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Java, Spring Boot & Microservices Interview Questions

This comprehensive guide covers high-level architectural and technical questions frequently asked of Senior Java Developers.

1. Can you use HashMap in a multi-threaded environment?​

It depends on the scenario:

  • Read-only (CQRS Query Side): If your microservice only performs GET requests and the map is never modified after initialization, a standard HashMap is safe and offers the best performance.
  • Read-Write: In a typical web application where concurrent updates occur, HashMap will throw a ConcurrentModificationException. In this case, use ConcurrentHashMap. While slower than HashMap, it provides thread safety via lock stripping.

2. String Literal vs. new String()​

  • String Literal ("hello"): Created only in the String Constant Pool (SCP).
  • new String("hello"): Creates two objects: one in the Heap memory and one in the SCP (if it doesn't already exist).
  • Benefit: Literal assignment facilitates reusability and saves memory by allowing multiple references to point to the same object in the SCP.

3. Java 8 Features & Real-world Usage​

Commonly used features in enterprise projects include:

  • Stream API: For functional-style processing of collections.
  • Lambda Expressions: To provide concise implementations for functional interfaces used in Streams.
  • Optional: To prevent NullPointerException by explicitly handling the absence of values in CRUD operations (e.g., findById).
  • Default Methods: To add new functionality to interfaces without breaking existing implementing classes.

4. Spring Boot Starter Dependencies​

Spring Boot starters are Maven templates that group related dependencies and their transitive dependencies.

  • Problem Solved: Manually adding jars often leads to version conflicts and missing dependencies (e.g., Tomcat needs X, X needs Y).
  • Solution: Adding a single starter like spring-boot-starter-web brings in everything needed for a web app with verified, compatible versions.

5. What is Spring Boot Actuator?​

Actuator provides production-ready features to monitor and manage your application. It exposes HTTP endpoints to check:

  • /health: Application status.
  • /metrics: Performance data.
  • /env: Environment variables.
  • /beans: All registered Spring beans.

6. Profiles in Spring Boot​

Profiles allow you to segregate configuration for different environments (Dev, Test, Prod).

  • Configuration: Use application-dev.properties or YAML blocks separated by ---.
  • Activation: The recommended way is via a JVM parameter during deployment: -Dspring.profiles.active=prod.

7. API Documentation with Swagger​

Swagger (OpenAPI) is used to document the API Contract. It provides a UI where clients can see:

  • Available endpoints (GET, POST, etc.).
  • Required headers and request bodies.
  • Expected response structures and status codes.

8. Microservices: Monolithic to Microservices​

When decomposing a monolith, follow these principles:

  • Business Capability: Divide services based on domains (e.g., Order Service, User Service).
  • Single Responsibility: Each service should do one thing well.
  • Loose Coupling/High Cohesion: Services should be independent and bounded by context.

9. Fault Tolerance: Hystrix vs. Resilience4j​

To make microservices robust against failures:

  • Circuit Breaker: Prevents a service from constantly retrying a failing call, protecting system resources.
  • Fallback: Provides a user-friendly message (e.g., "Service temporarily unavailable") instead of a raw 500 error.
  • Current Trend: Resilience4j is preferred over the deprecated Hystrix because it is lightweight, modular, and designed for Java 8+.

10. Synchronous vs. Asynchronous Communication​

  • Synchronous: Client waits for a response (e.g., calling a Product service to get details before adding to a cart). Uses REST/OpenFeign.
  • Asynchronous: Client sends a message and moves on (e.g., Order service sends a message to an Email service). Uses message brokers like Kafka or RabbitMQ.

11. SQL Joins Summary​

Join TypeDescription
Inner JoinReturns only matching rows from both tables.
Left JoinAll rows from the left table + matching rows from the right.
Right JoinAll rows from the right table + matching rows from the left.
Full JoinAll rows from both tables (not natively supported in MySQL; use UNION of Left and Right).

12. Hibernate: First Level vs. Second Level Cache​

  • First Level: Associated with the Session. Always enabled. Destroyed when the session closes.
  • Second Level: Associated with the SessionFactory. Optional (disabled by default). Persists across multiple sessions until the application shuts down.