Validate that @ConfigMapping annotation can only be placed in interfaces

[angelozerr]

@ConfigMaping can be used only for interface and not for class.

• Using interface is valid:

@ConfigMapping(prefix = "server")
public interface Server {

}

• Using class should higlight the Server as an error:

@ConfigMapping(prefix = "server")
public class Server {

}

Here the Quarkus stack trace when we try to annotated a Java class with ConfigMapping:

java.lang.IllegalStateException: SRCFG00043: The @ConfigMapping annotation can only be placed in interfaces, class org.acme.Server is a class
	at io.smallrye.config.ConfigMappingLoader.validateAnnotations(ConfigMappingLoader.java:104)
	at io.smallrye.config.ConfigMappingLoader.getConfigMappingClass(ConfigMappingLoader.java:44)
	at io.smallrye.config.ConfigMappingLoader.getConfigMappingInterface(ConfigMappingLoader.java:40)
	at io.smallrye.config.ConfigMappingLoader.getConfigMappingsMetadata(ConfigMappingLoader.java:34)
	at io.quarkus.deployment.configuration.ConfigMappingUtils.generateConfigClasses(ConfigMappingUtils.java:79)
	at io.quarkus.arc.deployment.ConfigBuildStep.generateConfigClasses(ConfigBuildStep.java:246)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.base/java.lang.reflect.Method.invoke(Method.java:566)
	at io.quarkus.deployment.ExtensionLoader$2.execute(ExtensionLoader.java:820)
	at io.quarkus.builder.BuildContext.run(BuildContext.java:277)
	at org.jboss.threads.ContextHandler$1.runWith(ContextHandler.java:18)
	at org.jboss.threads.EnhancedQueueExecutor$Task.run(EnhancedQueueExecutor.java:2449)
	at org.jboss.threads.EnhancedQueueExecutor$ThreadBody.run(EnhancedQueueExecutor.java:1478)
	at java.base/java.lang.Thread.run(Thread.java:834)
	at org.jboss.threads.JBossThread.run(JBossThread.java:501)
Resulted in: io.quarkus.builder.BuildException: Build failure: Build failed due to errors
	[error]: Build step io.quarkus.arc.deployment.ConfigBuildStep#generateConfigClasses threw an exception: java.lang.IllegalStateException: SRCFG00043: The @ConfigMapping annotation can only be placed in interfaces, class org.acme.Server is a class
	at io.smallrye.config.ConfigMappingLoader.validateAnnotations(ConfigMappingLoader.java:104)
	at io.smallrye.config.ConfigMappingLoader.getConfigMappingClass(ConfigMappingLoader.java:44)
	at io.smallrye.config.ConfigMappingLoader.getConfigMappingInterface(ConfigMappingLoader.java:40)
	at io.smallrye.config.ConfigMappingLoader.getConfigMappingsMetadata(ConfigMappingLoader.java:34)
	at io.quarkus.deployment.configuration.ConfigMappingUtils.generateConfigClasses(ConfigMappingUtils.java:79)
	at io.quarkus.arc.deployment.ConfigBuildStep.generateConfigClasses(ConfigBuildStep.java:246)