Top Java Interview Questions with Scenario-Based Answers (Spring Boot + SQL)

This article covers 20 must-know Java and Spring Boot interview questions, including scenario-based questions, advanced SQL concepts, interface vs abstract class differences, important Spring Boot annotations, and hard-level questions frequently asked in real company interviews. These questions are based on practical development experience and commonly asked for backend and microservices roles.Spring Boot & Java Interview Questions

1. REST API returns old cached data. How do you fix it?

Answer:

• Check if response caching is enabled.
• Use proper cache keys.
• Add cache expiration (TTL).
• Use @CacheEvict to clear old cache.
• Verify CacheManager configuration.
• Check browser/API gateway cache also.

Example:

@CacheEvict(value = "users", allEntries = true)

2. Two microservices communicate using REST API. One service is slow. How do you debug it?

Answer:

• Check API response time.
• Analyze logs and server health.
• Verify database queries.
• Check load balancer configuration.
• Verify timeout settings.
• Analyze loops/heavy processing in code.
• Monitor CPU and memory usage.

3. One public API should not require authentication. How do you manage it?

Answer:

Use Spring Security configuration.

http.authorizeHttpRequests(auth -> auth
    .requestMatchers("/public/**").permitAll()
    .anyRequest().authenticated()
);

• Public APIs use permitAll()
• Protected APIs require authentication.

4. Application works locally but not on server. What do you check?

Answer:

• Database credentials
• Active Spring profile (prod)
• Environment variables
• Hardcoded paths
• SSL configuration
• Server logs
• Network/port access
• Dependency version mismatch

5. API is slow because of large data. How do you improve performance?

Answer:

• Use pagination
• Add database indexing
• Avoid unnecessary data fetch
• Use projections/DTOs
• Optimize SQL queries
• Enable caching if needed

Pagination example:

Pageable pageable = PageRequest.of(0, 10);

6. How do you secure configuration values?

Answer:

• Store configs in external files
• Use environment variables
• Encrypt sensitive data
• Never hardcode passwords
• Use Spring Cloud Config/Vault

7. Scheduled job runs multiple times in Spring Boot. Why?

Answer:

Possible reasons:

• Multiple application instances running
• Multiple schedulers enabled
• Cluster environment issue

Solution:

• Use distributed locking
• Configure single scheduler instance

Example:

@EnableScheduling

8. File upload API fails for large files. How do you fix it?

Answer:

Increase multipart file size limit.

spring.servlet.multipart.max-file-size=50MB
spring.servlet.multipart.max-request-size=50MB

• Check server upload limit
• Verify reverse proxy limits (Nginx/Apache)

9. After Spring Boot upgrade, custom bean stops working. Why?

Answer:

• Deprecated APIs removed
• Auto-configuration conflict
• Bean initialization changes
• Dependency incompatibility

Fix:

• Update deprecated code
• Check migration guide
• Use exclusions if required

Example:

@SpringBootApplication(exclude = SecurityAutoConfiguration.class

10. API returns empty response but data exists in DB. What do you check?

Answer:

• Verify query conditions
• Check mapping issues
• Validate request parameters
• Debug service layer
• Check transaction handling
• Verify serialization issues

11. How do you manage centralized configuration for multiple microservices?

Answer:

Use Spring Cloud Config Server.

Benefits:

• Centralized config management
• Shared configuration
• Environment-specific configs
• Dynamic refresh support

12. Two auto-configurations create the same bean. What happens?

Answer:

Spring throws:

NoUniqueBeanDefinitionException;

Fix:

• Use @Primary
• Use @Qualifier
• Disable one auto-configuration

Example:

@Primary
@Bean
public MyService myService() {
    return new MyService();
}

13. What happens if no active profile is set in Spring Boot?

Answer:

Spring Boot uses the default profile.

Example:

spring.profiles.active=prod

14. Two repositories use the same entity. How do you manage conflicts?

Answer:

Use:

• @Primary
• @Qualifier
• Separate repository configuration

15. Why is String immutable in Java?

Answer:

Reasons:

• Security
• Thread safety
• Memory optimization (String Pool)
• Hashcode caching

Example:

String s = "Java";
s.concat("Code");

Original string does not change.

16. How does HashMap work internally?

Answer:

• HashMap stores data as key-value pairs.
• Hashcode is calculated for the key.
• Bucket index is identified.
• Data is stored in buckets.
• Collision handling:
• LinkedList (Java 7)
• Balanced Tree (Java 8+)

Steps:

1. Calculate hash
2. Find bucket
3. Store/retrieve value

17. What is ConcurrentHashMap?

Answer:

ConcurrentHashMap is thread-safe.

Features:

• Supports concurrent read/write
• Better performance than Hashtable
• Uses segment locking/internal synchronization

Example:

ConcurrentHashMap<Integer, String> map = new ConcurrentHashMap<>();

18. What are Design Patterns in Java?

Answer:

Design patterns are reusable solutions for common software problems.

Common patterns:

• Singleton
• Factory
• Builder
• Strategy
• Observer

19. Difference between Lazy Singleton and Eager Singleton.

Lazy Singleton

Object created only when needed.

class Singleton {
    private static Singleton instance;

    private Singleton() {}

    public static Singleton getInstance() {
        if(instance == null) {
            instance = new Singleton();
        }
        return instance;
    }
}

Eager Singleton

Object created during class loading.

class Singleton {
    private static final Singleton instance = new Singleton();

    private Singleton() {}

    public static Singleton getInstance() {
        return instance;
    }
}

20. Why is Enum Singleton considered best?

Answer:

Because it:

• Is thread-safe
• Prevents serialization issues
• Prevents reflection attacks
• Easy to implement

Example:

public enum Singleton {
    INSTANCE;
}

21. What is an Immutable Class?

Answer:

An immutable class object cannot be modified after creation.

Rules:

• Make class final
• Fields should be private final
• No setter methods
• Initialize fields using constructor

Example:

final class Employee {
    private final String name;

    public Employee(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }
}