Java + Spring Boot Knowledge Base
A job-market-oriented Java backend system: from Java basics, through the SQL / MySQL / Maven toolchain, to Spring / Spring Boot in practice and the 3→4 migration. Every chapter is organized around what employers actually ask for.
1. What kind of Java developer does the market want?
Java backend is one of the highest-demand roles. Junior-to-mid JDs (1-5 years) are remarkably similar. Here is the jargon translated into plain terms:
| What the JD says | What it really means | Chapter |
|---|---|---|
| "Solid Java fundamentals, understand JVM and concurrency" | Collection internals (HashMap above all), thread pool parameters, JMM, synchronized/volatile, GC & tuning | Java Basics / Java Advanced |
| "Familiar with MySQL: indexes, transactions, locks" | B+ tree, clustered/secondary indexes, leftmost prefix, isolation levels, MVCC, EXPLAIN tuning | SQL in Practice / MySQL |
| "Proficient with MyBatis / MyBatis-Plus" | Dynamic SQL, #{} vs ${}, L1/L2 caches, pagination plugin | Spring Boot in Action data access section |
| "Understand Spring IoC and AOP" | Three-level cache & circular dependencies, bean lifecycle, transaction failure scenarios | Spring Core |
| "Experience with Spring Boot" | Auto-configuration internals, starter mechanism, config system, layered web development | Spring Boot in Action |
| "Have used Maven / Gradle" | Dependency scopes, conflict resolution, multi-module builds, mirror setup | Maven |
| "Redis / message queues / microservices a plus" | Data types, the three cache problems, persistence, HA; registry & discovery, gateway, circuit breaking/limiting, distributed transactions/locks/idempotency, cache consistency | Redis / Microservices / Distributed Topics |
| "Works well under pressure, team player" | Prove it with one complete project on your résumé, not empty words | Project roadmap below |
A reality check for 2026: "Spring Boot 3 to 4", "virtual threads" and "Java 21 features" now show up in a large share of JDs and interviews — legacy systems still run 2.x/3.x while new projects start at 4.x. Being able to explain the version evolution is a differentiator. That is exactly what Spring Boot 3 vs 4 covers.
2. Knowledge map (suggested order)
| # | Chapter | Contents | Positioning |
|---|---|---|---|
| 1 | Java Basics | Syntax, OOP, collections, generics, exceptions, IO, lambdas | Foundation — ~30% of interview questions come from here |
| 2 | Java Advanced | Concurrency, JVM, reflection, Java 8/17/21 features | Deep end; separates mid from senior |
| 3 | SQL from Basics to Practice | DDL/DML/DQL, JOINs, subqueries, window functions | Writing complex queries is a daily requirement |
| 4 | MySQL Deep Dive | Architecture, index internals, transactions & MVCC, locks, tuning | Top-3 interview topic |
| 5 | Redis Cache | Data types, penetration/breakdown/avalanche, persistence, replication/Sentinel/Cluster | Top-3 interview topic; caching is a daily battleground |
| 6 | Maven | Coordinates, dependencies, lifecycle, multi-module, conflicts | Toolchain — read any pom.xml afterwards |
| 7 | Spring Core | IoC/DI, bean lifecycle, AOP, transactions | Framework internals; prerequisite for Boot |
| 8 | Spring Boot in Action | Auto-config, web dev, MyBatis integration, config system | 80% of your actual working time |
| 9 | Spring Boot 3 vs 4 | Version evolution, breaking changes, migration playbook | The hot 2026 interview topic |
| 10 | Microservices & Spring Cloud | Nacos, OpenFeign, Gateway, Sentinel, tracing | Where senior architecture questions live |
| 11 | Distributed Systems Core Topics | Distributed locks/transactions, idempotency, MQ, cache consistency, sharding | The capstone question pool |
| 12 | Spring AI Application Development | ChatClient, tool calling, RAG, MCP, conversation memory | The new 2026 JD hot spot: Java + AI apps |
3. A résumé-worthy project roadmap
Before applying, build one project end to end. An e-commerce / content admin system is ideal because it naturally exercises every exam point:
- Phase 1 — CRUD backbone: Spring Boot + MyBatis-Plus + MySQL for product/order management APIs. Practice layering, a unified response envelope, validation, global exception handling.
- Phase 2 — Concurrency & caching: Load-test, find problems → add Redis caching, async via thread pools, optimistic locking to prevent overselling. Every step maps to a frequent interview question.
- Phase 3 — Engineering: Maven multi-module split, logging & monitoring, Docker deployment. All your résumé "highlights" come from the bugs you meet in these phases.
Write résumé bullets as verb + technology + quantified result: "Cached hot product data with Redis, cutting API P99 from 800ms to 45ms" beats "familiar with Redis" by an order of magnitude.
4. Learning resources
JDK 21 API docsSpring Boot officialSpring Framework officialMaven guidesMySQL 8.4 manualMyBatis docsOpenRewrite (auto migration)
5. Frequent interview questions (a taste of the difficulty)
- HashMap put flow? Why is it not thread-safe? How does ConcurrentHashMap fix it? → Java Basics / Java Advanced
- What do the 7 thread pool parameters mean? Which rejection strategy? → Java Advanced
- Why does InnoDB use a B+ tree instead of a hash index? What is a table lookback? → MySQL
- How does RR isolation prevent phantom reads? → MySQL
- How does Spring resolve circular dependencies? Why three caches? → Spring Core
- When does
@Transactionalsilently fail? → Spring Core - How does Spring Boot auto-configuration work? → Spring Boot in Action
- What breaking changes does Spring Boot 4 bring over 3? → Spring Boot 3 vs 4
- Limiting vs breaking vs degradation — disambiguate. Why does Nacos config hot-refresh? → Microservices
- Penetration vs breakdown vs avalanche — tell them apart and prevent each. RDB or AOF? → Redis
- How does a distributed lock evolve? Cache–DB consistency? → Distributed Topics
- What is Spring AI's ChatClient/advisor architecture? What changed in 2.0 tool calling? → Spring AI