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Ankur Mhatre 86246dc860 Three new article modules: configuration binding, profiles and config data, Spring AOP

configuration-properties/  @ConfigurationProperties vs @Value on Spring Boot 4.1.1.
  The relaxed-binding matrix is generated by binding each spelling rather than
  transcribed, and re-checked against real processes -- the in-process probe was
  wrong twice before it was right. Records the three findings that came out of it:
  @Value does get relaxed resolution inside Spring Boot (Boot attaches
  ConfigurationPropertySources), the configuration processor silently stops
  generating metadata on JDK 23+ when declared as a plain dependency, and @Valid is
  not what makes nested constraints run.

profiles-and-config/       Precedence, profiles, spring.config.import and config trees.
  /precedence reports every source holding a property in rank order with file and
  line, which turns "my profile file had no effect" into a two-line answer. Also
  pins the counterintuitive one: an imported file outranks the file that imported it.

spring-aop/                Designators, proxy types, and aspects that do not fire.
  One advice per supported designator so the reference table is generated from real
  matches; all fourteen unsupported designators fed to the parser. Two corrections to
  the reference documentation: unsupported designators throw
  UnsupportedPointcutPrimitiveException (extends RuntimeException, not
  IllegalArgumentException), and spring-boot-starter-aop was renamed to
  spring-boot-starter-aspectj in Boot 4.

19 contract tests across the three modules, 15 captured transcripts, all regenerated
by scripts/run-all.sh. Verified on Spring Boot 4.1.1, Spring Framework 7.0.9,
JDK 25.0.4.1.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Gip4srpzMwjgoba6uEfbr5

2026-09-08 16:47:48 +00:00

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2. Relaxed binding, measured

Everything here is generated by RelaxedBindingProbe and re-checked against real processes by demo-env-binding.sh. Transcripts: 01-relaxed-matrix.txt, 02-env-var-binding.txt.

The canonical form

Every property has one canonical name: lower case, words separated by -, levels separated by .. For RelaxedProperties.apiKey under prefix demo.relaxed, that is demo.relaxed.api-key.

What matches it

Spelling	Source	Binder	`${}` plain Spring	`${}` in Spring Boot
`demo.relaxed.api-key`	file / `-D`	bound	bound	bound
`demo.relaxed.apiKey`	file / `-D`	bound	MISS	bound
`demo.relaxed.api_key`	file / `-D`	bound	MISS	bound
`demo.relaxed.APIKEY`	file / `-D`	bound	MISS	bound
`DEMO.RELAXED.API-KEY`	file / `-D`	bound	MISS	bound
`demo.relaxed.apikey`	file / `-D`	bound	MISS	bound
`demo.relaxed.api.key`	file / `-D`	MISS	MISS	MISS
`DEMO_RELAXED_API_KEY`	env var	bound	bound	bound
`DEMO_RELAXED_APIKEY`	env var	bound	MISS	bound
`demo_relaxed_api_key`	env var	bound	bound	bound

Reading it

Dashes, underscores and case are noise. ConfigurationPropertyName compares elements in a "uniform" form with separators removed and case folded, so api-key, apiKey, api_key and apikey are the same name.

A dot is not noise. demo.relaxed.api.key is four name elements; demo.relaxed.api-key is three. Nothing relaxed will ever join them. This is the one spelling that genuinely fails, and it is the one people write when they are guessing.

@Value gets relaxed resolution too — but only inside Spring Boot. The widely repeated claim that @Value does not support relaxed binding is a statement about the Spring Framework. Spring Boot calls ConfigurationPropertySources.attach(environment) while preparing every environment, which inserts a property source that resolves placeholders through the binder's name matching. Remove that call and the middle column is what you get.

That is worth knowing in both directions: you can rely on it in a Boot application, and you cannot rely on it in a plain ApplicationContext, a standalone Environment or a test that builds one by hand.

Environment variables

Both DEMO_RELAXED_API_KEY and DEMO_RELAXED_APIKEY work. Prefer the first: _ for every separator, upper case throughout. It is the form the binder generates when it goes looking, so it is the one that cannot depend on the reverse mapping.

A harness note that cost a wrong finding here: Spring Boot decides to apply environment-variable name mapping by comparing the property source's name against StandardEnvironment.SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME, not by checking its type. A SystemEnvironmentPropertySource registered under any other name is mapped with the ordinary rules. A test that builds one by hand and names it something descriptive will report that underscore spellings do not bind, and will be wrong.

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