ConfigurationClassEnhancer leniently allows for null bean references

Issue: SPR-15829

Author: jhoeller

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

@@ -309,6 +309,7 @@ private static class BeanMethodInterceptor implements MethodInterceptor, Conditi
 		 * super implementation of the proxied method i.e., the actual {@code @Bean} method
 		 */
 		@Override
+		@Nullable
 		public Object intercept(Object enhancedConfigInstance, Method beanMethod, Object[] beanMethodArgs,
 					MethodProxy cglibMethodProxy) throws Throwable {
 
@@ -360,10 +361,11 @@ public Object intercept(Object enhancedConfigInstance, Method beanMethod, Object
 				return cglibMethodProxy.invokeSuper(enhancedConfigInstance, beanMethodArgs);
 			}
 
-			return obtainBeanInstanceFromFactory(beanMethod, beanMethodArgs, beanFactory, beanName);
+			return resolveBeanReference(beanMethod, beanMethodArgs, beanFactory, beanName);
 		}
 
-		private Object obtainBeanInstanceFromFactory(Method beanMethod, Object[] beanMethodArgs,
+		@Nullable
+		private Object resolveBeanReference(Method beanMethod, Object[] beanMethodArgs,
 				ConfigurableBeanFactory beanFactory, String beanName) {
 
 			// The user (i.e. not the factory) is requesting this bean through a call to
@@ -390,18 +392,29 @@ private Object obtainBeanInstanceFromFactory(Method beanMethod, Object[] beanMet
 				Object beanInstance = (useArgs ? beanFactory.getBean(beanName, beanMethodArgs) :
 						beanFactory.getBean(beanName));
 				if (!ClassUtils.isAssignableValue(beanMethod.getReturnType(), beanInstance)) {
-					String msg = String.format("@Bean method %s.%s called as a bean reference " +
+					if (beanInstance.equals(null)) {
+						if (logger.isDebugEnabled()) {
+							logger.debug(String.format("@Bean method %s.%s called as bean reference " +
+									"for type [%s] returned null bean; resolving to null value.",
+									beanMethod.getDeclaringClass().getSimpleName(), beanMethod.getName(),
+									beanMethod.getReturnType().getName()));
+						}
+						beanInstance = null;
+					}
+					else {
+						String msg = String.format("@Bean method %s.%s called as bean reference " +
 								"for type [%s] but overridden by non-compatible bean instance of type [%s].",
 								beanMethod.getDeclaringClass().getSimpleName(), beanMethod.getName(),
 								beanMethod.getReturnType().getName(), beanInstance.getClass().getName());
-					try {
-						BeanDefinition beanDefinition = beanFactory.getMergedBeanDefinition(beanName);
-						msg += " Overriding bean of same name declared in: " + beanDefinition.getResourceDescription();
-					}
-					catch (NoSuchBeanDefinitionException ex) {
-						// Ignore - simply no detailed message then.
+						try {
+							BeanDefinition beanDefinition = beanFactory.getMergedBeanDefinition(beanName);
+							msg += " Overriding bean of same name declared in: " + beanDefinition.getResourceDescription();
+						}
+						catch (NoSuchBeanDefinitionException ex) {
+							// Ignore - simply no detailed message then.
+						}
+						throw new IllegalStateException(msg);
 					}
-					throw new IllegalStateException(msg);
 				}
 				Method currentlyInvoked = SimpleInstantiationStrategy.getCurrentlyInvokedFactoryMethod();
 				if (currentlyInvoked != null) {

spring-context/src/test/java/org/springframework/context/annotation/configuration/ConfigurationClassProcessingTests.java

@@ -205,6 +205,15 @@ public void configurationWithPrototypeScopedBeans() {
 		assertNotSame(bar.getSpouse(), baz);
 	}
 
+	@Test
+	public void configurationWithNullReference() {
+		BeanFactory factory = initBeanFactory(ConfigWithNullReference.class);
+
+		TestBean foo = factory.getBean("foo", TestBean.class);
+		assertTrue(factory.getBean("bar").equals(null));
+		assertNull(foo.getSpouse());
+	}
+
 	@Test
 	public void configurationWithAdaptivePrototypes() {
 		AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
@@ -307,7 +316,7 @@ static class ConfigWithBeanWithAliases {
 
 		static TestBean testBean = new TestBean(ConfigWithBeanWithAliases.class.getSimpleName());
 
-		@Bean(name = { "name1", "alias1", "alias2", "alias3" })
+		@Bean(name = {"name1", "alias1", "alias2", "alias3"})
 		public TestBean methodName() {
 			return testBean;
 		}
@@ -319,7 +328,7 @@ static class ConfigWithBeanWithAliasesConfiguredViaValueAttribute {
 
 		static TestBean testBean = new TestBean(ConfigWithBeanWithAliasesConfiguredViaValueAttribute.class.getSimpleName());
 
-		@Bean({ "enigma", "alias1", "alias2", "alias3" })
+		@Bean({"enigma", "alias1", "alias2", "alias3"})
 		public TestBean methodName() {
 			return testBean;
 		}
@@ -415,6 +424,16 @@ public TestBean adaptive2(DependencyDescriptor dd) {
 	}
 
 
+	@Configuration
+	static class ConfigWithNullReference extends ConfigWithPrototypeBean {
+
+		@Override
+		public TestBean bar() {
+			return null;
+		}
+	}
+
+
 	@Scope("prototype")
 	static class AdaptiveInjectionPoints {