diff --git a/spring-boot/src/main/java/org/springframework/boot/BeanDefinitionLoader.java b/spring-boot/src/main/java/org/springframework/boot/BeanDefinitionLoader.java
index 4546b76a50..be3149894b 100644
--- a/spring-boot/src/main/java/org/springframework/boot/BeanDefinitionLoader.java
+++ b/spring-boot/src/main/java/org/springframework/boot/BeanDefinitionLoader.java
@@ -326,7 +326,8 @@ class BeanDefinitionLoader {
 	/**
 	 * Source for Bean definitions defined in Groovy.
 	 */
-	interface GroovyBeanDefinitionSource {
+	@FunctionalInterface
+	protected interface GroovyBeanDefinitionSource {
 
 		Closure<?> getBeans();