Java Microservices Interview

Java Microservice Consultant Interview Experience: Questions and In-Depth Answers

If you're preparing for a Java Microservice Consultant role at any service-based/Big 4 firm, it's essential to understand the range and depth of questions you might face. Below, I provide detailed answers and explanations.

Technical Round 1

1. Project Architecture, Modules, and Responsibilities

Explanation:
Be ready to explain your project's overall design—how modules interact, their responsibilities, and your contributions. Discuss layers (controller, service, repository), major workflows, and scaling/maintenance strategies.

2. Java: Immutability, Mutable & Immutable Strings, Real-world Use Case

Immutable Objects:
- Their state can't change after creation.
- Example: String in Java is immutable.

Mutable Objects:
- Their state can change.
- Example: StringBuilder is mutable.

Real-world Use Case:
Immutability is crucial for thread safety. For example, using String as a key in HashMap is reliable because its value won't change, preventing issues with hash code calculations.

3. Threads: Ways to Implement & Lifecycle

Ways to create a thread:
- Extend Thread class

- Implement Runnable interface
- Implement Callable<V> interface (for threads that return results)

Thread Lifecycle:
- New
- Runnable
- Running
- Blocked/Waiting
- Terminated

4. Transaction Management and Handling Rollback on Exception

Explanation:
In frameworks like Spring, use @Transactional annotation. If an exception occurs, the transaction manager undoes all operations (rollback), ensuring data consistency.

5. Java 8: Streams, Lambdas, Functional Interfaces

• Streams: Declarative processing of collections.
  

  employees.stream().filter(e -> e.getAge() > 25).collect(Collectors.toList());
  
  

  
  
• Lambdas: Concise syntax for anonymous functions.
  

  (a, b) -> a + b
  
  

  
  
• Functional Interface: Interface with a single abstract method (e.g., Runnable, Comparator).

6. Sort Employee Objects Using Java 8 & Comparator

Explanation:
Use Comparator.comparing() and stream API:

employees.stream()
    .sorted(Comparator.comparing(Employee::getSalary))
    .collect(Collectors.toList());

7. SQL: Find Employee with 3rd Highest Salary

Query:

SELECT DISTINCT salary 
FROM Employee 
ORDER BY salary DESC 
LIMIT 1 OFFSET 2;

Or, using subquery:

SELECT MIN(salary) 
FROM (SELECT DISTINCT salary FROM Employee ORDER BY salary DESC LIMIT 3) AS temp;

8. Microservices vs Monolith – Advantages and Use Cases

	Monolith	Microservices
Structure	Single codebase	Multiple loosely-coupled services
Scalability	Vertical (harder)	Horizontal (easier, per service)
Deployment	All-or-nothing	Independent services
Use Case	Small apps, POCs	Large, dynamic, scalable systems

Advantages of microservices: Faster deployment, isolated failures, independent scaling.

9. How to Create a Microservice, Tools Used, and Monitoring

• Creation: Spring Boot to build lightweight RESTful services.
• Tools Used: Spring Boot, Docker, Kubernetes, Eureka, Zuul.
• Monitoring: Use tools like Prometheus, Grafana, Spring Actuator, ELK stack, or Zipkin for distributed tracing.

10. Coding Standards and Checklist

• Code readability
• Proper naming conventions
• Error handling
• Logging
• Unit tests
• Documentation

Technical Round 2

1. JVM vs JRE vs JDK

JVM	JRE	JDK
Java Virtual Machine	Java Runtime Env.	Java Dev Kit
Runs Java bytecode	JVM + libs for running	JRE + dev tools
Platform-dependent	Platform-dependent	Platform-dependent

2. Ways to Create a String & Their Storage

• Literal (e.g., "hello"): Stored in String pool.
• Using new String(): Created in heap, not pooled automatically.

3. Interthread Communication: wait(), notify(), Why in Object Class

wait(), notify() are in Object class because every Java object has a monitor (intrinsic lock) that threads can synchronize on. This allows any object—not just threads—to be used for interthread communication.

4. Design Patterns: Singleton Implementation and Usage

Singleton Implementation:

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

Real-time Use: Connection pools, logging, configuration managers.

5. Code: Connection Pool (Fixed Size, Acquire/Release)

Maintain a pool of connections using a collection (e.g., BlockingQueue). Provide methods to acquireConnection() and releaseConnection(). Ensure thread safety using synchronization or concurrent collections.

6. Spring Boot: Configure Multiple Databases

• Define two DataSource, EntityManagerFactory, and TransactionManager beans—one for each database.

Annotate repository/package with respective config.

7. Rollback Strategies Post Commit

After a commit, it's generally not possible to rollback at the database level. Can compensate by writing compensating transactions (SAGA pattern in microservices).

8. Project Testing, Deployment & Production Issue Debugging

• Testing: JUnit, Mockito, integration tests
• Deployment: CI/CD, Docker, Kubernetes
• Debugging: Analyze logs, use monitoring tools, replicate issues locally

Managerial Round

1. Detailed Project Discussion

Be ready to talk about your ownership, impact, team interaction, and architectural decisions.

2. Java Collections: List, Set, Map—Usage

• List: Ordered collection, allows duplicates (use: ordered data)
• Set: Unordered, no duplicates (use: unique items like user IDs)
• Map: Key-value pairs (use: dictionary, lookup tables)

3. Why Set Doesn't Allow Duplicates? What if I Add One?

A Set enforces uniqueness by comparing objects using equals() and hashCode(). If you add a duplicate, the set ignores the insertion.

4. Cloud Concepts: IAM, EC2, S3

• IAM: Manage users and permissions
• EC2: Virtual servers in AWS cloud
• S3: Object storage for files, backups, etc.

5. Exception Handling: Custom Exceptions & User-Friendly Messages

• Create subclasses of Exception or RuntimeException.

Catch low-level exceptions and throw custom ones with clear, user-readable messages.

6. Coding (Notepad): RESTful Person API (id, name, gender Enum)

Implement Controller, Service, Repository, and Model classes with a Gender enum type. Expose CRUD endpoints.

7. SQL: Get Manager Names from Employee Table (empId, name, mgrId)

Query:

SELECT DISTINCT mgr.name
FROM Employee emp
JOIN Employee mgr ON emp.mgrId = mgr.empId;

8. Logical Puzzle: Room with 100 Switches—Efficient Way to Find Your Light (O(logN))

Use a binary search strategy. Assume each room has a unique light indicator per switch position. Flip switches in halves, observe changes, and narrow down to your room in log₂(100) ≈ 7 tries (binary search method).

Final Takeaway

Like other top-tier consultancies, tests both technical knowledge and the ability to apply concepts to real-world scenarios. Focus on problem-solving, communication, and clarity, not just technical correctness. Good luck on your interview journey!