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Classloading and Runtime-Dev Modules

The Two ClassLoaders

1. Deployment ClassLoader — Used at build time. Has access to all dependencies including deployment-only libraries.
2. Runtime ClassLoader — Used at application runtime. Only has access to runtime dependencies.

Core Rules

• Runtime code MUST NOT reference deployment classes. Violations cause ClassNotFoundException at runtime.
• Deployment code CAN reference runtime classes.
• Recorders bridge the gap. A @Recorder lives in the runtime module but is invoked from deployment build steps.

Runtime-Dev Modules

Extensions providing dev-mode-only functionality (e.g., Dev UI pages) use a runtime-dev/ module. These classes are only loaded in dev mode, not in production. This requires two steps:

1. Register as conditional dev dependency

In the runtime module's pom.xml, configure quarkus-extension-maven-plugin:

<plugin>
    <groupId>io.quarkus</groupId>
    <artifactId>quarkus-extension-maven-plugin</artifactId>
    <executions>
        <execution>
            <phase>process-resources</phase>
            <goals>
                <goal>extension-descriptor</goal>
            </goals>
            <configuration>
                <conditionalDevDependencies>
                    <artifact>${project.groupId}:${project.artifactId}-dev:${project.version}</artifact>
                </conditionalDevDependencies>
            </configuration>
        </execution>
    </executions>
</plugin>

Without this, runtime-dev classes will NOT be loaded in dev mode — you get ClassNotFoundException.

2. Guard build steps with IsLocalDevelopment

@BuildSteps(onlyIf = IsLocalDevelopment.class)
class MyExtensionDevUIProcessor {
    // Dev UI build steps here
}

Why This Matters

Forgetting conditionalDevDependencies is particularly confusing because:

• The code compiles without errors
• It only fails at runtime in dev mode
• The error message doesn't hint at the missing configuration

Common Mistakes

• Importing a deployment class in runtime code
• Using a build-time-only library in a recorder
• Accessing runtime config during augmentation (build time)
• Putting user-facing API classes in deployment instead of runtime
• Forgetting conditionalDevDependencies for runtime-dev modules

Source: quarkusio/quarkus — distributed by TomeVault.