spring-boot-demo

Companion code for the Spring Boot articles on ankurm.com.

One directory per article. Each is a self-contained Maven project with its own README.md, numbered documentation chapters under docs/, and captured real output under docs/output/ that a single script regenerates. Every figure quoted in an article came out of one of those files.

Directory	Article	What it demonstrates
actuator-in-production/	Spring Boot Actuator in Production	endpoint exposure defaults, securing Actuator, custom health indicators and how they hang
spring-boot-startup-time/	Why Your Spring Boot App Takes 8 Seconds to Start	BufferingApplicationStartup, JFR startup events, self time vs total time, the classpath-scan tax, the JDK 25 AOT cache
configuration-properties/	@ConfigurationProperties vs @Value in Spring Boot 4	relaxed binding measured three ways, record constructor binding, validation, IDE metadata generation
profiles-and-config/	Spring Boot Profiles Done Right	the precedence stack made visible, config trees and ConfigMaps, why a profile file loses to an environment variable
spring-aop/	Spring AOP Explained	every pointcut designator with real matches, JDK vs CGLIB proxies, six aspects that do not fire
docker-images/	Dockerizing Spring Boot 4: Layered Jars, Buildpacks, Distroless and Image Size Benchmarks	one service packaged nine ways and measured: size on disk and pushed, rebuild delta, startup, PID 1 and signals, jlink, the JDK 25 AOT cache
kubernetes-deployment/	Deploying Spring Boot 4 on Kubernetes	probe groups under a dependency outage, graceful shutdown under load four ways, JVM ergonomics per pod shape, CPU limits throttling GC, HPA on a Micrometer metric
problem-details/	Global Exception Handling with ProblemDetail (RFC 9457) in Spring Boot 4	thirteen failures under five handling setups, validation errors, i18n, content negotiation, errors outside MVC, silent 500s, decoding on the client
resilience/	Spring Framework 7's Built-in Resilience: @Retryable, @ConcurrencyLimit, and What's Left for Resilience4j	@Retryable and @ConcurrencyLimit counted invocation by invocation, retries inside transactions, where Resilience4j still earns its place, migrating from Spring Retry
resilience4j-circuit-breaker/	Resilience4j Circuit Breaker in Spring Boot 4.1: What It's Still For	the circuit breaker verified through trip/open/half-open/recover, spring-boot-starter-aop renamed to spring-boot-starter-aspectj (not removed), and where Resilience4j still beats Framework 7's own @Retryable/@ConcurrencyLimit
observability/	A Practical Guide to Monitoring Spring Boot Microservices: Prometheus, Grafana, and Boot 4.1's OpenTelemetry Starter	real metrics and traces pushed via OTLP to a real grafana/otel-lgtm container, Docker Compose auto-wiring the endpoint with zero management.otlp.* properties, @Observed silently inert without an explicit ObservedAspect bean, and a dual-version (Boot 4.0 vs 4.1) proof of which OTEL_* environment variables are genuinely new
caching/	The Spring Cache Abstraction: @Cacheable, @CacheEvict, Key Generators and the Self-Invocation Trap	the self-invocation trap measured four ways, the key collision SimpleKeyGenerator makes easy, eviction timing under a thrown exception, a rollback the cache keeps, and where this sits next to Hibernate's L2 cache
redis/	Redis with Spring Boot 4.1: RedisTemplate, StringRedisTemplate, @RedisHash, Pub/Sub and TTL	the default JDK-serialised RedisTemplate writing keys redis-cli cannot read, before and after Jackson 3 serializers (class names read with javap, the @class allow-list), the six keys two @RedisHash entities create, expired hashes leaving index entries behind unless keyspace events are on, @RedisListener on Boot 4.1's auto-configured container, a plain set clearing a TTL, and a RedisCacheConfiguration bean silently overriding spring.cache.redis.time-to-live
spring-batch/	Spring Batch on Boot 4.1: Jobs, Steps, Chunk Processing and Restartability	a job that fails mid-chunk and resumes exactly where it left off across two separate JVMs, skip vs. restart on the same poisoned row, the resourceless job repository that forgets a restart ever happened, and the chunk(int) vs chunk(int, tx) builder split
spring-batch-partitioning/	Spring Batch Partitioning and Parallel Steps: Scaling a 10-Million-Row Job	the real grid-size sweep at 10M and 300K rows (best speedup 1.42x, on 2 cores), MultiResourcePartitioner ignoring gridSize entirely, a rejected partition's StepExecution stuck at STARTING forever, and Spring Batch 6.0's new JobOperator#recover unsticking it
db-migrations-flyway-liquibase/	Flyway vs Liquibase for Spring Boot 4: Migrations, Rollbacks and Baselines	Flyway Community's undo throwing FlywayRedgateEditionRequiredException at runtime, a real Liquibase 5.0.3 filename-caching defect that produces a phantom successful run, Liquibase's 10-second default lock-poll rate versus Flyway's near-instant row lock, the FSL license change and its ASF/Keycloak fallout, and what actually happens when both tools are enabled against one database
db-migrations-expand-contract/	Zero-Downtime Database Migrations: Expand-Contract in Practice with Spring Boot	a real 4-deploy rolling sequence against two live replicas with a load generator proving 99.98% success, H2's AUTO_SERVER single-point-of-failure trap, a NOT NULL constraint that fails every Stage 4 insert, and ALTER TABLE silently dropping a concurrently committed row with no exception thrown
openapi-versioning/	springdoc-openapi with Spring Boot 4.1: Generating, Customising and Versioning Your API Spec	springdoc 3.1.1 silently merging same-path, different-version handlers into one oneOf operation with an arbitrary operationId, a working per-version fix with GroupedOpenApi + OpenApiCustomizer, and the officially-versioning-supported functional-endpoint path turning out to document only one of two registered versions
graphql-dataloader/	Spring GraphQL 2.0: Schema-First APIs, DataLoader Batching and Killing N+1	a naive @SchemaMapping resolver measured at 21 SQL statements for 20 books versus a @BatchMapping resolver's flat 2, a dangling foreign key nulling an entire GraphQL response via non-null propagation identically under both resolver strategies, and two Spring Boot 4.1 packaging changes (DataSourceAutoConfiguration's new package, Jackson 3 by default) hit along the way
custom-validation/	Custom Validation in Spring Boot: Beyond the Basics!	the javax.validation to jakarta.validation namespace fix Boot 3 already required, Jakarta Validation 3.1's record-validation clarification proven on both field- and class-level custom constraints, a record validation failure's empty 400 body by default, and what spring.mvc.problemdetails.enabled does and does not fix
etag-caching/	Mastering Cache Control with ETag in Spring Boot RESTful APIs	spring-boot-starter-web's own POM now reading "deprecated in favor of spring-boot-starter-webmvc", ShallowEtagHeaderFilter and WebRequest.checkNotModified() re-verified unchanged on Spring Framework 7, deep cache vs shallow cache, and conditional PUT with If-Match as optimistic locking
restclient-basic-auth/	Spring Boot RestTemplate with Basic Auth: A Modern Guide	RestClient with Basic Auth two ways against a real embedded server, {noop} passwords confirmed to emit no runtime warning at all, spring-boot-starter-restclient as its own required Boot 4 module, and the RestClient.exchange() trap covered in depth by the RestTemplate to RestClient migration guide
core-di/	Dependency Injection in Spring Boot 4: Constructor vs Setter vs Field (and Why Field Injection Hurts) and @Autowired Explained: By-Type Resolution, @Qualifier, @Primary, ObjectProvider and List Injection	one OrderService in three injection styles built with plain new, a field-injected dependency used in a constructor, circular dependencies in plain Spring vs Boot with the real failure report, @Lazy injecting a proxy, and the @Autowired resolution ladder read from Spring 7.0.9 bytecode (name match beats @Priority), plus the empty-collection trap that only bites field and setter injection
core-beans/	Spring Bean Scopes: Singleton, Prototype, Request, Session and the Prototype-in-Singleton Trap Spring Bean Lifecycle in Boot 4: @PostConstruct, InitializingBean, SmartLifecycle and Shutdown Order and Circular Dependencies in Spring Boot 4: Why Startup Fails and 4 Ways to Fix It	instance counts per scope, five fixes for a prototype inside a singleton, request scope without a proxy failing at start-up, every lifecycle callback in order, @PostConstruct running before the proxy exists, SmartLifecycle phases measured, a SmartLifecycle at the default phase stopping before Tomcat has drained, and the circular-dependency start-up failure reproduced with four fixes
custom-starter/	Write Your Own Spring Boot 4 Starter: Auto-Configuration, @Conditional and Properties	a starter split into an auto-configuration module and a starter, the AutoConfiguration.imports registration, @ConditionalOn* conditions, configuration metadata, and tests with ApplicationContextRunner
core-events/	Spring Application Events: @EventListener, @TransactionalEventListener and Async Events	publishing and consuming events, listener ordering, conditional listeners, AFTER_COMMIT behaviour proven with a rollback, and async listeners
multi-datasource/	Multiple DataSources in Spring Boot 4 with Spring Data JPA	two H2 databases each with its own DataSource, Flyway, EntityManagerFactory and transaction manager, six ways of getting the wiring wrong, plain vs named @Transactional, and ChainedTransactionManager under a failing commit
i18n/	Internationalization (i18n) in Spring Boot 4: MessageSource, LocaleResolver and Localized ProblemDetail	English, Marathi and Hindi message bundles, Accept-Language vs a cookie resolver, apostrophes and Devanagari digits in MessageFormat, the JVM default locale as a hidden fallback, localized validation messages and ProblemDetail titles
aot-cache/	Faster Spring Boot Startup with the JDK AOT Cache (JEP 514/515): Benchmarks vs CDS and Native	the JDK 25 AOT cache trained two ways, against a plain JVM, AppCDS, Spring's own AOT output and a GraalVM native image (ten interleaved rounds each), first-request latency, 24 cases where the cache silently stops applying (jar, timestamp, JVM build, ZGC, compact headers), -XX:AOTMode=on, Docker layer arithmetic

core-di, core-beans, custom-starter, core-events, multi-datasource, i18n and aot-cache are the exception: they have no docs/ folder. Their deeper material lives in collapsible sections inside the articles themselves, and their captured output sits in a top-level output/ directory instead of docs/output/.

Articles whose text is kept here rather than only on the blog have it under <directory>/post/ — post.md for the body and meta.md for the title, excerpt and categories.

Running any of them

Each project needs a JDK 25 and Maven 3.9. redis also needs redis-server and redis-cli on the PATH. docker-images and observability also need Docker, and kubernetes-deployment Docker plus a Kubernetes cluster; their READMEs list the rest:

cd <directory>
export JAVA_HOME=/path/to/jdk-25
mvn -DskipTests package
./scripts/run-all.sh          # regenerate every transcript under docs/output/

Licence

MIT — see LICENSE.