Warn about non-static BeanDefinitionRegistryPostProcessor declarations on @Configuration classes

Issue: SPR-14234

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

@@ -159,7 +159,7 @@ class ConfigurationClassEnhancer {
 	 * Conditional {@link Callback}.
 	 * @see ConditionalCallbackFilter
 	 */
-	private static interface ConditionalCallback extends Callback {
+	private interface ConditionalCallback extends Callback {
 
 		boolean isMatch(Method candidateMethod);
 	}
@@ -343,7 +343,8 @@ class ConfigurationClassEnhancer {
 				// The factory is calling the bean method in order to instantiate and register the bean
 				// (i.e. via a getBean() call) -> invoke the super implementation of the method to actually
 				// create the bean instance.
-				if (BeanFactoryPostProcessor.class.isAssignableFrom(beanMethod.getReturnType())) {
+				if (logger.isWarnEnabled() &&
+						BeanFactoryPostProcessor.class.isAssignableFrom(beanMethod.getReturnType())) {
 					logger.warn(String.format("@Bean method %s.%s is non-static and returns an object " +
 							"assignable to Spring's BeanFactoryPostProcessor interface. This will " +
 							"result in a failure to process annotations such as @Autowired, " +

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

@@ -1,5 +1,5 @@
 /*
- * 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.
@@ -382,6 +382,12 @@ public class ConfigurationClassPostProcessor implements BeanDefinitionRegistryPo
 					throw new BeanDefinitionStoreException("Cannot enhance @Configuration bean definition '" +
 							beanName + "' since it is not stored in an AbstractBeanDefinition subclass");
 				}
+				else if (logger.isWarnEnabled() && beanFactory.containsSingleton(beanName)) {
+					logger.warn("Cannot enhance @Configuration bean definition '" + beanName +
+							"' since its singleton instance has been created too early. The typical cause " +
+							"is a non-static @Bean method with a BeanDefinitionRegistryPostProcessor " +
+							"return type: Consider declaring such methods as 'static'.");
+				}
 				configBeanDefs.put(beanName, (AbstractBeanDefinition) beanDef);
 			}
 		}