Polish

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/EmbeddedServletContainerAutoConfiguration.java

@@ -134,25 +134,21 @@ public class EmbeddedServletContainerAutoConfiguration {
 			if (this.beanFactory == null) {
 				return;
 			}
-			if (ObjectUtils.isEmpty(this.beanFactory.getBeanNamesForType(
-					EmbeddedServletContainerCustomizerBeanPostProcessor.class, true,
-					false))) {
-				RootBeanDefinition beanDefinition = new RootBeanDefinition(
-						EmbeddedServletContainerCustomizerBeanPostProcessor.class);
-				beanDefinition.setSynthetic(true);
-				registry.registerBeanDefinition(
+			registerSyntheticBeanIfMissing(registry,
 					"embeddedServletContainerCustomizerBeanPostProcessor",
-						beanDefinition);
-
-			}
-			if (ObjectUtils.isEmpty(this.beanFactory.getBeanNamesForType(
-					ErrorPageRegistrarBeanPostProcessor.class, true, false))) {
-				RootBeanDefinition beanDefinition = new RootBeanDefinition(
+					EmbeddedServletContainerCustomizerBeanPostProcessor.class);
+			registerSyntheticBeanIfMissing(registry,
+					"errorPageRegistrarBeanPostProcessor",
 					ErrorPageRegistrarBeanPostProcessor.class);
-				beanDefinition.setSynthetic(true);
-				registry.registerBeanDefinition("errorPageRegistrarBeanPostProcessor",
-						beanDefinition);
+		}
 
+		private void registerSyntheticBeanIfMissing(BeanDefinitionRegistry registry,
+				String name, Class<?> beanClass) {
+			if (ObjectUtils.isEmpty(
+					this.beanFactory.getBeanNamesForType(beanClass, true, false))) {
+				RootBeanDefinition beanDefinition = new RootBeanDefinition(beanClass);
+				beanDefinition.setSynthetic(true);
+				registry.registerBeanDefinition(name, beanDefinition);
 			}
 		}
 

spring-boot/src/main/java/org/springframework/boot/context/embedded/EmbeddedServletContainerCustomizerBeanPostProcessor.java

@@ -27,6 +27,7 @@ import org.springframework.beans.factory.BeanFactoryAware;
 import org.springframework.beans.factory.ListableBeanFactory;
 import org.springframework.beans.factory.config.BeanPostProcessor;
 import org.springframework.core.annotation.AnnotationAwareOrderComparator;
+import org.springframework.util.Assert;
 
 /**
  * {@link BeanPostProcessor} that applies all {@link EmbeddedServletContainerCustomizer}s
@@ -45,6 +46,9 @@ public class EmbeddedServletContainerCustomizerBeanPostProcessor
 
 	@Override
 	public void setBeanFactory(BeanFactory beanFactory) {
+		Assert.isInstanceOf(ListableBeanFactory.class, beanFactory,
+				"EmbeddedServletContainerCustomizerBeanPostProcessor can only be used "
+						+ "with a ListableBeanFactory");
 		this.beanFactory = (ListableBeanFactory) beanFactory;
 	}
 

spring-boot/src/main/java/org/springframework/boot/web/servlet/ErrorPageRegistrarBeanPostProcessor.java

@@ -27,6 +27,7 @@ import org.springframework.beans.factory.BeanFactoryAware;
 import org.springframework.beans.factory.ListableBeanFactory;
 import org.springframework.beans.factory.config.BeanPostProcessor;
 import org.springframework.core.annotation.AnnotationAwareOrderComparator;
+import org.springframework.util.Assert;
 
 /**
  * {@link BeanPostProcessor} that applies all {@link ErrorPageRegistrar}s from the bean
@@ -45,6 +46,9 @@ public class ErrorPageRegistrarBeanPostProcessor
 
 	@Override
 	public void setBeanFactory(BeanFactory beanFactory) {
+		Assert.isInstanceOf(ListableBeanFactory.class, beanFactory,
+				"ErrorPageRegistrarBeanPostProcessor can only be used "
+						+ "with a ListableBeanFactory");
 		this.beanFactory = (ListableBeanFactory) beanFactory;
 	}