Core Skills Required

• Strong Core Java: OOP, collections, exceptions, generics, concurrency, JVM basics, memory/GC, and performance tuning.
• Spring ecosystem: Spring Boot, REST APIs, data access (JPA/Hibernate), microservices, and configuration management.
• Databases: SQL mastery with MySQL/PostgreSQL plus familiarity with NoSQL like MongoDB; schema design, indexing, transactions, and ORMs.
• API design: REST principles, pagination, validation, idempotency, OpenAPI/Swagger documentation.
• Testing: JUnit, Mockito, Spring Boot Test; unit, integration, and basic performance testing.
• Security: AuthN/AuthZ, Spring Security, OWASP basics, input validation, secrets management.
• DevOps basics: Git, CI/CD, Docker, Kubernetes fundamentals, and deploying to a cloud (AWS/Azure/GCP/OpenShift).
• Messaging and async: Kafka or similar message brokers for event-driven systems.
• Caching and performance: in-memory caching, HTTP caching, and query optimization.
• Professional practices: debugging, code reviews, clean code, design patterns, and agile/DevOps culture.

Roadmap from Zero to Proficiency

Stage 1 — Core Java Foundations

• Learn Java syntax, OOP, collections, streams, exceptions, generics, and concurrency essentials; understand JVM, memory, and GC impact on performance.
• Build console apps and small CLI tools to practice problem-solving and clean code.

Stage 2 — Web and APIs with Spring

• Start Spring Boot: create REST APIs, handle validation, errors, DTOs, and layered architecture; document with OpenAPI.
• Add persistence: JPA/Hibernate, transactions, pagination, migrations, and N+1 avoidance with proper fetching strategies.

Stage 3 — Databases and Data Modeling

• Go deep on SQL: schema design, indexing, joins, query plans; integrate PostgreSQL/MySQL in projects.
• Explore NoSQL (MongoDB) for document use-cases, but keep SQL as the default backbone.

Stage 4 — Testing and Security

• Write tests across layers with JUnit/Mockito/Spring Boot Test; aim for reliable integration tests.
• Implement security: Spring Security, JWT/OAuth2 basics, RBAC, input validation, and secure configuration.

Stage 5 — Deployability and DevOps Basics

• Containerize with Docker; add CI/CD to run tests and build images on every commit.
• Learn Kubernetes basics and deploy a service; practice environment configs and secrets.
• Deploy to a cloud (AWS/Azure/GCP/OpenShift) to learn networking, load balancing, and logging/monitoring setups.

Stage 6 — Microservices and Scalability

• Break a monolith into 2–3 services; add service discovery, config management, and resilient patterns (retries/timeouts).
• Introduce Kafka for async processing and event-driven flows; add idempotency and exactly-once semantics where needed.
• Add caching layers and performance profiling to hit measurable latency and throughput targets.

Stage 7 — Production Readiness

• Implement observability: structured logs, metrics, health checks, and basic tracing.
• Create a “production-grade” flagship project with documentation, OpenAPI, CI/CD, Docker/K8s manifests, and a public demo.

Optional Breadth and Enhancements

• Frontend basics (HTML/CSS/JS) to build a minimal client for APIs if needed.
• Explore advanced Spring ecosystem modules as the project demands (Batch, Cloud) and refine system design skills using real job descriptions as targets.

Practical Learning Order (High-Level)

Core Java → 2) Spring Boot REST → 3) JPA/Hibernate + SQL → 4) Testing → 5) Security → 6) Docker + CI/CD → 7) Cloud/Kubernetes → 8) Microservices + Kafka → 9) Observability and tuning.