BeanMethodInterceptor does not pass on null arguments for singleton beans

Issue: SPR-13887

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

 /*
- * Copyright 2002-2015 the original author or authors.
+ * Copyright 2002-2016 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.
@@ -364,8 +364,20 @@ class ConfigurationClassEnhancer {
 					if (alreadyInCreation) {
 						beanFactory.setCurrentlyInCreation(beanName, false);
 					}
-					Object beanInstance = (!ObjectUtils.isEmpty(beanMethodArgs) ?
-							beanFactory.getBean(beanName, beanMethodArgs) : beanFactory.getBean(beanName));
+					boolean useArgs = !ObjectUtils.isEmpty(beanMethodArgs);
+					if (useArgs && beanFactory.isSingleton(beanName)) {
+						// Stubbed null arguments just for reference purposes,
+						// expecting them to be autowired for regular singleton references?
+						// A safe assumption since @Bean singleton arguments cannot be optional...
+						for (Object arg : beanMethodArgs) {
+							if (arg == null) {
+								useArgs = false;
+								break;
+							}
+						}
+					}
+					Object beanInstance = (useArgs ? beanFactory.getBean(beanName, beanMethodArgs) :
+							beanFactory.getBean(beanName));
 					if (beanInstance != null && !ClassUtils.isAssignableValue(beanMethod.getReturnType(), beanInstance)) {
 						String msg = String.format("@Bean method %s.%s called as a bean reference " +
 									"for type [%s] but overridden by non-compatible bean instance of type [%s].",

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

 /*
- * Copyright 2002-2015 the original author or authors.
+ * Copyright 2002-2016 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.
@@ -592,17 +592,23 @@ public class ConfigurationClassPostProcessorTests {
 	}
 
 	@Test
-	public void testPrototypeArgumentsThroughBeanMethodCall() {
+	public void testPrototypeArgumentThroughBeanMethodCall() {
 		ApplicationContext ctx = new AnnotationConfigApplicationContext(BeanArgumentConfigWithPrototype.class);
 		ctx.getBean(FooFactory.class).createFoo(new BarArgument());
 	}
 
 	@Test
-	public void testSingletonArgumentsThroughBeanMethodCall() {
+	public void testSingletonArgumentThroughBeanMethodCall() {
 		ApplicationContext ctx = new AnnotationConfigApplicationContext(BeanArgumentConfigWithSingleton.class);
 		ctx.getBean(FooFactory.class).createFoo(new BarArgument());
 	}
 
+	@Test
+	public void testNullArgumentThroughBeanMethodCall() {
+		ApplicationContext ctx = new AnnotationConfigApplicationContext(BeanArgumentConfigWithNull.class);
+		ctx.getBean("aFoo");
+	}
+
 
 	// -------------------------------------------------------------------------
 
@@ -1137,7 +1143,7 @@ public class ConfigurationClassPostProcessorTests {
 
 		@Bean
 		@Scope("prototype")
-		public DependingFoo foo(final BarArgument bar) {
+		public DependingFoo foo(BarArgument bar) {
 			return new DependingFoo(bar);
 		}
 
@@ -1145,7 +1151,7 @@ public class ConfigurationClassPostProcessorTests {
 		public FooFactory fooFactory() {
 			return new FooFactory() {
 				@Override
-				public DependingFoo createFoo(final BarArgument bar) {
+				public DependingFoo createFoo(BarArgument bar) {
 					return foo(bar);
 				}
 			};
@@ -1156,7 +1162,7 @@ public class ConfigurationClassPostProcessorTests {
 	static class BeanArgumentConfigWithSingleton {
 
 		@Bean @Lazy
-		public DependingFoo foo(final BarArgument bar) {
+		public DependingFoo foo(BarArgument bar) {
 			return new DependingFoo(bar);
 		}
 
@@ -1164,19 +1170,39 @@ public class ConfigurationClassPostProcessorTests {
 		public FooFactory fooFactory() {
 			return new FooFactory() {
 				@Override
-				public DependingFoo createFoo(final BarArgument bar) {
+				public DependingFoo createFoo(BarArgument bar) {
 					return foo(bar);
 				}
 			};
 		}
 	}
 
+	@Configuration
+	static class BeanArgumentConfigWithNull {
+
+		@Bean
+		public DependingFoo aFoo() {
+			return foo(null);
+		}
+
+		@Bean @Lazy
+		public DependingFoo foo(BarArgument bar) {
+			return new DependingFoo(bar);
+		}
+
+		@Bean
+		public BarArgument bar() {
+			return new BarArgument();
+		}
+	}
+
 	static class BarArgument {
 	}
 
 	static class DependingFoo {
 
 		DependingFoo(BarArgument bar) {
+			Assert.notNull(bar);
 		}
 	}