# @ConfigurationProperties vs @Value

Companion project for [**@ConfigurationProperties vs @Value in Spring Boot 4**](https://ankurm.com/)
on ankurm.com.

Every table and transcript quoted in that article was produced by running something here.
Three of them were produced twice, because the first answer was wrong — see
[chapter 2](docs/02-relaxed-binding.md).

## Versions

| | |
|---|---|
| Spring Boot | 4.1.1 |
| Spring Framework | 7.0.9 |
| JDK | Eclipse Temurin 25.0.4.1 (LTS) |
| Validation | Hibernate Validator via `spring-boot-starter-validation` |

## Quickstart

```bash
export JAVA_HOME=/path/to/jdk-25
mvn -DskipTests package
./scripts/run-all.sh          # regenerate every transcript in docs/output/
mvn test                      # 7 contract tests
```

## Profiles

| Profile | What it does |
|---|---|
| *(none)* | binds `demo.*` from `application.yaml` |
| `probe` | runs the relaxed-binding matrix and exits |
| `envprobe` | reports one property as seen by the binder and by `@Value` |
| `badvalidation` | values that violate every constraint, so startup fails |
| `csvlist` | the recipients list as a comma-separated string |

## Endpoints

| Endpoint | Purpose |
|---|---|
| `GET /diag/bound` | the bound objects, and the same values via `@Value` |
| `GET /diag/origin?name=` | every source holding a property, in precedence order, with file and line |

Both are diagnostics. Delete them before shipping.

## Documentation

1. [Two mechanisms, not two styles](docs/01-two-mechanisms.md)
2. [Relaxed binding, measured](docs/02-relaxed-binding.md)
3. [Getting the bean registered](docs/03-registration.md)
4. [Records, constructor binding and defaults](docs/04-records-and-defaults.md)
5. [Validation](docs/05-validation.md)
6. [When `@Value` is still the right answer](docs/06-when-value-still-wins.md)
7. [IDE metadata, and the JDK 23 change that silently breaks it](docs/07-ide-metadata.md)
8. [Diagnosing a value](docs/08-diagnosing-a-value.md)

## Captured output

| File | Produced by |
|---|---|
| [`00-versions.txt`](docs/output/00-versions.txt) | `scripts/demo-versions.sh` |
| [`01-relaxed-matrix.txt`](docs/output/01-relaxed-matrix.txt) | `scripts/demo-relaxed-matrix.sh` |
| [`02-env-var-binding.txt`](docs/output/02-env-var-binding.txt) | `scripts/demo-env-binding.sh` |
| [`03-value-vs-binding.txt`](docs/output/03-value-vs-binding.txt) | `scripts/demo-value-vs-binding.sh` |
| [`04-validation-failure.txt`](docs/output/04-validation-failure.txt) | `scripts/demo-validation.sh` |
| [`05-metadata-generation.txt`](docs/output/05-metadata-generation.txt) | `scripts/demo-metadata-generation.sh` |

## Three findings worth the trip

- **`@Value` gets relaxed binding inside Spring Boot.** The universal claim that it does not is
  a statement about the Spring Framework; Boot attaches a property source that changes it.
- **The configuration processor silently stops on JDK 23+** when declared as a plain
  dependency, so IDE auto-completion quietly dies while the build stays green.
- **`@Valid` is not what makes nested constraints run.** Boot's `ValidationBindHandler`
  validates every object it binds.