The Top 20 Hibernate Interview Questions for Java Developers

Hibernate is one of the most popular Java frameworks for mapping Java objects to database tables. As an ORM (Object-Relational Mapping) tool, Hibernate simplifies database operations by abstracting away complex SQL queries behind a simple and elegant API.

With over 10 years of active development and a strong community, Hibernate has become an essential skill for Java developers working with relational databases This means you can expect Hibernate interview questions in just about any Java developer interview

In this guide, I’ll share the top 20 Hibernate interview questions Java developers should prepare for. Knowing the concepts behind these questions will help you demonstrate your practical Hibernate skills during interviews and land you your dream Java job!

Core Hibernate Interview Questions

Q1: What is Hibernate?

Hibernate is a Java framework that simplifies database programming by mapping application domain model objects to database tables using configuration files. It handles object-relational mapping, database connection management, and performs CRUD operations without writing complex SQL queries.

Q2 What is ORM and how does Hibernate implement it?

ORM (Object-Relational Mapping) is a technique to map Java classes to database tables. Hibernate implements ORM by providing APIs to perform CRUD operations on objects, which then reflect in the database.

For example, we can get a Hibernate Session and call session.save(obj) to persist an object. Hibernate handles converting the object to corresponding rows in SQL tables behind the scenes.

Q3: What are the key interfaces in the Hibernate framework?

The key interfaces in Hibernate are:

• Configuration: Used to configure Hibernate settings like database connection details.
• SessionFactory: Creates Session instances and caches mappings. Only one instance is required per database.
• Session: Main runtime interface to perform CRUD operations. Wraps JDBC connection. Not thread safe.
• Transaction: Used to group operations into transactions. Abstracts JDBC transactions.
• Query: Used to create and execute HQL/SQL queries.

Q4: What is the role of SessionFactory and Session in Hibernate?

• SessionFactory is a heavyweight object that is initialized only once per database. It caches mappings, manages connections, and builds Session instances.

• Session is a lightweight interface used at runtime to perform CRUD operations. It is not thread safe and acts as a wrapper to the underlying JDBC connection. We get it from the SessionFactory.

Q5: How does Hibernate handle object state?

Hibernate defines the following object states:

• Transient: Object is never persisted and not associated with any session.
• Persistent: Object is associated with an open session and persisted in the database.
• Detached: Object was persistent but session was closed. Still contains database state.
• Removed: Object was deleted from the database using session operations.

Hibernate tracks state changes using these states and syncs the object graph to the database.

Q6: What is the Hibernate architecture?

The Hibernate architecture consists of the following key components:

• SessionFactory: Heavyweight singleton object for creating sessions.
• Session: Main interface for performing CRUD operations. Not thread safe.
• Transaction: Used to group operations into transactions.
• Connection Provider: Interface to provide JDBC connections. Default is DataSource.
• Transaction Factory: Creates Transaction objects for Session. Default is JDBC Connection transaction.
• Dialect: Translates Hibernate calls to database specific calls. E.g. MySQLDialect.

Advanced Hibernate Questions

Q7: How does Hibernate generate primary keys?

Hibernate can generate primary keys in the following ways:

• identity: Primary key is generated by the database. Hibernate gets the generated id after insert.
• sequence: Hibernate uses a sequence object to generate unique ids.
• table: Ids are generated using an id table managed by Hibernate.
• assigned: Application manually assigns ids before adding objects.

We can specify the key generation strategy using @GeneratedValue annotation on the property.

Q8: What is the Hibernate session cache?

Hibernate uses session caching to minimize database hits for repeated read operations on the same object graph. The first level cache is associated with the Session while the second level cache is associated with the SessionFactory.

• First level cache exists per Session instance. Hibernate tries to minimize DB hits by caching immutable objects, entities, and collections in the session cache.

• Second level cache is configurable and exists per SessionFactory. It can be used to cache objects across sessions. But it is not enabled by default.

Q9: How is caching handled in a clustered environment?

In a clustered environment, multiple instances may access the same database. The second level cache can result in stale data in this scenario.

Hibernate provides integration with various distributed caching solutions like Ehcache, Infinispan, and Hazelcast which provide coherent and transactional caches for clustered environments.

Q10: What is lazy loading in Hibernate?

Lazy loading is a design pattern to defer initialization of object properties until needed. In Hibernate, lazy loading can be used to avoid unnecessary fetching of child collections and associations.

By default, Hibernate uses lazy loading for collections like lists, maps, and sets. So they won’t be initialized when the parent object is loaded. Hibernate will load the collection when it is accessed.

Q11: How can lazy loading be enabled in Hibernate?

Lazy loading does not require any special configuration in Hibernate. It works out of the box for collections like sets, lists, maps. We can also enable lazy loading for entity associations by setting the fetch attribute to LAZY or EXTRA_LAZY.

@ManyToOne(fetch = FetchType.LAZY)Customer customer; 

This ensures Hibernate will not load the Customer object when the parent is loaded.

Q12: What is N+1 query issue in Hibernate?

N+1 query issue refers to a situation where Hibernate ends up making multiple 1 query to load each collection/association when loading N parent objects.

For example, if we load 50 Order objects, Hibernate might make 1 query to fetch orders and then 50 queries to fetch the customers for each order.

We can avoid N+1 queries by using joins, batch fetching, and tweaking of fetch strategies.

Q13: How can the N+1 query problem be solved?

Some ways to solve the N+1 query problem include:

• Using JPA and Hibernate joined/fetch joins to load associations in a single query
• Enabling Hibernate batch fetching of collections and associations
• Loading collections eagerly instead of lazy loading
• Using DTO projections and queries instead of entities
• Caching associations after first load

Q14: What is the difference between get() and load() method?

• get() hits the database immediately and returns the entity instance. If not found, it returns null.

• load() returns a proxy without hitting database immediately. Database is hit only when methods are called on the proxy. Throws exception if record not found.

Q15: What are the different inheritance mapping strategies in Hibernate?

Hibernate supports the following inheritance mapping strategies:

• SINGLE_TABLE: All classes share one table with a discriminator column
• TABLE_PER_CLASS: Each class has its own table. No polymorphism.
• JOINED: Base class table is joined with subclass tables.
• SUBCLASS: Base class and subclass tables are joined. Duplicate columns.

We can choose the strategy using the @Inheritance annotation.

Q16: What are the different types of Hibernate mappings?

Hibernate supports following types of mappings between Java classes and database tables:

• Many-to-one: Maps a many valued association with one-sided foreign key. E.g. @ManyToOne
• One-to-many: Maps a one valued association with a foreign key on many side. E.g. @OneToMany
• Many-to-many: Maps associations between entities using a join table with foreign keys. E.g. @ManyToMany
• One-to-one: Maps two entities to share the same primary key value. E.g. @OneToOne

These annotations help Hibernate map relationships efficiently.

Q17: What is Hibernate Validator?

Hibernate Validator is a validation framework that provides declarative validation constraints for entity attributes. Some common constraints are:

• @NotNull – Cannot be null
• @Size – String length between min and max
• @Email – Must be a valid email format
• @Pattern – Must match the regex pattern

We can cascade validation from root to related entities automatically.

Hibernate Query Language Questions

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