Improve Servlet, Filter, and EventListener configuration diagnostics

ServletContextInitializerBeans handles existing registration beans and
also creates initializers for existing Servlet, Filter, etc beans.
Debug logging has been added to this class to improve things in three
main areas:

 - Distinguish between existing registration beans and those that are
   created to wrap a Servlet or Filter bean
 - Provide information about the resource (configuration class or XML)
   that resulted in the creation of the bean
 - Log information about EventListeners

Closes gh-2177

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

@@ -33,7 +33,10 @@ import javax.servlet.Filter;
 import javax.servlet.MultipartConfigElement;
 import javax.servlet.Servlet;
 
+import org.apache.commons.logging.Log;
+import org.apache.commons.logging.LogFactory;
 import org.springframework.beans.factory.ListableBeanFactory;
+import org.springframework.beans.factory.support.BeanDefinitionRegistry;
 import org.springframework.core.annotation.AnnotationAwareOrderComparator;
 import org.springframework.util.LinkedMultiValueMap;
 import org.springframework.util.MultiValueMap;
@@ -57,6 +60,8 @@ class ServletContextInitializerBeans extends
 
 	static final String DISPATCHER_SERVLET_NAME = "dispatcherServlet";
 
+	private final Log log = LogFactory.getLog(getClass());
+
 	private final Set<Object> seen = new HashSet<Object>();
 
 	private final MultiValueMap<Class<?>, ServletContextInitializer> initializers;
@@ -79,36 +84,55 @@ class ServletContextInitializerBeans extends
 	private void addServletContextInitializerBeans(ListableBeanFactory beanFactory) {
 		for (Entry<String, ServletContextInitializer> initializerBean : getOrderedBeansOfType(
 				beanFactory, ServletContextInitializer.class)) {
-			addServletContextInitializerBean(initializerBean.getValue());
+			addServletContextInitializerBean(initializerBean.getKey(),
+					initializerBean.getValue(), beanFactory);
 		}
 	}
 
-	private void addServletContextInitializerBean(ServletContextInitializer initializer) {
+	private void addServletContextInitializerBean(String beanName,
+			ServletContextInitializer initializer, ListableBeanFactory beanFactory) {
 		if (initializer instanceof ServletRegistrationBean) {
-			addServletContextInitializerBean(Servlet.class, initializer,
-					((ServletRegistrationBean) initializer).getServlet());
+			addServletContextInitializerBean(Servlet.class, beanName, initializer,
+					beanFactory, ((ServletRegistrationBean) initializer).getServlet());
 		}
 		else if (initializer instanceof FilterRegistrationBean) {
-			addServletContextInitializerBean(Filter.class, initializer,
-					((FilterRegistrationBean) initializer).getFilter());
+			addServletContextInitializerBean(Filter.class, beanName, initializer,
+					beanFactory, ((FilterRegistrationBean) initializer).getFilter());
 		}
 		else if (initializer instanceof ServletListenerRegistrationBean) {
-			addServletContextInitializerBean(EventListener.class, initializer,
+			addServletContextInitializerBean(EventListener.class, beanName, initializer,
+					beanFactory,
 					((ServletListenerRegistrationBean<?>) initializer).getListener());
 		}
 		else {
-			addServletContextInitializerBean(ServletContextInitializer.class,
-					initializer, null);
+			addServletContextInitializerBean(ServletContextInitializer.class, beanName,
+					initializer, beanFactory, null);
 		}
 	}
 
-	private void addServletContextInitializerBean(Class<?> type,
-			ServletContextInitializer initializer, Object source) {
+	private void addServletContextInitializerBean(Class<?> type, String beanName,
+			ServletContextInitializer initializer, ListableBeanFactory beanFactory,
+			Object source) {
 		this.initializers.add(type, initializer);
 		if (source != null) {
 			// Mark the underlying source as seen in case it wraps an existing bean
 			this.seen.add(source);
 		}
+		if (this.log.isDebugEnabled()) {
+			String resourceDescription = getResourceDescription(beanName, beanFactory);
+			int order = getOrder(initializer);
+			this.log.debug("Added existing " + type.getSimpleName()
+					+ " initializer bean '" + beanName + "'; order=" + order
+					+ ", resource=" + resourceDescription);
+		}
+	}
+
+	private String getResourceDescription(String beanName, ListableBeanFactory beanFactory) {
+		if (beanFactory instanceof BeanDefinitionRegistry) {
+			BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
+			return registry.getBeanDefinition(beanName).getResourceDescription();
+		}
+		return "unknown";
 	}
 
 	@SuppressWarnings("unchecked")
@@ -141,12 +165,21 @@ class ServletContextInitializerBeans extends
 		List<Map.Entry<String, B>> beans = getOrderedBeansOfType(beanFactory, beanType);
 		for (Entry<String, B> bean : beans) {
 			if (this.seen.add(bean.getValue())) {
+				int order = getOrder(bean.getValue());
+				String beanName = bean.getKey();
 				// One that we haven't already seen
-				RegistrationBean registration = adapter.createRegistrationBean(
-						bean.getKey(), bean.getValue(), beans.size());
-				registration.setName(bean.getKey());
-				registration.setOrder(getOrder(bean.getValue()));
+				RegistrationBean registration = adapter.createRegistrationBean(beanName,
+						bean.getValue(), beans.size());
+				registration.setName(beanName);
+				registration.setOrder(order);
 				this.initializers.add(type, registration);
+
+				if (this.log.isDebugEnabled()) {
+					this.log.debug("Created " + type.getSimpleName()
+							+ " initializer for bean '" + beanName + "'; order=" + order
+							+ ", resource="
+							+ getResourceDescription(beanName, beanFactory));
+				}
 			}
 		}
 	}