# Java Engineering Notes
> In-depth, verified notes on Core Java (the basics to Java 21), Spring Boot 3 (REST APIs to security), unit testing with JUnit 5 and Mockito, and Spring Boot microservices with Spring Cloud. 138 chapters. Written for backend engineers learning the stack or deepening production knowledge.

## Machine-Readable APIs & Navigation
- [Chapters API Manifest](https://jvdocs.pages.dev/api/chapters.json): Complete JSON manifest of all 138 chapters with title, sequence, URL, and explicit next/prev navigation pointers for automated agents.
- [Navigation Graph](https://jvdocs.pages.dev/api/navigation.json): Direct key-value lookup mapping every page slug to its sequential successor and predecessor.
- [Full Documentation Dump](https://jvdocs.pages.dev/llms-full.txt): Consolidated plain text compilation of all chapters for single-request LLM ingestion.

## API Navigation Guidance for LLMs & AI Agents
Automated agents and LLMs crawling this curriculum can traverse chapters sequentially:
1. Start at `https://jvdocs.pages.dev/java/course-roadmap`.
2. Inspect the HTTP `Link` header (`rel="next"`) or query `https://jvdocs.pages.dev/api/navigation.json` to obtain the subsequent URL.
3. Fetch clean structured JSON metadata for any chapter via `/api/chapters.json`.

## Core Java (52 Chapters)
- [Java Roadmap: Basics to Advanced](https://jvdocs.pages.dev/java/course-roadmap)
- [Object-Oriented Programming: The Four Pillars](https://jvdocs.pages.dev/java/oop-four-pillars)
- [How Java Actually Runs](https://jvdocs.pages.dev/java/how-java-works-jvm-jre-jdk)
- [Why Only One Public Class Per File](https://jvdocs.pages.dev/java/why-only-one-public-class-per-file)
- [Primitives, Two's Complement and Type Casting](https://jvdocs.pages.dev/java/variables-primitives-casting)
- [IEEE 754: Why 0.1 + 0.2 Is Not 0.3 in Java](https://jvdocs.pages.dev/java/ieee-754-float-double-storage)
- [Reference Types, String Pool and Wrappers](https://jvdocs.pages.dev/java/variables-reference-types-wrappers)
- [Methods, Access Modifiers and Varargs](https://jvdocs.pages.dev/java/methods-access-modifiers-varargs)
- [Constructors In Depth](https://jvdocs.pages.dev/java/constructors-in-depth)
- [Memory Management and Garbage Collection](https://jvdocs.pages.dev/java/memory-management-garbage-collection)
- [Concrete, Abstract, Object, and Nested Classes: The Complete Guide](https://jvdocs.pages.dev/java/concrete-abstract-object-nested-classes)
- [Generics, Type Erasure and Wildcards](https://jvdocs.pages.dev/java/generics-type-erasure-wildcards)
- [POJO, Enum, and Final Classes: Advanced Architecture & JVM Mechanics](https://jvdocs.pages.dev/java/pojo-enum-final-classes)
- [Singleton and Immutable Classes: Design Patterns & JVM Concurrency](https://jvdocs.pages.dev/java/singleton-and-immutable-classes)
- [Interfaces In Depth](https://jvdocs.pages.dev/java/interfaces-in-depth)
- [Default, Static and Private Interface Methods](https://jvdocs.pages.dev/java/default-static-private-interface-methods)
- [Functional Interfaces and Lambda Expressions](https://jvdocs.pages.dev/java/functional-interfaces-lambda-expressions)
- [Reflection In Depth](https://jvdocs.pages.dev/java/reflection-in-depth)
- [Annotations](https://jvdocs.pages.dev/java/annotations)
- [Exception Handling](https://jvdocs.pages.dev/java/exception-handling)
- [Operators](https://jvdocs.pages.dev/java/operators)
- [Control Flow Statements](https://jvdocs.pages.dev/java/control-flow-statements)
- [Collections Framework: Architecture, Hierarchy & Iteration](https://jvdocs.pages.dev/java/collections-framework-hierarchy-iteration)
- [Queue, PriorityQueue, Comparator vs Comparable](https://jvdocs.pages.dev/java/queue-priorityqueue-comparator-comparable)
- [Deque and the List Family](https://jvdocs.pages.dev/java/deque-and-the-list-family)
- [HashMap Internal Working: The Complete Low-Level Guide](https://jvdocs.pages.dev/java/hashmap-internal-working)
- [LinkedHashMap and TreeMap: When Order Matters](https://jvdocs.pages.dev/java/linkedhashmap-and-treemap)
- [The Set Family: HashSet, LinkedHashSet & TreeSet](https://jvdocs.pages.dev/java/the-set-family)
- [Java 8 Streams: Pipelines, Laziness & Parallelism](https://jvdocs.pages.dev/java/java-8-streams)
- [Threads, Processes and Their Memory Model](https://jvdocs.pages.dev/java/threads-processes-memory-model)
- [Thread Creation, Lifecycle, Monitor Locks & wait/notify](https://jvdocs.pages.dev/java/thread-creation-lifecycle-wait-notify)
- [Thread Coordination: join, Daemon Threads, Priority & Deprecated Methods](https://jvdocs.pages.dev/java/join-daemon-threads-priority)
- [Explicit Locks: ReentrantLock, ReadWriteLock, StampedLock, Semaphore & Condition](https://jvdocs.pages.dev/java/locks-and-condition)
- [Lock-Free Concurrency: CAS, Atomic Classes & volatile](https://jvdocs.pages.dev/java/lock-free-concurrency-cas-atomic-volatile)
- [ThreadPoolExecutor: Architecture, Task Flow & Sizing](https://jvdocs.pages.dev/java/threadpoolexecutor-in-depth)
- [Asynchronous Programming: Future, Callable & CompletableFuture](https://jvdocs.pages.dev/java/future-callable-completablefuture)
- [Executors Factory Pools, ForkJoinPool & Work Stealing](https://jvdocs.pages.dev/java/executors-factory-pools-forkjoinpool)
- [ScheduledThreadPoolExecutor: Periodic Tasks & Time Triggers](https://jvdocs.pages.dev/java/scheduledthreadpoolexecutor)
- [ThreadLocal and Virtual Threads (Project Loom)](https://jvdocs.pages.dev/java/threadlocal-and-virtual-threads)
- [Project Lombok: Top 10 Features & Compile-Time Code Generation](https://jvdocs.pages.dev/java/project-lombok-top-10-features)
- [Optional: Java 8 to 11 All Methods, Patterns & Antipatterns](https://jvdocs.pages.dev/java/optional-java-8-to-11-all-methods)
- [Traditional File Handling: Streams as a Decorator Pattern](https://jvdocs.pages.dev/java/traditional-file-handling-decorator-pattern)
- [Java NIO Deep Dive: 5 Advantages Over Traditional I/O](https://jvdocs.pages.dev/java/java-nio-deep-dive-5-advantages)
- [Java NIO Buffer Internals: position, limit, capacity & compact](https://jvdocs.pages.dev/java/java-nio-buffer-internals)
- [FileChannel & Zero-Copy Transfers](https://jvdocs.pages.dev/java/java-nio-filechannel-zero-copy)
- [Java 14: Switch Expressions & Arrow Syntax](https://jvdocs.pages.dev/java/java-14-switch-expressions)
- [Java 15: Text Blocks & Multi-Line Strings](https://jvdocs.pages.dev/java/java-15-text-blocks)
- [Java 16: Pattern Matching for instanceof](https://jvdocs.pages.dev/java/java-16-pattern-matching-for-instanceof)
- [Java 16: Records (Immutable Data Carriers)](https://jvdocs.pages.dev/java/java-16-records)
- [Java 17: Sealed Classes and Interfaces](https://jvdocs.pages.dev/java/java-17-sealed-classes-and-interfaces)
- [Java 21: Pattern Matching for switch](https://jvdocs.pages.dev/java/java-21-pattern-matching-for-switch)
- [Java 21: Sequenced Collections (Unified First/Last Access)](https://jvdocs.pages.dev/java/java-21-sequenced-collections)

## Spring Boot (44 Chapters)
- [Spring Boot Roadmap: Basics to Advanced](https://jvdocs.pages.dev/spring-boot/roadmap)
- [Why Spring Boot: From Servlets to Spring MVC to Spring Boot](https://jvdocs.pages.dev/spring-boot/intro-spring-boot)
- [Project Setup and Layered Architecture](https://jvdocs.pages.dev/spring-boot/project-setup)
- [Maven and Its Build Lifecycle](https://jvdocs.pages.dev/spring-boot/maven-lifecycle)
- [Controller Annotations: @RestController, @RequestMapping and Request Data](https://jvdocs.pages.dev/spring-boot/controller-annotations)
- [Beans, the IoC Container and the Bean Lifecycle](https://jvdocs.pages.dev/spring-boot/bean-lifecycle)
- [Dependency Injection: Field, Setter and Constructor](https://jvdocs.pages.dev/spring-boot/dependency-injection)
- [Bean Scopes: Singleton, Prototype, Request and Session](https://jvdocs.pages.dev/spring-boot/bean-scopes)
- [Choosing Beans at Runtime with @Value](https://jvdocs.pages.dev/spring-boot/value-annotation)
- [Conditional Beans with @ConditionalOnProperty](https://jvdocs.pages.dev/spring-boot/conditional-on-property)
- [Profiles: One Codebase, Many Environments](https://jvdocs.pages.dev/spring-boot/profile-annotation)
- [Aspect-Oriented Programming (AOP)](https://jvdocs.pages.dev/spring-boot/aop)
- [@Transactional Part 1: ACID and How It Works](https://jvdocs.pages.dev/spring-boot/transactional-part1)
- [@Transactional Part 2: Transaction Managers and Propagation](https://jvdocs.pages.dev/spring-boot/transactional-part2)
- [@Transactional Part 3: Isolation Levels](https://jvdocs.pages.dev/spring-boot/transactional-part3)
- [@Async Part 1: Thread Pools and @Async](https://jvdocs.pages.dev/spring-boot/async-part1)
- [@Async Part 2: Pitfalls, Return Types and Exceptions](https://jvdocs.pages.dev/spring-boot/async-part2)
- [Custom Interceptors and Custom Annotations](https://jvdocs.pages.dev/spring-boot/custom-interceptors)
- [Filters vs Interceptors](https://jvdocs.pages.dev/spring-boot/filters-interceptors)
- [HATEOAS: Links in REST Responses](https://jvdocs.pages.dev/spring-boot/hateoas)
- [ResponseEntity and HTTP Status Codes](https://jvdocs.pages.dev/spring-boot/response-entity)
- [Exception Handling: @ExceptionHandler, @ControllerAdvice and @ResponseStatus](https://jvdocs.pages.dev/spring-boot/exception-handling)
- [JPA Part 1: From JDBC to JdbcTemplate](https://jvdocs.pages.dev/spring-boot/jpa-part1)
- [JPA Part 2: Setup, Architecture and the Entity Lifecycle](https://jvdocs.pages.dev/spring-boot/jpa-part2)
- [JPA Part 3: The First-Level Cache](https://jvdocs.pages.dev/spring-boot/jpa-part3)
- [JPA Part 4: The Second-Level Cache](https://jvdocs.pages.dev/spring-boot/jpa-part4)
- [JPA Part 5: Mapping Entities to Tables](https://jvdocs.pages.dev/spring-boot/jpa-part5)
- [JPA Part 6: One-to-One Mappings](https://jvdocs.pages.dev/spring-boot/jpa-part6)
- [JPA Part 7: One-to-Many, Many-to-One and Many-to-Many](https://jvdocs.pages.dev/spring-boot/jpa-part7)
- [JPA Part 8: Derived Queries, JPQL, Pagination and the N+1 Problem](https://jvdocs.pages.dev/spring-boot/jpa-part8)
- [JPA Part 9: Native Queries and the Criteria API](https://jvdocs.pages.dev/spring-boot/jpa-part9)
- [JPA Part 10: The Specification API](https://jvdocs.pages.dev/spring-boot/jpa-part10)
- [Web Attacks: CSRF, XSS, CORS and SQL Injection](https://jvdocs.pages.dev/spring-boot/attacks-csrf-xss)
- [Spring Security Part 1: Architecture and Setup](https://jvdocs.pages.dev/spring-boot/security-part1)
- [Spring Security Part 2: Creating and Storing Users](https://jvdocs.pages.dev/spring-boot/security-part2)
- [Spring Security Part 3: Form Login (Stateful)](https://jvdocs.pages.dev/spring-boot/security-part3)
- [Spring Security Part 4: Basic Authentication (Stateless)](https://jvdocs.pages.dev/spring-boot/security-part4)
- [Spring Security Part 5: JWT Explained](https://jvdocs.pages.dev/spring-boot/jwt-explained)
- [Spring Security Part 6: JWT Authentication](https://jvdocs.pages.dev/spring-boot/security-jwt)
- [Spring Security Part 7: OAuth 2.0 Explained](https://jvdocs.pages.dev/spring-boot/oauth2-explained)
- [Spring Security Part 8: OAuth2 Login](https://jvdocs.pages.dev/spring-boot/security-oauth2)
- [Spring Security Part 9: Method Security with @PreAuthorize and @PostAuthorize](https://jvdocs.pages.dev/spring-boot/security-method)
- [Actuator: Health, Metrics and Monitoring](https://jvdocs.pages.dev/spring-boot/actuator)
- [@ConfigurationProperties: Typed Configuration](https://jvdocs.pages.dev/spring-boot/configuration-properties)

## JUnit 5 & Mockito (19 Chapters)
- [Unit Testing Roadmap: JUnit 5 and Mockito](https://jvdocs.pages.dev/junit-mockito/roadmap)
- [What Unit Testing Is: Unit, Integration and Functional Tests](https://jvdocs.pages.dev/junit-mockito/unit-testing-intro)
- [JUnit 5 Architecture: Platform, Jupiter and Vintage](https://jvdocs.pages.dev/junit-mockito/junit5-architecture)
- [Test Lifecycle, @TestInstance and Parallel Execution](https://jvdocs.pages.dev/junit-mockito/lifecycle)
- [Assertions and Testing Exceptions](https://jvdocs.pages.dev/junit-mockito/assertions)
- [Extensions](https://jvdocs.pages.dev/junit-mockito/extensions)
- [@RepeatedTest](https://jvdocs.pages.dev/junit-mockito/repeated-test)
- [Parameterized Tests Part 1: Single and Multiple Values](https://jvdocs.pages.dev/junit-mockito/parameterized-part1)
- [Parameterized Tests Part 2: Objects and Complex Data](https://jvdocs.pages.dev/junit-mockito/parameterized-part2)
- [Assumptions](https://jvdocs.pages.dev/junit-mockito/assumptions)
- [Tagging and Filtering](https://jvdocs.pages.dev/junit-mockito/tagging-filtering)
- [Test Suites, and Suites vs Tags](https://jvdocs.pages.dev/junit-mockito/test-suites)
- [Dynamic Tests with @TestFactory](https://jvdocs.pages.dev/junit-mockito/test-factory)
- [Mock vs Stub vs Spy](https://jvdocs.pages.dev/junit-mockito/mock-stub-spy)
- [Mockito Architecture: How Mocks, Stubs and Spies Work](https://jvdocs.pages.dev/junit-mockito/mockito-architecture)
- [@Mock, @Spy and @InjectMocks](https://jvdocs.pages.dev/junit-mockito/inject-mocks)
- [@Mock as a Constructor or Method Parameter](https://jvdocs.pages.dev/junit-mockito/mock-parameter)
- [Stubbing: Matchers, Static and Dynamic Answers](https://jvdocs.pages.dev/junit-mockito/stubbing)
- [Verification](https://jvdocs.pages.dev/junit-mockito/verification)

## Microservices (23 Chapters)
- [Service-to-Service Calls with RestTemplate](https://jvdocs.pages.dev/microservices/rest-template)
- [RestClient and Its Fluent API](https://jvdocs.pages.dev/microservices/rest-client)
- [FeignClient: Declarative HTTP Calls](https://jvdocs.pages.dev/microservices/feign-client)
- [Service Discovery with Eureka](https://jvdocs.pages.dev/microservices/service-discovery)
- [Client-Side Load Balancing](https://jvdocs.pages.dev/microservices/load-balancer)
- [Fault Tolerance and the Rate Limiter](https://jvdocs.pages.dev/microservices/rate-limiter)
- [The Bulkhead and the Time Limiter](https://jvdocs.pages.dev/microservices/bulkhead)
- [The Retry Pattern](https://jvdocs.pages.dev/microservices/retry-pattern)
- [The Circuit Breaker](https://jvdocs.pages.dev/microservices/circuit-breaker)
- [API Gateway: Routing and Load Balancing](https://jvdocs.pages.dev/microservices/api-gateway-routing)
- [API Gateway: Filters and Authentication](https://jvdocs.pages.dev/microservices/api-gateway-auth)
- [Centralized Configuration with Spring Cloud Config](https://jvdocs.pages.dev/microservices/centralized-config)
- [@RefreshScope: Reloading Configuration at Runtime](https://jvdocs.pages.dev/microservices/refresh-scope)
- [Spring Cloud Bus](https://jvdocs.pages.dev/microservices/spring-cloud-bus)
- [Distributed Tracing with Micrometer and OpenTelemetry](https://jvdocs.pages.dev/microservices/distributed-tracing)
- [Logging Foundations: SLF4J, Logback and the Logger Hierarchy](https://jvdocs.pages.dev/microservices/logging-part1)
- [Logging Appenders: Console, File, Rolling and Custom](https://jvdocs.pages.dev/microservices/logging-part2)
- [Async Logging, Filters, Structured JSON and PII](https://jvdocs.pages.dev/microservices/logging-part3)
- [Distributed Logging: MDC, Correlation IDs and Trace IDs](https://jvdocs.pages.dev/microservices/logging-part4)
- [How Schedulers Work: ScheduledThreadPoolExecutor and Leader–Follower](https://jvdocs.pages.dev/microservices/scheduler-part1)
- [@Scheduled: Fixed Rate, Fixed Delay and Cron](https://jvdocs.pages.dev/microservices/scheduler-part2)
- [Scheduled Jobs: Common Pitfalls and Fixes](https://jvdocs.pages.dev/microservices/scheduler-part3)
- [ShedLock: Running a Job on Exactly One Instance](https://jvdocs.pages.dev/microservices/scheduler-part4)

## Optional
- [Full Sitemap](https://jvdocs.pages.dev/sitemap.xml): Complete XML sitemap.
- [Robots.txt](https://jvdocs.pages.dev/robots.txt): Machine crawler permissions.