example. This technique has become the default since Hibernate version 3.

24. What are the concurrency strategies?

Concurrency strategies are mediators responsible for storing and retrieving cached items. The software developer must decide which cache concurrency to implement for each persistent class and collection when enabling a second-level cache.

The concurrency strategies are:

• Nonstrict-Read-Write: This strategy works with data that can be altered and tolerates a small chance of stale data. It offers no guarantee of consistency between the database and the cache.
• Read-Only: This strategy works best with data that can't be changed and is only used to reference data.
• Transactional: This strategy is used primarily for read-only data in cases where it's essential to prevent stale data in concurrent transactions, such as in rare instances of an update.
• Read-Write: This strategy is like the transactional strategy.

25. Define Hibernate's validator framework.

Data validation is an integral part of any application and is used in the presentation layer when using JavaScript and server-side code before processing. Validation is a cross-cutting task that occurs before making it persistent to adhere to the correct format.

26. Explain the hibernate mapping file.

A hibernate mapping file is an XML file that defines the relationship between a Java class and a database table. It specifies the mapping of the class properties to the table columns and describes any additional connections between the class and other classes or tables. 

27. What are the most commonly used annotations available to support hibernate mapping? 

The most commonly used annotations for hibernate mapping are @Entity, @Table, @Column, @Id, @GeneratedValue, @ManyToOne, and @OneToMany. 

28. Explain Hibernate architecture.

The Hibernate architecture consists of several components: the Session Factory, Session, Transaction, and Query. The Session Factory is responsible for creating and managing sessions, interacting with the database, and executing queries.

29. Can you tell the difference between the getCurrentSession and openSession methods? 

The getCurrentSession method creates or retrieves the current Session from the current session context. In contrast, the open Session begins with a new session that is not bound to the current context. 

30. Differentiate between save() and saveOrUpdate() methods in hibernate Session. 

The save() method persists a new object in the database. In contrast, the saveOrUpdate() method continues a new object or updates an existing object in the database. 

31. Differentiate between get() and load() in Hibernate session.

The get() method retrieves an object from the database by its primary key and throws an exception if it is not found. The load() method is also used to retrieve an object from the database by its primary key, but it returns a proxy object if it is not found. 

32. What are the criteria for API in hibernate? 

The criteria API in hibernate is a programmatic way of creating and executing queries. It allows developers to build complex queries using a fluent interface rather than writing raw SQL.  

33. Can you tell me something about one too many associations and how we can use them in Hibernate? 

A one-to-many association occurs when one entity is associated with multiple other entities. In Hibernate, one-to-many associations can be implemented using the @OneToMany annotation, and the @JoinColumn annotation is used to specify the column that will be used to join the two entities.

34. What are Many to Many associations? 

A many-to-many association occurs when multiple entities are associated with various other entities. In Hibernate, many-to-many associations can be implemented using the @ManyToMany annotation, and a join table is used to store the relationship between the two entities. 

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35. What does a Session.Lock () method in hibernate do?

The SessionSession.lock() method is used to acquire a lock on an object in the current Session. This is useful for preventing concurrent updates to the same thing. 

36. What is hibernate caching? 

Hibernate caching refers to storing data in memory so that it can be retrieved quickly without hitting the database again. This improves performance and reduces the load on the database.

37. What are the different types of Hibernate Caching?

There are two types of hibernate caching: first-level caching and second-level caching. First-level caching is enabled by default and is associated with the Session object. Second-level caching is optional and is associated with the SessionFactory object. 

38. When is the merge() method for the hibernate Session proper? 

The hibernate Session's merge() method is proper when you want to update an existing object in the database without reattaching it to the Session. 

39. Collection mapping can be done using One-to-One and Many-to-One Associations. What do you think? 

Yes, collection mapping can be done using both One-to-One and Many-to-One associations. A one-to-one association is used when one object is associated with another object, while a Many-to-One association is used when one object is related to multiple other objects. 

40. What is the difference between setMaxResults() and setFetchSize() for Query?

The setMaxResults() method limits the number of results a query returns, while the setFetchSize() method controls the number of rows retrieved from the database at a time. setMaxResults() limits the total number of results returned, while setFetchSize() controls the number of rows retrieved at a time to avoid memory issues. 

41. Does Hibernate support Native SQL Queries? 

Yes, Hibernate supports Native SQL Queries, which allow you to use SQL statements directly to interact with the database. This can be useful when you want to perform complex, impossible queries with HQL or Criteria API.

42. Explain the concept of ORM (Object-Relational Mapping).

Object-relational mapping (ORM) is a programming method that first emerged to allow programmers to interact with a relational database using an object-oriented approach. ORM represents database tables as classes in the targeted programming language, where each row of the table represents an instance of that class. Correspondingly, the columns of the table represent the class's fields or properties.

ORM frameworks, such as Hibernate, make the conversion process between the relational database schema and the object-oriented code automatic. It empowers the developer to work with objects in their code, relegating the handling of a great bulk of database operations such as SQL generation, data retrieval, transactions, and more to the ORM tool running behind the scenes.

This enables you to deal primarily with the objects inside the ORM framework, which automatically maps them to the underlying database instead of devising SQL queries to retrieve data. This leads to less work with data access, as time is spent preparing boilerplate code, so there is less code to keep in sync.

In summary, ORMs help interact between the object-oriented world of developing applications and the real-world act of keeping states.

Now that we have learned some of the intermediate-level Hibernate interview questions let's increase the difficulty level and look at some of the advanced-level Hibernate interview questions and answers.

Advanced Level Hibernate Interview Questions

We round out the Hibernate interview questions with eight expert questions.

43. What design patterns does the Hibernate framework use?

Some design patterns include:

• Data Mapper, which moves data between objects and a database, keeping them independent of each other and the mapper
• Domain Model Pattern, which is a domain object model that incorporates both behavior and data
• Proxy Pattern, for lazy loading
• Factory pattern in SessionFactory

44. What is Hibernate tuning?

The process of Hibernate tuning is designed to optimize Hibernate applications' performance. The three strategies are:

• SQL Optimization
• Session Management
• Data Caching

45. Name the states in which a persistent entity exists.

Persistent entities exist in only three states:

• Transient
• Persistent
• Detached

46. How can you view the Hibernate-generated SQL on a console?

To enable viewing SQL on a console for debugging purposes, you must add the following in the Hibernate configuration file:

1 <property name="show_sql">true</property>

47. What's the difference between Session and SessionFactory?

A Session provides the first-level cache and is a single-threaded, short-lived object. A Session Factory provides the second-level cache, which is immutable and shared by all Sessions. It lives until Hibernate is running.

48. How many ways can an object be fetched from Hibernate's database?

There are four ways to fetch objects from Hibernate's database:

• Criteria API
• HQL
• The identifier
• Standard SQL

49. How many ways can you turn off Hibernate's second-level cache?

There are three ways to turn off the cache:

• By setting hibernate. Cache. use_second_level_cache property to false
• By using CACHEMODE.IGNORE
• Using a cache provider such as org.hibernate.cache.NoCacheProvider

50. Describe the differences between Hibernate's transient, persistent, and detached states.

Here is how the states differ:

• Transient. This state describes new objects created in Java but not associated with a Hibernate session.
• Persistent. This state describes objects associated with a Hibernate session.
• Detached. This state describes a formerly Persistent object associated with a Hibernate session. Developers can reattach the object to a Hibernate session using either update() or saveOrUpdate().

51. What happens when the no-args constructor is absent in the Entity bean?  

If the no-args constructor is absent in the Entity bean, Hibernate cannot instantiate the object and throw an exception. Therefore, it is always recommended to have a no-args constructor in the Entity bean for Hibernate to work correctly.

52. Can we declare the Entity class final? 

No, the Entity class cannot be declared final, as Hibernate uses runtime proxies to enhance the class for persistence. Therefore, a final class cannot be subclassed and thus cannot be proxied. 

53. Explain Query Cache. 

The Query Cache is a second-level cache that stores a query's results so that they can be reused later. It improves performance by avoiding multiple database hits for the same Query. 

54. Can you say something about the N+1 SELECT problem in Hibernate? 

The N+1 SELECT problem in Hibernate refers to a single SELECT statement being executed to retrieve the parent object. Then, N SELECT statements are executed to retrieve the child objects, resulting in poor performance. 

55. How to solve the N+1 SELECT problem in Hibernate? 

The N+1 SELECT problem can be solved using fetching strategies such as eager loading, lazy loading, or batch fetching. Another solution is to use the JOIN FETCH clause in your HQL or JPQL Query. 

56. What is a Single Table Strategy? 

The Single Table Strategy maps multiple classes that inherit from a single root class to a single database table. Each row in the table represents an instance of one of the classes in the hierarchy. 

57. Can you say something about the Table Per Class Strategy? 

The Table Per Class Strategy maps classes in an inheritance hierarchy to separate tables in the database. Each class in the hierarchy has its table, and the data for that class is stored in the corresponding table. 

58. Can you say something about Named SQL Query? 

Named SQL Query is a feature in Hibernate that allows you to define and reuse a named query throughout your application. It is described in the mapping file or by using the @NamedQuery annotation. 

59. What are the benefits of NamedQuery? 

The benefits of NamedQuery include the following:

• Improved code maintainability and readability.
• Reduced code duplication.
• Ability to easily change the Query without affecting its code.

It also improves performance when the same Query is used multiple times by caching the query results. 

We hope you would have got a better understanding of hibernate interview questions that are frequently asked and it would help you during the interview process.

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