If you’re at a job interview, the last thing you want is to get stumped on a question. It’s especially frustrating if you’re well-versed on the subject in question, but you could have benefitted from a refresher beforehand.

That’s why it’s a sound strategy to review some old material and brush up on the more common interview questions. This way, you won’t get caught off-guard and possibly tarnish that crucial first impression.

If you’re applying for a Hibernate-related position, then you’ve come to the right place. Here are two dozen of the most popular interview questions, broken down by difficulty level. You will find entry-level information, intermediate concepts, and Hibernate interview questions for experienced applicants.

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Let’s start off with the Hibernate interview questions by taking a brief look at Hibernate.

What is Hibernate, and Why Should We Care?

Hibernate ORM, also known as just “Hibernate,” is a lightweight, open-source object-relational mapping tool used in the Java programming language, providing a framework that maps object-oriented domain models to relational databases.

Hibernate makes it easier to create objects independent of database software and develop persistence logic. Put in simple, non-technical terms, Hibernate 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 now begin with the basic or the entry-level Hibernate interview questions and answers.

Basic Level Hibernate Interview Questions

Let’s start with the fundamentals in these hibernate interview questions. These first eight Hibernate-related questions cover simple, entry-level basics.

1. What is an ORM tool?

An Object Relational Mapping (ORM) tool helps to simplify data creation, manipulation, and access by internally using Java API to interact with the databases. It’s a technique that maps 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, 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
  • It’s easy to integrate with other Java Enterprise Edition (EE) frameworks.

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 boiler-plate 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 the context of 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 that it uses objects instead of table names. HQL is a very simple, efficient, and flexible query language used to do various operations on a relational database without the need for complex database queries.

8. Name the four ORM levels in Hibernate.

Hibernate’s four 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 statement - "session being a thread-safe object"? 

The statement is incorrect. A Session in Hibernate is a lightweight and non-thread-safe object and 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 is used to store the currently loaded objects in memory. The second level cache is associated with a SessionFactory and is used to hold 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 is used to represent the inheritance relationships between classes in a relational database. It allows the developer to map a single table to multiple classes using techniques like 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.

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 and answers.

17. Name Hibernate’s five collection types used in one-to-many relationship mappings.

The five collection types are:

  • Array
  • Bag
  • List
  • Map
  • Set

18. What is “dirty checking”?

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

19. What is Hibernate’s default cache service?

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

20. 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.

21. List and describe the Hibernate framework’s essential interfaces.

Hibernate’s important interfaces are:

  • SessionFactory (org.hibernate.SessionFactory). SessionFactory is an immutable thread-safe cache of compiled mappings meant 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 that represents 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, then closed as soon as the user is finished.
  • Transaction (org.hibernate.transaction). The transaction is a single-threaded, short-lived object that the application uses to specify atomic units of work.

22. What is lazy loading?

Lazy loading is a technique where objects are loaded as needed, instead of an entire page, for example. This technique became default since Hibernate version 3.

23. What are the concurrency strategies?

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

The concurrency strategies are:

  • Non-strict-Read-Write: This strategy works with data that can be altered and can tolerate a small chance of stale data. This strategy 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 consequently, is only used to reference data.
  • Transactional: This strategy is used primarily for read-mostly data in cases where it’s essential to prevent stale data in concurrent transactions, in those rare instances of an update.
  • Read-Write: This strategy is like the transactional strategy.

24. 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 so that it adheres to the correct format.

25. Explain 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. 

26. 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. 

27. Explain Hibernate architecture 

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

28. 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 process begins a new session that is not bound to the current context. 

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

The save() method is used to persist a new object in the database. In contrast, the saveOrUpdate() method is used to continue a new object or update an existing object in the database. 

30. 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 the object 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. 

31. 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.  

32. 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.

33. 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. 

34. What does 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. 

35. What is hibernate caching? 

Hibernate caching refers to storing data in memory to retrieve it quickly without hitting the database again. This improves performance and reduces the load on the database.

36. 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. 

37. When is the merge() method of the hibernate Session useful? 

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

38. 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. One-to-One association is used when one object is associated with one other object. At the same time, the Many-to-One association is used when one object is associated with multiple other objects. 

39. Can you tell the difference between setMaxResults() and setFetchSize() of Query?

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

40. 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 queries that are impossible with HQL or Criteria API.

Now that we have learned some of the intermediate level Hibernate interview questions, let us next increase the difficulty level and look into 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.

41. 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

42. 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

43. Name the states that a persistent entity exists in.

Persistent entities exist in only three states:

  • Transient
  • Persistent
  • Detached

44. 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>

45. What’s the difference between Session and SessionFactory?

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

46. 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

47. How many ways can you disable Hibernate’s second-level cache?

There are three ways to disable the cache:

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

48. Describe the differences between Hibernate’s transient, persistent, and detached states.

Here is how the states differ:

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

49. 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 properly.

50. 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. 

51. Explain Query Cache 

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

52. 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. 

53. 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. 

54. What is a Single Table Strategy? 

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

55. 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. 

56. Can you say something about Named SQL Query? 

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

57. 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.

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