Validate that @ConfigMapping and @ConfigProperties can only be placed in interfaces and classes

implementation/src/main/java/io/smallrye/config/ConfigMappingLoader.java

@@ -7,6 +7,8 @@
 import java.util.ArrayList;
 import java.util.List;
 
+import org.eclipse.microprofile.config.inject.ConfigProperties;
+
 import io.smallrye.common.classloader.ClassDefiner;
 
 public final class ConfigMappingLoader {
@@ -39,6 +41,8 @@ static ConfigMappingInterface getConfigMappingInterface(final Class<?> type) {
     }
 
     static Class<?> getConfigMappingClass(final Class<?> type) {
+        validateAnnotations(type);
+
         final ConfigMappingClass configMappingClass = ConfigMappingClass.getConfigurationClass(type);
         if (configMappingClass == null) {
             return type;
@@ -86,6 +90,16 @@ static Class<?> loadClass(final Class<?> parent, final ConfigMappingMetadata con
         }
     }
 
+    static void validateAnnotations(Class<?> type) {
+        if (!type.isInterface() && type.isAnnotationPresent(ConfigMapping.class)) {
+            throw ConfigMessages.msg.mappingAnnotationNotSupportedInClass(type);
+        }
+
+        if (type.isInterface() && type.isAnnotationPresent(ConfigProperties.class)) {
+            throw ConfigMessages.msg.propertiesAnnotationNotSupportedInInterface(type);
+        }
+    }
+
     /**
      * Do not remove this method or inline it. It is keep separate on purpose, so it is easier to substitute it with
      * the GraalVM API for native image compilation.

implementation/src/main/java/io/smallrye/config/ConfigMessages.java

@@ -146,4 +146,9 @@ IllegalArgumentException converterException(@Cause Throwable converterException,
     @Message(id = 42, value = "Value does not match the expected map format \"<key1>=<value1>;<key2>=<value2>...\" (value was \"%s\")")
     NoSuchElementException valueNotMatchMapFormat(String value);
 
+    @Message(id = 43, value = "The @ConfigMapping annotation can only be placed in interfaces, %s is a class")
+    IllegalStateException mappingAnnotationNotSupportedInClass(Class<?> type);
+
+    @Message(id = 44, value = "The @ConfigProperties annotation can only be placed in classes, %s is an interface")
+    IllegalStateException propertiesAnnotationNotSupportedInInterface(Class<?> type);
 }

implementation/src/test/java/io/smallrye/config/ConfigMappingsTest.java

@@ -8,6 +8,7 @@
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import java.util.Map;
 import java.util.Objects;
@@ -128,6 +129,31 @@ void validateDisableOnConfigProperties() {
         assertEquals(8080, server.port);
     }
 
+    @Test
+    void validateAnnotations() {
+        SmallRyeConfig config = new SmallRyeConfigBuilder().build();
+        IllegalStateException exception = assertThrows(IllegalStateException.class,
+                () -> registerConfigMappings(config, singleton(configClassWithPrefix(ServerMappingClass.class, "server"))));
+        assertTrue(exception.getMessage()
+                .startsWith("SRCFG00043: The @ConfigMapping annotation can only be placed in interfaces"));
+
+        exception = assertThrows(IllegalStateException.class,
+                () -> new SmallRyeConfigBuilder().withMapping(ServerMappingClass.class, "server").build());
+        assertTrue(exception.getMessage()
+                .startsWith("SRCFG00043: The @ConfigMapping annotation can only be placed in interfaces"));
+
+        exception = assertThrows(IllegalStateException.class,
+                () -> registerConfigMappings(config,
+                        singleton(configClassWithPrefix(ServerPropertiesInterface.class, "server"))));
+        assertTrue(exception.getMessage()
+                .startsWith("SRCFG00044: The @ConfigProperties annotation can only be placed in classes"));
+
+        exception = assertThrows(IllegalStateException.class,
+                () -> new SmallRyeConfigBuilder().withMapping(ServerPropertiesInterface.class, "server").build());
+        assertTrue(exception.getMessage()
+                .startsWith("SRCFG00044: The @ConfigProperties annotation can only be placed in classes"));
+    }
+
     @ConfigMapping(prefix = "server")
     interface Server {
         String host();
@@ -150,6 +176,16 @@ static class ServerClass {
         Map<String, Integer> numbers;
     }
 
+    @ConfigMapping(prefix = "server")
+    static class ServerMappingClass {
+        String host;
+    }
+
+    @ConfigProperties(prefix = "server")
+    interface ServerPropertiesInterface {
+        String host();
+    }
+
     static class Version {
         int id;
         String name;
@@ -176,7 +212,6 @@ public int hashCode() {
     }
 
     static class VersionConverter implements Converter<Version> {
-
         @Override
         public Version convert(String value) {
             return new Version(Integer.parseInt(value.substring(0, 1)), value.substring(2));