spring-boot-demo/custom-starter/README.md

custom-starter

A Spring Boot 4 starter written from scratch, and the application that uses it. Companion to Write Your Own Spring Boot 4 Starter: Auto-Configuration, @Conditional and Properties.

The starter provides one thing, a Masker that hides all but the last few characters of a value, plus an optional Jackson integration that writes a Sensitive value masked. The subject is the packaging, not the masking.

Spring Boot	4.1.1 (parent pom, published 20 Aug 2026)
Spring Framework	7.0.9
Java	25
Maven	3.9

There is no docs/ folder. The explanations live in the article, in collapsible sections next to the paragraph each one extends, and every console block in the article is quoted from a file in output/.

Layout

Module	What it is
masker-spring-boot-autoconfigure/	MaskerAutoConfiguration, MaskerJacksonAutoConfiguration, MaskerProperties, the AutoConfiguration.imports file, the configuration metadata, and the tests
masker-spring-boot-starter/	The dependency an application adds. No code, only a pom
masker-demo/	An application that adds the starter and nothing else

masker-spring-boot-autoconfigure/src/test/java/com/ankurm/traps/ holds auto-configurations that are wrong on purpose. src/broken/ holds a source file that is made not to compile on purpose. Neither is part of the shipped jar.

Run it

export JAVA_HOME=/path/to/jdk-25
./scripts/run-all.sh          # tests, then the jar and failure captures: regenerates all of output/
mvn -B test                   # only the tests

Captured output

File	What it shows	Made by
01	the auto-configuration with no properties and no user beans	MaskerAutoConfigurationTests
02	masker.* properties and masker.enabled=false	MaskerAutoConfigurationTests
03	@ConditionalOnMissingBean backing off, and the registration order that decides it	MaskerAutoConfigurationTests
04	the Jackson module registered, a Sensitive written masked, the recorded condition outcomes	JacksonIntegrationTests
05	Jackson hidden with FilteredClassLoader: no failure, no module	JacksonIntegrationTests
06	@ConditionalOnBean with and without an ordering constraint, and the sorted order	ConditionTrapTests
07	@ConditionalOnClass on a @Bean method, on a classpath with no Jackson	capture-jar-facts.sh
08	an auto-configuration missing from the imports file: passes a runner test, absent everywhere	ForgottenImportsEntryTests
09	ImportCandidates, the guard test for the omission	ForgottenImportsEntryTests
10	an application with only the starter dependency, and Boot's recorded conditions	DemoApplicationTests
11	the same application with properties, then switched off	DemoApplicationTests
12	what is in the two jars, and what the starter pulls in	capture-jar-facts.sh
13	the generated and the hand-written configuration metadata, and the auto-configuration conditions metadata	capture-jar-facts.sh
14	Boot 4's split of spring-boot-autoconfigure, and the Boot 3 import that no longer compiles	capture-jar-facts.sh
15	the processor path failing when the version is taken from your own parent	capture-jar-facts.sh
17	which Boot 4.1.1 jar holds each class this starter imports	capture-jar-facts.sh
16	@ConditionalOnProperty without matchIfMissing	ConditionTrapTests

Transcripts are deterministic; only build-directory paths are rewritten, to <custom-starter>.