Every console block quoted in the article came out of docs/output/. 03-08 are raw curl captures against a running instance; 01, 02 and 09 are real build/startup transcripts. BookControllerTest.kt turns the null-safety claims into assertions, so a change in Spring, Kotlin or JSpecify behaviour breaks the build, not just the prose.

There is deliberately no docs/NN-topic.md chapter folder: the deeper material lives in collapsible "going deeper" sections inside the article itself, next to the paragraph each one extends. docs/output/ (captured transcripts) is the one thing that still lives under docs/.

Versions


Spring Boot	4.1.1
Spring Framework	7.0.9
Kotlin	2.3.21 (Spring Boot 4.1.1's own managed version; latest actual GA is 2.4.20 — see the article)
kotlinx.coroutines	1.10.2
jackson-module-kotlin	2.22.3 (not BOM-managed; pin it explicitly)
JSpecify	1.0.0
JDK	25 (GraalVM CE 25.4.4.1.1)
Maven	3.9

Quickstart

export JAVA_HOME=/path/to/jdk-25
mvn test                     # runs BookControllerTest, re-verifying every null-safety claim
mvn spring-boot:run           # starts the app on :8090 with 3 seeded books

What's demonstrated

Area	Where
allopen/noarg via the `spring`/`jpa` kotlin compiler plugins	`pom.xml`'s `kotlin-maven-plugin` config; `BookService.kt`, `Book.kt`, `Application.kt`
Real build failure without the `spring` plugin (`@Configuration class may not be final`, then `@Bean method must not be private or final`)	`docs/output/01-build-failure-without-allopen.txt`
Clean startup with both plugins restored	`docs/output/02-fixed-startup.txt`
`suspend fun` / `Flow<T>` controller methods under plain `spring-boot-starter-webmvc` (Tomcat, not WebFlux)	`BookController.kt` (`/books`, `/books/flow`)
Which thread actually resumes a suspended controller method	`/books/thread-check`, `docs/output/05-thread-check.txt`
Kotlin platform types (`String!`) for un-annotated Java APIs	`legacy/LegacyJavaApi.java`, `/books/null-demo/legacy/{id}`, `docs/output/06-null-demo-legacy.txt`
JSpecify `@NullMarked`/`@Nullable` interop, and that it is a static contract, not a runtime guarantee	`legacy/NullMarkedJavaApi.java`, `/books/null-demo/jspecify/{id}`, `/books/null-demo/jspecify-honest/{id}`, `docs/output/07-08`
Mixed Kotlin+Java compilation in one Maven module	`pom.xml` (`kotlin-maven-plugin` compiles `src/main/kotlin` + `src/main/java`; `maven-compiler-plugin`'s `default-compile`/`default-testCompile` disabled, `java-compile` picks up the default `src/main/java`)
`TestRestTemplate` no longer shipping in Spring Boot 4.1's `spring-boot-test` module	`docs/output/09-no-testresttemplate.txt`, `BookControllerTest.kt` (plain `java.net.http.HttpClient` instead)

Captured output

File	What it shows
`01-build-failure-without-allopen.txt`	both real failures when the `spring` compiler plugin is removed
`02-fixed-startup.txt`	clean startup once it's restored
`03-books-endpoint.txt`	`GET /books`, a `suspend fun` controller method, real JSON
`04-books-flow-endpoint.txt`	`GET /books/flow`, a `Flow<Book>` controller method
`05-thread-check.txt`	three runs of `/books/thread-check`: before/after thread names across a real suspension point
`06-null-demo-legacy.txt`	the platform-type NPE, with no compiler hint anywhere, plus its stack trace
`07-null-demo-jspecify.txt`	the `@NullMarked` Java method that still NPEs, plus its stack trace
`08-null-demo-jspecify-honest.txt`	the `@Nullable`-annotated method the Kotlin compiler forces a null check on — no NPE
`09-no-testresttemplate.txt`	`TestRestTemplate` verified absent from `spring-boot-test-4.1.1.jar`

Endpoints default to http://localhost:8090. H2 is in-memory (jdbc:h2:mem:kotlindemo), reseeded on every restart with three books, one of them with a null title.