Java Backend Developer Interview Questions and Answers

1. What is JasperReports and how is it used in Java applications?

JasperReports is an open-source reporting engine written in Java. It's widely used to generate dynamic content like PDFs, Excel files, or HTML reports directly from Java applications. It relies on JRXML templates (XML-based report designs), which define the layout, fields, parameters, and data sources. These templates are compiled to .jasper files and filled with data at runtime.

Use cases:

Generating invoices
Exporting database query results into PDF
Creating charts, dashboards, and business reports

It integrates well with Spring Boot using libraries like jasperreports and spring-boot-starter-data-jpa.

2. What is the compatibility matrix between Java versions and Spring Boot versions?

Spring Boot versions are tightly coupled with specific Java versions due to dependency and bytecode compatibility.

Spring Boot Version	Supported Java Versions
2.3 to 2.7	Java 8 to Java 17
3.x	Java 17 and above only

Spring Boot 3 and onwards use Jakarta EE 9+, so all javax.* imports have changed to jakarta.*.

3. How can a Spring Boot application be deployed on AWS?

Several options exist:

Elastic Beanstalk – Upload the .jar, AWS handles provisioning and scaling.
EC2 – Manually install Java and run the .jar on a virtual machine.
ECS or EKS – Containerize the app and deploy via ECS or Kubernetes.
Lambda – Deploy as a function using AWS Serverless Java container.

You can also use RDS for managed databases and S3 for static assets.

4. What is the difference between a Docker image and a Spring Boot JAR file?

A Spring Boot JAR is a standalone Java executable requiring a JVM.
A Docker image includes the JAR, JVM, OS, and dependencies — it's portable and environment-independent.

Docker is better suited for CI/CD and cloud deployments due to its consistency.

5. Should you run a database inside a Docker container? What are the pros and cons?

Yes, for development and testing. No, or cautiously in production.

Pros:

Easy to set up
Consistent across environments
Useful in CI pipelines

Cons:

Risk of data loss
Backup/restore challenges
Lower performance
Not ideal for clustered DBs

6. What are intermediate and terminal operations in Java Streams?

Intermediate operations: Lazy, return a stream (map, filter, sorted, distinct) Terminal operations: Eager, trigger processing (collect, forEach, reduce, count)

List<String> names = List.of("Tom", "Jerry", "Anna", "Tom");
List<String> result = names.stream()
    .filter(name -> name.length() > 3)
    .distinct()
    .collect(Collectors.toList());

7. What are the different repository interfaces in Spring Data JPA?

CrudRepository<T, ID> – Basic CRUD
PagingAndSortingRepository<T, ID> – Adds pagination/sorting
JpaRepository<T, ID> – Adds JPA features like flush, deleteInBatch

Prefer JpaRepository for full capabilities.

8. What are ACID properties in databases?

Atomicity – All or nothing
Consistency – Valid state before/after transaction
Isolation – Concurrent transactions don’t interfere
Durability – Persisted changes survive crashes

Essential for transactional reliability.

9. How can you improve the performance of a slow SQL query?

Use indexes
Avoid SELECT *
Optimize joins
Analyze using EXPLAIN
Avoid nested subqueries
Cache frequent queries

10. What is a database view? What are its types?

A view is a virtual table from a SQL query.

Types:

Simple View: Single table
Complex View: Joins/aggregations
Materialized View: Physically stored query results

Useful for abstraction and reusability.

11. What is indexing in a database and what are its benefits?

Indexing creates a sorted data structure (e.g., B-Tree) for quick lookups.

Benefits:

Faster queries
Efficient filtering, sorting
Better JOIN performance

12. When can indexing hurt performance instead of improving it?

Write-heavy operations (INSERT/UPDATE/DELETE)
Index on low-cardinality fields
Too many indexes increase storage and overhead

13. How to find the frequency of numbers in a list and sort by count?

List<Integer> list = List.of(1, 2, 1, 3, 3, 3, 2, 2);
Map<Integer, Long> freq = list.stream()
    .collect(Collectors.groupingBy(n -> n, Collectors.counting()));

freq.entrySet().stream()
    .sorted(Map.Entry.<Integer, Long>comparingByValue().reversed())
    .forEach(e -> System.out.println(e.getKey() + ": " + e.getValue()));

14. What is the purpose of the @Transactional annotation in Spring?

Marks a method or class to be executed within a transaction. Supports rollback on exceptions, propagation control, and isolation levels.

Maintains data integrity by ensuring consistent execution.

15. What is the difference between @Component, @Service, and @Repository?

@Component: Generic bean
@Service: Business logic layer
@Repository: Data access layer; adds exception translation

All are picked up by component scanning.

16. What is the N+1 problem in JPA and how can it be avoided?

Occurs when fetching a list of entities also loads each related entity separately.

Avoid using:

@EntityGraph
JOIN FETCH
DTO projections

17. How does Spring Boot auto-configure your application?

Uses @EnableAutoConfiguration to scan the classpath and automatically register beans.

Example: If H2 and Spring Data JPA are on the classpath, Spring auto-configures a datasource and repository support.

18. What is the difference between eager and lazy loading in JPA?

Eager: Fetch related entities immediately
Lazy: Load on access

Prefer lazy to avoid unnecessary DB hits.

19. How do you implement pagination in Spring Data JPA?

Use Pageable interface:

Page<Employee> page = repository.findAll(PageRequest.of(0, 10, Sort.by("name")));

Provides total elements, pages, and content.

20. How do you secure a Spring Boot application?

Use Spring Security
JWT-based authentication
Method-level protection (@PreAuthorize)
HTTPS enforcement

Supports OAuth2, LDAP, and basic auth as well.