Working with Configuration

Use @ConfigMapping Interfaces (Not Legacy Classes)

Quarkus configuration must use @ConfigMapping interfaces. The legacy @ConfigRoot-annotated classes with @ConfigItem have been fully removed (see adr/0008-phasing-out-config-classes.adoc).

Correct (current approach)

@ConfigMapping(prefix = "quarkus.my-extension")
@ConfigRoot(phase = ConfigPhase.RUN_TIME)
public interface MyExtensionRuntimeConfig {

    /** Description of the property. */
    @WithDefault("default-value")
    String myProperty();

    /** Optional property (absent means not set by the user). */
    Optional<String> optionalProperty();
}

Do NOT Use Optional to Define Default Values

Never use Optional and provide a default via .orElse(...) in application code. The default must be declared in the config mapping itself via @WithDefault:

// BAD — do not do this
String name = config.optName().orElse("theDefaultName");

// GOOD — declare the default in the config mapping
@WithDefault("theDefaultName")
String name();

Non-root @ConfigMapping (without @ConfigRoot)

@ConfigMapping can also be used without @ConfigRoot for non-root mappings. These are not auto-discovered — you must register them explicitly (e.g., via @Inject in a CDI bean or programmatically with SmallRye Config). Use this when mapping a sub-tree of configuration that does not represent an extension's top-level config root.

@ConfigMapping(prefix = "some.custom.prefix")
public interface MyCustomConfig {

    /** A non-root config property. */
    String someValue();
}

Wrong (legacy approach, do not use)

// DO NOT use this pattern
@ConfigRoot(name = "my-extension", phase = ConfigPhase.RUN_TIME)
public class MyExtensionConfig {
    @ConfigItem(defaultValue = "default-value")
    String myProperty;
}

Config Phases

@ConfigRoot defaults to BUILD_TIME when no phase is specified. Always set the phase explicitly — omitting it for a config injected at runtime will cause UnsatisfiedResolutionException because BUILD_TIME configs are not registered as CDI beans.

• BUILD_TIME — Baked into the binary at build time, immutable at runtime. Use for settings that affect code generation. Only available in @BuildStep methods — not injectable via @Inject in runtime CDI beans.
• BUILD_AND_RUN_TIME_FIXED — Read at build time, also available at runtime. Cannot change without rebuilding. Available in both @BuildStep methods and runtime CDI beans.
• RUN_TIME — Read at application startup. Can change between runs. Available in runtime CDI beans via @Inject. Not available in @BuildStep methods.

Naming

• Config interfaces: <Feature>Config.java, or split as <Feature>BuildTimeConfig.java / <Feature>RuntimeConfig.java
• Config prefix: quarkus.<extension-name>

Annotation Processor (quarkus-extension-processor)

The quarkus-extension-processor annotation processor is required for all extension modules, not just those with @ConfigRoot. It generates metadata files (including META-INF/quarkus-config-roots.list for config discovery) that Quarkus needs to wire extensions correctly. Without it, most extension functionality will silently fail.

Both runtime/ and deployment/ POMs must include:

<plugin>
    <artifactId>maven-compiler-plugin</artifactId>
    <executions>
        <execution>
            <id>default-compile</id>
            <configuration>
                <annotationProcessorPaths>
                    <path>
                        <groupId>io.quarkus</groupId>
                        <artifactId>quarkus-extension-processor</artifactId>
                        <version>${project.version}</version>
                    </path>
                </annotationProcessorPaths>
            </configuration>
        </execution>
    </executions>
</plugin>

Most modules already have this. If you create a new module or notice missing metadata, verify the annotation processor is present in its pom.xml.

Key Rules

• Always specify phase explicitly on @ConfigRoot — the default is BUILD_TIME, which is not injectable via CDI at runtime
• Configuration is immutable — never mutate config objects
• Use RUN_TIME phase unless the config genuinely affects build-time behavior
• Do NOT access RUN_TIME config during build steps — it is not available yet
• BUILD_TIME config changes require a rebuild
• When adding new quarkus.* properties, always register them in a @ConfigMapping interface. Reading them programmatically via ConfigProvider.getConfig().getOptionalValue(...) is fine, but the corresponding key must still be defined in a @ConfigMapping — otherwise SmallRye Config will emit "unknown property" warnings at startup.