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# 7. IDE metadata, and the JDK 23 change that silently breaks it

Transcript: [`05-metadata-generation.txt`](output/05-metadata-generation.txt).

## What the metadata is

`spring-boot-configuration-processor` is an annotation processor. At compile time it reads your
`@ConfigurationProperties` types and writes `META-INF/spring-configuration-metadata.json`:

```json
{
  "groups": [
    { "name": "demo.mail", "type": "com.ankurm.configprops.props.MailProperties" }
  ],
  "properties": [
    { "name": "demo.mail.port", "type": "java.lang.Integer", "defaultValue": 587 }
  ]
}
```

That file is what makes property names auto-complete in an IDE, and what shows the Javadoc on
a record component as hover documentation. Nothing at runtime reads it.

## The failure

Declaring the processor as a dependency — the way every tutorial written before 2024 shows —
stops working on JDK 23 and later:

```xml
<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-configuration-processor</artifactId>
  <optional>true</optional>
</dependency>
```

`javac` used to discover annotation processors on the classpath. That discovery was deprecated
with a warning in JDK 21 and switched off in JDK 23. A processor that is only on the classpath
is now simply not run.

The build still succeeds. The jar is still valid. The application behaves identically. The only
symptom is that property auto-completion quietly stops working, which is the kind of thing
people blame on the IDE.

Two compilations of identical sources with the identical processor jar:

```
A) javac -cp <deps>:spring-boot-configuration-processor.jar -d a $SOURCES
   spring-configuration-metadata.json files produced: 0

B) javac -proc:full -cp <deps>:spring-boot-configuration-processor.jar -d b $SOURCES
   spring-configuration-metadata.json files produced: 1
```

## The fix

Declare it as an annotation processor path, which puts it on `--processor-path` where discovery
was never disabled:

```xml
<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-compiler-plugin</artifactId>
  <configuration>
    <annotationProcessorPaths>
      <path>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-configuration-processor</artifactId>
        <version>${project.parent.version}</version>
      </path>
    </annotationProcessorPaths>
  </configuration>
</plugin>
```

`-proc:full` also works and is a smaller change, but it re-enables classpath scanning for every
processor, which is the behaviour that was turned off for a reason.

## Checking

`ls target/classes/META-INF/spring-configuration-metadata.json`. If it is not there, the
processor did not run. Add `@ConfigurationProperties` metadata to your build's definition of
done, because nothing else will tell you.