Most Frequently asked hibernate Interview Questions (2024)

Question: What is the concept of HQL (Hibernate Query Language) in Hibernate?

Answer:

Hibernate Query Language (HQL) is a database-independent query language used in Hibernate for querying the database. HQL is similar to SQL (Structured Query Language) but it operates on objects rather than database tables, and it is designed to work seamlessly with the object-oriented nature of Hibernate. HQL allows you to write queries that interact with Java objects (entities) rather than database tables and columns directly, which helps in maintaining a layer of abstraction between the application and the database.

HQL is object-oriented, meaning it works with entities and their properties rather than database tables and columns. This enables developers to write queries using domain models (Java classes) instead of having to deal with SQL directly.

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Key Features of HQL:

1. Object-Oriented:

   • HQL operates on persistent objects and their properties, not on database tables and columns. It allows querying by the class names and object properties.

2. Database Independence:

   • Unlike SQL, which is tightly coupled with the underlying database’s syntax, HQL abstracts the database details and is database-independent. This allows you to switch between different databases (like MySQL, PostgreSQL, Oracle) without changing the query logic in your code.

3. Similar to SQL:

   • HQL syntax is quite similar to SQL, making it easy to learn for developers familiar with SQL. However, it differs in that HQL operates on entities, not on database tables directly.

4. Support for Aggregates:

   • HQL supports the use of aggregate functions (e.g., COUNT, SUM, AVG, MAX, MIN) to perform calculations on the result set.

5. Support for Joins:

   • HQL allows you to write join queries to fetch related entities, including support for inner joins, left joins, and fetch joins.

6. Dynamic Queries:

   • HQL can be used to create dynamic queries based on user input or other conditions. Hibernate provides tools like Criteria API and Query objects to help with building dynamic queries.

7. Type Safety:

   • HQL provides type safety because it operates on persistent objects. For example, queries return Java objects (entities), and it ensures the types are compatible with the mapped entity classes.

8. Support for Pagination:

   • HQL allows for pagination (fetching results in batches) through the setFirstResult() and setMaxResults() methods, which can be very useful for optimizing performance in large result sets.

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Basic Syntax of HQL:

1. Basic Query:

   • In HQL, you query entities using their class names, not table names.

   // Fetch all records from the Employee entity
   String hql = "FROM Employee";
   Query query = session.createQuery(hql);
   List<Employee> employees = query.list();

2. WHERE Clause:

   • You can apply conditions using the WHERE clause based on entity properties.

   // Fetch employees with a specific condition
   String hql = "FROM Employee e WHERE e.salary > :salary";
   Query query = session.createQuery(hql);
   query.setParameter("salary", 50000);
   List<Employee> employees = query.list();

3. JOIN Clause:

   • HQL supports joining related entities in the query.

   // Fetch employees along with their department (assuming Employee has a Department reference)
   String hql = "FROM Employee e JOIN e.department d WHERE d.name = :departmentName";
   Query query = session.createQuery(hql);
   query.setParameter("departmentName", "IT");
   List<Employee> employees = query.list();

4. SELECT Clause:

   • You can use SELECT to query specific properties of an entity or perform aggregate functions.

   // Fetch only the employee names
   String hql = "SELECT e.name FROM Employee e";
   Query query = session.createQuery(hql);
   List<String> employeeNames = query.list();

5. ORDER BY Clause:

   • You can use the ORDER BY clause to sort the results.

   // Fetch employees ordered by salary
   String hql = "FROM Employee e ORDER BY e.salary DESC";
   Query query = session.createQuery(hql);
   List<Employee> employees = query.list();

6. Pagination:

   • HQL supports pagination with setFirstResult() and setMaxResults() to limit the number of records fetched.

   // Fetch employees with pagination (first 10 records)
   String hql = "FROM Employee e";
   Query query = session.createQuery(hql);
   query.setFirstResult(0); // Start at the first record
   query.setMaxResults(10); // Limit to 10 records
   List<Employee> employees = query.list();

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Advantages of Using HQL:

1. Database Independence:

   • Since HQL abstracts away database-specific SQL syntax, it allows you to switch databases without changing the query logic. This can be especially useful in cross-platform applications or for migrating between databases.

2. Object-Oriented Queries:

   • With HQL, you can query domain model objects rather than directly dealing with database tables and columns. This leads to cleaner and more maintainable code since you are dealing with your application’s business objects directly.

3. Automatic Mapping:

   • HQL queries automatically map the results to Java objects. There’s no need to manually extract data from a result set, as Hibernate takes care of object-relational mapping (ORM).

4. Efficient Data Fetching:

   • HQL allows you to write efficient queries to fetch only the required data (e.g., specific fields or aggregates). You can also use fetch joins to eagerly load related entities, thus reducing the number of queries made to the database.

5. Readability:

   • HQL is more readable and maintainable than raw SQL queries because it uses Java object names rather than database table names. This keeps the query syntax aligned with the object-oriented design of the application.

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Differences Between HQL and SQL:

Feature	HQL	SQL
Target	Works with Java objects (entities)	Works with database tables
Syntax	Similar to SQL but works on Java classes	Standard SQL syntax
Database Dependency	Database-independent	Database-dependent
Joins	Joins are done using entity relationships (associative properties)	Joins are done using table relationships
Query Result	Returns entities (or selected fields from entities)	Returns result sets (rows)

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Example Use Cases for HQL:

1. Fetching All Data:

   • Retrieve all employees from the database:

   String hql = "FROM Employee";
   Query query = session.createQuery(hql);
   List<Employee> employees = query.list();

2. Conditional Query:

   • Fetch employees with a specific salary range:

   String hql = "FROM Employee e WHERE e.salary BETWEEN :minSalary AND :maxSalary";
   Query query = session.createQuery(hql);
   query.setParameter("minSalary", 50000);
   query.setParameter("maxSalary", 100000);
   List<Employee> employees = query.list();

3. Aggregate Query:

   • Get the total salary of all employees:

   String hql = "SELECT SUM(e.salary) FROM Employee e";
   Query query = session.createQuery(hql);
   Double totalSalary = (Double) query.uniqueResult();

4. Join Query:

   • Get employees with their department details:

   String hql = "FROM Employee e JOIN e.department d WHERE d.name = :departmentName";
   Query query = session.createQuery(hql);
   query.setParameter("departmentName", "HR");
   List<Employee> employees = query.list();

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Conclusion:

Hibernate Query Language (HQL) is a powerful and flexible query language that provides an object-oriented abstraction over traditional SQL, allowing you to interact with database records in terms of Java objects. HQL is easy to learn for developers familiar with SQL, and it significantly improves application portability by decoupling the query logic from the underlying database. By using HQL, you can efficiently query, retrieve, and manipulate data while adhering to Hibernate’s object-relational mapping principles.

Question: How does Hibernate handle lazy loading and eager loading?

Answer:

In Hibernate, lazy loading and eager loading are two techniques used to control how related entities are fetched from the database. These mechanisms help manage the performance and efficiency of database queries by determining when and how associated data is loaded.

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1. Lazy Loading:

Lazy loading is the default fetching strategy in Hibernate. It means that related entities are loaded only when they are accessed for the first time (on demand). Hibernate uses proxy objects to delay loading the associated data until it is actually needed.

Key Characteristics of Lazy Loading:

• On-Demand Loading: Related entities or collections are fetched only when their data is specifically accessed, not when the parent entity is loaded.
• Proxy Objects: Hibernate uses proxy objects (placeholders) to represent the related entities. These proxies are only initialized (loaded) when a property or method of the associated entity is accessed.
• Performance Optimization: Lazy loading reduces the initial load time of the parent entity by deferring the loading of related entities. This is especially useful for reducing unnecessary database queries when related data is not needed.

Example:

Consider a Student entity that has a List<Course> as a relation:

@Entity
public class Student {
    @Id
    private int id;

    private String name;

    @OneToMany(fetch = FetchType.LAZY)
    private List<Course> courses;
}

• In the above example, the courses list will not be loaded when the Student entity is retrieved. Instead, a proxy object will be created, and the actual data will only be fetched when you access student.getCourses().

Advantages of Lazy Loading:

• Reduced Memory Usage: Related entities are only loaded when needed, which reduces memory usage for large datasets.
• Performance: It avoids unnecessary database queries, improving performance when only certain parts of the entity graph are needed.

Disadvantages of Lazy Loading:

• N+1 Problem: If you load a list of entities and later access their related collections, you may unintentionally trigger multiple queries, one for each entity’s related collection (the “N+1 select problem”).
• LazyInitializationException: If the session that fetched the entity is closed (for example, outside a transaction), any attempt to access a lazily-loaded collection will throw a LazyInitializationException.

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2. Eager Loading:

Eager loading is the opposite of lazy loading. It means that related entities are immediately loaded when the parent entity is fetched, regardless of whether they are actually needed. This is useful when you know that the associated data will be required immediately after fetching the parent entity.

Key Characteristics of Eager Loading:

• Immediate Loading: All related entities or collections are loaded immediately when the parent entity is fetched, resulting in one or more additional queries at the time of loading.
• Fewer Proxies: Since data is fetched upfront, proxy objects are not used. The related entities are fully initialized at the time the parent is loaded.
• Performance Cost: Eager loading can lead to performance overhead because unnecessary data may be loaded even if it’s not needed. It can also lead to large result sets that could be inefficient.

Example:

Consider the same Student entity, but this time using eager loading:

@Entity
public class Student {
    @Id
    private int id;

    private String name;

    @OneToMany(fetch = FetchType.EAGER)
    private List<Course> courses;
}

• In this case, when you retrieve a Student entity, Hibernate will immediately fetch the courses collection from the database along with the Student entity.

Advantages of Eager Loading:

• Immediate Access: All related data is available immediately, so there’s no need for lazy initialization or additional queries when the data is accessed.
• Fewer Database Queries: It can reduce the number of database queries when you know that the related data will be used right away.

Disadvantages of Eager Loading:

• Performance Overhead: If the related entities are large and not needed immediately, eager loading may result in unnecessary database calls and excessive memory usage.
• Query Complexity: Fetching all related data eagerly can lead to very large, complex queries (especially for deep object graphs), which can impact performance.

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How Lazy and Eager Loading are Configured:

• @OneToMany, @ManyToOne, @OneToOne, @ManyToMany Annotations:
  • The fetch attribute can be used to define the fetching strategy.
    • FetchType.LAZY: Uses lazy loading.
    • FetchType.EAGER: Uses eager loading.

@OneToMany(fetch = FetchType.LAZY)
private List<Course> courses; // Lazy loading

@ManyToOne(fetch = FetchType.EAGER)
private Student student; // Eager loading

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Fetch Strategies in Hibernate:

1. Lazy Initialization:
   • Hibernate uses proxy objects to delay the initialization of related entities.
   • When you access the related entity, Hibernate issues a database query to fetch the data. This is often used in large-scale applications where related data might not always be required.
2. Eager Fetching with JOINs:
   • Eager loading often uses JOIN queries to fetch the parent and related entities in a single query.
   • For example, with @OneToMany(fetch = FetchType.EAGER), Hibernate might execute a JOIN between the parent and related tables.

   SELECT * FROM student s
   INNER JOIN course c ON s.id = c.student_id;

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Use Cases for Lazy and Eager Loading:

1. Use Lazy Loading When:

   • The related data is large, and you don’t need it all at once.
   • You want to minimize memory consumption and database queries by only loading data when it’s needed.
   • You can deal with potential LazyInitializationException (by managing session boundaries appropriately).

2. Use Eager Loading When:

   • You know you’ll need the related data immediately after loading the parent entity.
   • You want to avoid the N+1 query problem in cases where you are sure you’ll use all the related data.
   • You prefer to load all related data upfront to ensure you can work with fully initialized objects throughout your session.

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Choosing Between Lazy and Eager Loading:

• Performance Considerations: In many cases, lazy loading is preferred to avoid the overhead of loading unnecessary data. However, for specific use cases where related data is required immediately (e.g., rendering a web page with multiple associated entities), eager loading is more appropriate.

• Best Practices:

  • Lazy loading is typically used in large applications where related entities are not always needed.
  • Eager loading is often used when you’re dealing with smaller datasets or when you need to display the associated data in the same context (e.g., when fetching a product along with its details and reviews).

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Conclusion:

• Lazy Loading helps optimize performance by deferring the loading of associated data until it is explicitly needed, thus reducing the number of queries executed and minimizing memory usage.
• Eager Loading ensures all related entities are loaded immediately, which is useful when the associated data is essential for the operation but can increase query complexity and memory usage.

By understanding how and when to use lazy or eager loading, developers can make better decisions about how to fetch related data and optimize performance in their Hibernate-based applications.

Question: What are the advantages of using Hibernate’s Criteria API?

Answer:

Hibernate’s Criteria API is a powerful, object-oriented alternative to Hibernate’s native HQL (Hibernate Query Language) for querying the database. It allows you to build queries dynamically in a programmatic way, rather than writing static queries as strings. This approach provides several key advantages over using traditional HQL or SQL.

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Advantages of Using Hibernate’s Criteria API:

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1. Type-Safe Queries:

• The Criteria API allows for type-safe querying by using Java objects and classes. It ensures that the fields, properties, and relationships in the query are checked at compile time, which can significantly reduce runtime errors that are common with string-based queries (like HQL or SQL).
• The Criteria API utilizes Java’s reflection to generate queries, making it less prone to errors like spelling mistakes in field names or incorrect data types.

Example:

CriteriaBuilder builder = session.getCriteriaBuilder();
CriteriaQuery<Student> query = builder.createQuery(Student.class);
Root<Student> root = query.from(Student.class);
query.select(root).where(builder.equal(root.get("name"), "John Doe"));

In this example, the query is type-safe, and any errors will be detected at compile time.

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2. Dynamic Query Generation:

• One of the main advantages of the Criteria API is its ability to create dynamic queries. You can programmatically modify or construct the query depending on runtime conditions, such as filtering, ordering, and adding joins.
• This is particularly useful when the exact structure of the query is not known in advance and may vary based on user input or other dynamic factors.

Example:

CriteriaQuery<Student> query = builder.createQuery(Student.class);
Root<Student> root = query.from(Student.class);
List<Predicate> predicates = new ArrayList<>();

if (age != null) {
    predicates.add(builder.equal(root.get("age"), age));
}
if (name != null) {
    predicates.add(builder.like(root.get("name"), "%" + name + "%"));
}

query.select(root).where(predicates.toArray(new Predicate[0]));

This example shows how different conditions can be dynamically added based on the values of age and name.

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3. Improved Readability and Maintainability:

• Criteria API enables you to construct queries in a more structured and readable way, which can be easier to maintain in the long run, compared to writing complex HQL or SQL queries as strings.
• Queries are expressed using the object model (i.e., entity objects), which makes them more intuitive and closely aligned with the domain model.

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4. Support for Complex Queries:

• The Criteria API provides extensive support for creating complex queries involving joins, grouping, sorting, and aggregations in an easy-to-read, object-oriented manner.
• Unlike HQL or SQL, which can become unwieldy for complex queries with multiple joins, conditions, and groupings, the Criteria API can handle this efficiently and more intuitively.

Example:

CriteriaQuery<Object[]> query = builder.createQuery(Object[].class);
Root<Student> studentRoot = query.from(Student.class);
Join<Student, Course> courseJoin = studentRoot.join("courses");
query.multiselect(studentRoot.get("name"), builder.count(courseJoin))
     .groupBy(studentRoot.get("name"));

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5. Easier Refactoring and Reducing Hardcoding:

• Since the Criteria API relies on the entity model and the class structure, it helps avoid hardcoded strings in the queries. This makes it much easier to refactor and maintain, particularly in large applications with many entities and relationships.
• You don’t need to manually update queries in multiple places when the structure of the domain model changes (e.g., renaming fields or changing relationships), as the Criteria API automatically uses the entity’s metadata.

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6. No SQL Injection Risk:

• Since the Criteria API relies on parameterized queries and is generated through Java code, it helps protect against SQL injection attacks. Queries are not written directly as strings, which reduces the risk of malicious input altering the structure of the query.

Example:

CriteriaQuery<Student> query = builder.createQuery(Student.class);
Root<Student> root = query.from(Student.class);
query.select(root).where(builder.equal(root.get("name"), name));

In this example, the query is parameterized, so user input like name is automatically sanitized by the Criteria API.

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7. Integration with JPA (Java Persistence API):

• The Criteria API is part of the JPA specification (introduced in JPA 2.0), meaning it can be used across various JPA providers, not just Hibernate.
• If you’re using Hibernate as your JPA provider, it’s often more consistent to use the Criteria API as it works seamlessly with the rest of the JPA annotations and configuration.

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8. Improved Performance in Some Cases:

• In some cases, using the Criteria API can lead to improved performance, especially for queries that need to be built dynamically based on conditions that are only known at runtime.
• It can also enable the use of query caching, making certain types of complex queries more efficient.

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9. Support for Pagination and Sorting:

• The Criteria API allows you to easily apply pagination (using setFirstResult() and setMaxResults()) and sorting (using Order), which is useful for applications that display data in pages or need to support sorting by different attributes.
• This is especially useful when dealing with large datasets or building scalable applications that need to load data in chunks.

Example:

CriteriaQuery<Student> query = builder.createQuery(Student.class);
Root<Student> root = query.from(Student.class);
query.select(root).orderBy(builder.asc(root.get("name")));

TypedQuery<Student> typedQuery = session.createQuery(query);
typedQuery.setFirstResult(0);
typedQuery.setMaxResults(10);

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10. Compatibility with CriteriaBuilder:

• The CriteriaBuilder API allows for the construction of type-safe predicates and conditions. It offers a fluent, object-oriented interface for building complex conditions, thus avoiding the issues that often arise with string-based queries.
• With CriteriaBuilder, developers can construct conditions like equal, like, greaterThan, etc., in a type-safe manner.

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Conclusion:

The Criteria API provides several significant advantages over traditional HQL or SQL queries, including:

• Type-safety: Compile-time checking of field names and data types.
• Dynamic query creation: Build queries programmatically based on runtime conditions.
• Maintainability: Easier to maintain and refactor without worrying about string-based queries.
• Protection against SQL injection: Automatically parameterized queries.
• Complex queries: Efficient handling of joins, groupings, and aggregations.

Using the Criteria API in Hibernate promotes cleaner, more secure, and more maintainable code, especially in large or complex applications. However, for simpler queries, HQL might still be preferred due to its more straightforward syntax.

Question: What is the difference between SessionFactory and Session in Hibernate?

Answer:

In Hibernate, both SessionFactory and Session are key components used to interact with the database. However, they serve distinct purposes and are used at different stages of the application’s lifecycle. Here’s a breakdown of their differences:

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1. SessionFactory:

Definition:

• The SessionFactory is a thread-safe, immutable object responsible for creating Session objects in Hibernate.
• It is created once at the application startup and can be used throughout the application’s lifecycle to open multiple Session instances.

Key Characteristics:

• Singleton: The SessionFactory is typically created once per application (or per database). It is a singleton object and is expensive to create, so it should be reused throughout the application.
• Thread-Safe: SessionFactory is designed to be used by multiple threads concurrently. It is thread-safe, meaning it can be shared across threads.
• Heavy Object: It contains configuration details, metadata (such as mappings, entity configurations, and caching strategies), and connection pools.
• Costly Creation: Creating a SessionFactory is a relatively costly operation (in terms of memory and processing), so it is generally created once during application startup and reused.

Usage:

• The SessionFactory is used to configure Hibernate (via configuration files or annotations) and to create Session objects.

Example:

SessionFactory sessionFactory = new Configuration().configure().buildSessionFactory();

Key Methods:

• openSession(): Used to create a new Session object.
• getCurrentSession(): Returns the current session associated with the current thread (for environments using transaction management).

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2. Session:

Definition:

• The Session is a lightweight, non-thread-safe object that represents a single unit of work with the database in Hibernate.
• A Session provides an interface to interact with the database, perform CRUD (Create, Read, Update, Delete) operations, and manage the state of persistent objects.

Key Characteristics:

• Non-Thread-Safe: Unlike the SessionFactory, the Session is not thread-safe, meaning each thread should have its own Session instance.
• Transactional: A Session represents a transaction boundary and is used for managing the persistence context, which keeps track of entities and their state during the transaction.
• Short-Lived: A Session is typically opened for the duration of a single transaction or a single unit of work. Once the work is complete, the Session is closed.
• Caching: A Session includes a first-level cache (Session Cache) which stores entities loaded within the session. This helps to optimize performance by reducing the number of database queries.
• Lightweight: A Session is lightweight and inexpensive to create. It should be opened and closed as needed within a transaction or unit of work.

Usage:

• The Session is used to create, read, update, and delete entities, and also to manage the lifecycle of persistent objects during the scope of the session.

Example:

Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();

// Perform CRUD operations
Student student = session.get(Student.class, 1);

// Commit the transaction
tx.commit();
session.close();

Key Methods:

• save(), saveOrUpdate(), delete(), load(), get(): Used for performing CRUD operations on entities.
• beginTransaction(): Starts a new transaction.
• close(): Closes the session after use.
• createQuery(): Creates HQL or native SQL queries.

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Summary of Key Differences:

Aspect	SessionFactory	Session
Purpose	Factory for creating Session objects.	Represents a single unit of work with the database.
Creation	Created once, typically during application startup.	Created for each transaction or unit of work.
Thread-Safety	Thread-safe, can be shared among threads.	Not thread-safe, used by a single thread.
Lifecycle	Long-lived, created once per application.	Short-lived, created and closed per transaction.
Memory & Performance	More memory-intensive, should be reused.	Lightweight, created and discarded frequently.
Methods	openSession(), getCurrentSession().	save(), get(), createQuery(), close().
Scope	Global, typically a singleton in the application.	Local, created and closed for each transaction.
Cache	No caching directly, but can configure second-level cache.	Has first-level cache (session cache).

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When to Use SessionFactory vs. Session:

• SessionFactory:
  • Use it to configure and initialize Hibernate.
  • Create it once during the application startup and reuse it throughout the application’s lifecycle.
• Session:
  • Use it for any interaction with the database (CRUD operations, querying, transaction management).
  • Create it within the context of a transaction, and close it once the transaction is completed.

Conclusion:

• The SessionFactory is responsible for the creation of Session objects and serves as the main configuration point for Hibernate. It is a heavy, thread-safe object that should be created once and reused throughout the application.
• The Session is a lightweight, short-lived object used to interact with the database. It is not thread-safe and should be created and closed within the scope of a single transaction.

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