Detect nested configuration classes even for empty outer classes

Issue: SPR-16839

spring-context/src/main/java/org/springframework/context/annotation/ConfigurationClassUtils.java

@@ -78,7 +78,9 @@ abstract class ConfigurationClassUtils {
 	 * @param metadataReaderFactory the current factory in use by the caller
 	 * @return whether the candidate qualifies as (any kind of) configuration class
 	 */
-	public static boolean checkConfigurationClassCandidate(BeanDefinition beanDef, MetadataReaderFactory metadataReaderFactory) {
+	public static boolean checkConfigurationClassCandidate(
+			BeanDefinition beanDef, MetadataReaderFactory metadataReaderFactory) {
+
 		String className = beanDef.getBeanClassName();
 		if (className == null || beanDef.getFactoryMethodName() != null) {
 			return false;
@@ -103,7 +105,8 @@ abstract class ConfigurationClassUtils {
 			}
 			catch (IOException ex) {
 				if (logger.isDebugEnabled()) {
-					logger.debug("Could not find class file for introspecting configuration annotations: " + className, ex);
+					logger.debug("Could not find class file for introspecting configuration annotations: " +
+							className, ex);
 				}
 				return false;
 			}
@@ -116,7 +119,7 @@ abstract class ConfigurationClassUtils {
 			beanDef.setAttribute(CONFIGURATION_CLASS_ATTRIBUTE, CONFIGURATION_CLASS_LITE);
 		}
 		else {
-			return false;
+			return hasNestedConfigurationClass(metadata, metadataReaderFactory);
 		}
 
 		// It's a full or lite configuration candidate... Let's determine the order value, if any.
@@ -128,6 +131,40 @@ abstract class ConfigurationClassUtils {
 		return true;
 	}
 
+	/**
+	 * Check whether the specified class declares a nested configuration class.
+	 */
+	private static boolean hasNestedConfigurationClass(
+			AnnotationMetadata metadata, MetadataReaderFactory metadataReaderFactory) {
+
+		// Potentially nested configuration classes...
+		if (metadata instanceof StandardAnnotationMetadata) {
+			Class<?> beanClass = ((StandardAnnotationMetadata) metadata).getIntrospectedClass();
+			for (Class<?> memberClass : beanClass.getDeclaredClasses()) {
+				if (isConfigurationCandidate(new StandardAnnotationMetadata(memberClass))) {
+					return true;
+				}
+			}
+		}
+		else {
+			for (String memberName : metadata.getMemberClassNames()) {
+				try {
+					MetadataReader metadataReader = metadataReaderFactory.getMetadataReader(memberName);
+					if (isConfigurationCandidate(metadataReader.getAnnotationMetadata())) {
+						return true;
+					}
+				}
+				catch (IOException ex) {
+					if (logger.isDebugEnabled()) {
+						logger.debug("Could not find class file for introspecting configuration annotations: " +
+								memberName, ex);
+					}
+				}
+			}
+		}
+		return false;
+	}
+
 	/**
 	 * Check the given metadata for a configuration class candidate
 	 * (or nested component class declared within a configuration/component class).

spring-context/src/test/java/org/springframework/context/annotation/ImportVersusDirectRegistrationTests.java

@@ -0,0 +1,78 @@
+/*
+ * Copyright 2002-2018 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.context.annotation;
+
+import org.junit.Test;
+
+import org.springframework.beans.factory.support.RootBeanDefinition;
+
+/**
+ * @author Andy Wilkinson
+ */
+public class ImportVersusDirectRegistrationTests {
+
+	@Test
+	public void thingIsAvailableWhenOuterConfigurationIsRegisteredDirectly() {
+		try (AnnotationConfigApplicationContext directRegistration = new AnnotationConfigApplicationContext()) {
+			directRegistration.register(AccidentalLiteConfiguration.class);
+			directRegistration.refresh();
+			directRegistration.getBean(Thing.class);
+		}
+	}
+
+	@Test
+	public void thingIsAvailableWhenOuterConfigurationIsRegisteredWithClassName() {
+		try (AnnotationConfigApplicationContext directRegistration = new AnnotationConfigApplicationContext()) {
+			directRegistration.registerBeanDefinition("config",
+					new RootBeanDefinition(AccidentalLiteConfiguration.class.getName()));
+			directRegistration.refresh();
+			directRegistration.getBean(Thing.class);
+		}
+	}
+
+	@Test
+	public void thingIsAvailableWhenOuterConfigurationIsImported() {
+		try (AnnotationConfigApplicationContext viaImport = new AnnotationConfigApplicationContext()) {
+			viaImport.register(Importer.class);
+			viaImport.refresh();
+			viaImport.getBean(Thing.class);
+		}
+	}
+
+}
+
+
+@Import(AccidentalLiteConfiguration.class)
+class Importer {
+}
+
+
+class AccidentalLiteConfiguration {
+
+	@Configuration
+	class InnerConfiguration {
+
+		@Bean
+		public Thing thing() {
+			return new Thing();
+		}
+	}
+}
+
+
+class Thing {
+}