prgrmmng.com Logo prgrmmng

Java and Microservices Interview Questions and Answers (Top 25+)

Illustration for Java and Microservices Interview Questions and Answers (Top 25+)
By Last updated:
Spring BootJavaRest ApiKafkaThreadsOauth2InterviewMicroservices

📺 Watch the video on YouTube

1. Explain the architecture of microservices and its pros and cons. What are the components in a microservice?

Architecture: Microservices architecture is a design approach where an application is composed of loosely coupled, independently deployable services. Each service focuses on a specific business capability and communicates over standard protocols (usually HTTP/REST or messaging).

Core Components:

  • Service Registry (e.g., Eureka): Keeps track of service instances.
  • API Gateway: Entry point for client requests.
  • Configuration Server: Manages service configurations centrally.
  • Database per Service: Each microservice manages its own data.
  • Communication Mechanism: REST, gRPC, messaging queues.
  • Monitoring & Logging: Tools like Prometheus, Grafana, ELK stack.
  • CI/CD Pipeline: Automated testing and deployment tools.

Pros:

  • Independent deployment and scaling
  • Fault isolation
  • Technology diversity
  • Better alignment with agile and DevOps

Cons:

  • Increased complexity
  • Difficult distributed debugging
  • Higher operational overhead
  • Data consistency and transaction management issues

2. What is an API Gateway?

An API Gateway is a server that acts as a single entry point into a microservices system. It handles requests by routing them to the appropriate service, aggregating results, and enforcing cross-cutting concerns like authentication, logging, rate limiting, and CORS.

Popular Tools: Netflix Zuul, Spring Cloud Gateway, Kong, NGINX.

3. How to communicate between two microservices?

Options:

  • Synchronous: RESTful APIs using HTTP (client blocks until a response).
  • Asynchronous: Messaging queues like RabbitMQ, Kafka.

4. Synchronous and Asynchronous Way of Communication

  • Synchronous:

    • Immediate request/response
    • Tight coupling
    • Example: REST APIs over HTTP

  • Asynchronous:

    • Message-based communication
    • Loose coupling
    • Example: Kafka, RabbitMQ

5. What is a Messaging Queue?

A messaging queue is a form of asynchronous service-to-service communication used in microservices architecture. Producers send messages to a queue, and consumers process them independently.

Examples: RabbitMQ, Apache Kafka, ActiveMQ

6. How to secure a microservice?

  • Use HTTPS
  • OAuth2 and JWT for authentication and authorization
  • API gateway for request filtering
  • Role-based access control (RBAC)
  • Secure service-to-service communication (e.g., mTLS)
  • Input validation and sanitization

7. What is OAuth?

OAuth (Open Authorization) is an open standard protocol for token-based authorization. It allows a third-party application to access a user's resources without exposing credentials.

8. What is a Token? Explain various tokens in Java OAuth

A token is a digitally signed piece of data that represents the user's authentication and authorization information.

Types:

  • Access Token: Grants access to protected resources.
  • Refresh Token: Used to obtain a new access token.
  • ID Token (OIDC): Contains user profile info.

In Java:

  • Libraries like Spring Security OAuth2 or Keycloak handle token creation and validation.

9. What is a Stream?

Streams in Java (introduced in Java 8) are a new abstraction for processing sequences of data in a functional style.

Key operations:

  • map(), filter(), reduce(), collect()
  • Lazy evaluation and pipelining
  • Parallel processing support

10. How is memory allocation done in Java 8?

Java 8 uses heap and non-heap memory:

  • Heap: Divided into Young and Old Generation
  • Non-Heap: Metaspace (replaces PermGen)

Memory allocation strategies are handled by the JVM's garbage collectors like G1, CMS.

11. What is Metaspace?

Metaspace is a memory area introduced in Java 8 that replaces PermGen. It stores class metadata and can grow dynamically based on the system's memory.

12. What are Functional Interfaces?

A functional interface is an interface with a single abstract method (SAM), used primarily in lambda expressions.

Examples:

  • Runnable
  • Callable
  • Comparator
  • Custom interfaces with @FunctionalInterface

13. What are Threads in Java?

A thread is a lightweight process and the smallest unit of CPU execution in Java. It allows concurrent execution of tasks.

14. How many ways can we create a thread in Java?

  1. Extending Thread class
  2. Implementing Runnable interface
  3. Using ExecutorService
  4. Using Callable with Future

15. What are the different stages of thread lifecycle and what methods are called in those stages?

  1. New – Created but not started (new Thread()).
  2. Runnable – start() called, waiting for CPU.
  3. Running – Thread is executing.
  4. Blocked/Waiting – Waiting for monitor lock or signal.
  5. Timed Waiting – sleep(), join(), wait(timeout)
  6. Terminated – Thread execution completed.

16. What is a RESTful Web Service?

RESTful web services use HTTP methods to expose CRUD operations on resources.

Principles:

  • Stateless
  • URI-based
  • Use of standard HTTP methods (GET, POST, PUT, DELETE)

17. Difference between @PathParam and @PathVariable?

  • @PathParam is used in JAX-RS (Jakarta/Java EE).
  • @PathVariable is used in Spring MVC. Both are used to extract values from the URI path.

18. Explain Polymorphism and its types?

Polymorphism allows objects to be treated as instances of their parent type.

Types:

  • Compile-time (Method Overloading)
  • Runtime (Method Overriding)

19. Explain Runtime Polymorphism and how to implement it?

Runtime polymorphism is achieved through method overriding where a subclass provides a specific implementation of a method declared in its superclass.

class Animal {
    void sound() { System.out.println("Animal sound"); }
}

class Dog extends Animal {
    void sound() { System.out.println("Bark"); }
}

20. What is Maven?

Maven is a build automation and dependency management tool for Java projects.

Key Features:

  • pom.xml for managing dependencies
  • Lifecycle phases: clean, compile, test, install, deploy

21. What are Different Joins?

  • INNER JOIN: Common rows in both tables
  • LEFT JOIN: All from left + matching from right
  • RIGHT JOIN: All from right + matching from left
  • FULL JOIN: All rows from both tables
  • CROSS JOIN: Cartesian product

22. What does the repository tag in Maven do?

The <repository> tag in pom.xml defines external repositories from which dependencies should be downloaded if not found in Maven Central.

23. How to print odd numbers using Stream?

List<Integer> list = List.of(1, 2, 3, 4, 5);
list.stream().filter(n -> n % 2 != 0).forEach(System.out::println);

24. Difference between Spring and Spring Boot?

Feature Spring Spring Boot
Setup Manual configuration Auto-configuration
Deployment WAR files Embedded servers (Tomcat, Jetty)
Complexity More boilerplate Reduced boilerplate

25. What is a ConcurrentHashMap?

A thread-safe variant of HashMap introduced in Java 5. It allows concurrent reads and updates without locking the entire map.

Key Features:

  • High performance under concurrent access
  • Segment-based locking in older versions
  • Bucket-based locking in newer versions

26. What is Test Driven Development (TDD)?

TDD is a software development approach where tests are written before the actual code.

Cycle:

  1. Write a failing test
  2. Write minimal code to pass the test
  3. Refactor the code

Benefits:

  • Better design
  • Fewer bugs
  • Easier refactoring
← Previous: Java and Spring Boot Interview Questions and Answers (Top 60+) Next: Java 17 and Design Patterns Interview Questions (LTS Edition) →