Correctly format class name in default package

Closes gh-27247

spring-messaging/src/main/java/org/springframework/messaging/handler/invocation/AbstractMethodMessageHandler.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -53,6 +53,7 @@ import org.springframework.util.ClassUtils;
 import org.springframework.util.CollectionUtils;
 import org.springframework.util.LinkedMultiValueMap;
 import org.springframework.util.MultiValueMap;
+import org.springframework.util.StringUtils;
 import org.springframework.util.concurrent.ListenableFuture;
 import org.springframework.util.concurrent.ListenableFutureCallback;
 
@@ -319,9 +320,12 @@ public abstract class AbstractMethodMessageHandler<T>
 	}
 
 	private String formatMappings(Class<?> userType, Map<Method, T> methods) {
-		String formattedType = Arrays.stream(ClassUtils.getPackageName(userType).split("\\."))
-				.map(p -> p.substring(0, 1))
-				.collect(Collectors.joining(".", "", "." + userType.getSimpleName()));
+		String packageName = ClassUtils.getPackageName(userType);
+		String formattedType = (StringUtils.hasText(packageName) ?
+				Arrays.stream(packageName.split("\\."))
+						.map(packageSegment -> packageSegment.substring(0, 1))
+						.collect(Collectors.joining(".", "", "." + userType.getSimpleName())) :
+				userType.getSimpleName());
 		Function<Method, String> methodFormatter = method -> Arrays.stream(method.getParameterTypes())
 				.map(Class::getSimpleName)
 				.collect(Collectors.joining(",", "(", ")"));

spring-messaging/src/main/java/org/springframework/messaging/handler/invocation/reactive/AbstractMethodMessageHandler.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -54,6 +54,7 @@ import org.springframework.util.CollectionUtils;
 import org.springframework.util.MultiValueMap;
 import org.springframework.util.ObjectUtils;
 import org.springframework.util.RouteMatcher;
+import org.springframework.util.StringUtils;
 
 /**
  * Abstract base class for reactive HandlerMethod-based message handling.
@@ -341,9 +342,12 @@ public abstract class AbstractMethodMessageHandler<T>
 	}
 
 	private String formatMappings(Class<?> userType, Map<Method, T> methods) {
-		String formattedType = Arrays.stream(ClassUtils.getPackageName(userType).split("\\."))
-				.map(p -> p.substring(0, 1))
-				.collect(Collectors.joining(".", "", "." + userType.getSimpleName()));
+		String packageName = ClassUtils.getPackageName(userType);
+		String formattedType = (StringUtils.hasText(packageName) ?
+				Arrays.stream(packageName.split("\\."))
+						.map(packageSegment -> packageSegment.substring(0, 1))
+						.collect(Collectors.joining(".", "", "." + userType.getSimpleName())) :
+				userType.getSimpleName());
 		Function<Method, String> methodFormatter = method -> Arrays.stream(method.getParameterTypes())
 				.map(Class::getSimpleName)
 				.collect(Collectors.joining(",", "(", ")"));

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/AbstractHandlerMethodMapping.java

@@ -42,6 +42,7 @@ import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.LinkedMultiValueMap;
 import org.springframework.util.MultiValueMap;
+import org.springframework.util.StringUtils;
 import org.springframework.web.cors.CorsConfiguration;
 import org.springframework.web.cors.reactive.CorsUtils;
 import org.springframework.web.method.HandlerMethod;
@@ -222,9 +223,12 @@ public abstract class AbstractHandlerMethodMapping<T> extends AbstractHandlerMap
 	}
 
 	private String formatMappings(Class<?> userType, Map<Method, T> methods) {
-		String formattedType = Arrays.stream(ClassUtils.getPackageName(userType).split("\\."))
-				.map(p -> p.substring(0, 1))
-				.collect(Collectors.joining(".", "", "." + userType.getSimpleName()));
+		String packageName = ClassUtils.getPackageName(userType);
+		String formattedType = (StringUtils.hasText(packageName) ?
+				Arrays.stream(packageName.split("\\."))
+						.map(packageSegment -> packageSegment.substring(0, 1))
+						.collect(Collectors.joining(".", "", "." + userType.getSimpleName())) :
+				userType.getSimpleName());
 		Function<Method, String> methodFormatter = method -> Arrays.stream(method.getParameterTypes())
 				.map(Class::getSimpleName)
 				.collect(Collectors.joining(",", "(", ")"));

spring-webmvc/src/main/java/org/springframework/web/servlet/handler/AbstractHandlerMethodMapping.java

@@ -44,6 +44,7 @@ import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.LinkedMultiValueMap;
 import org.springframework.util.MultiValueMap;
+import org.springframework.util.StringUtils;
 import org.springframework.web.cors.CorsConfiguration;
 import org.springframework.web.cors.CorsUtils;
 import org.springframework.web.method.HandlerMethod;
@@ -301,9 +302,12 @@ public abstract class AbstractHandlerMethodMapping<T> extends AbstractHandlerMap
 	}
 
 	private String formatMappings(Class<?> userType, Map<Method, T> methods) {
-		String formattedType = Arrays.stream(ClassUtils.getPackageName(userType).split("\\."))
-				.map(p -> p.substring(0, 1))
-				.collect(Collectors.joining(".", "", "." + userType.getSimpleName()));
+		String packageName = ClassUtils.getPackageName(userType);
+		String formattedType = (StringUtils.hasText(packageName) ?
+				Arrays.stream(packageName.split("\\."))
+						.map(packageSegment -> packageSegment.substring(0, 1))
+						.collect(Collectors.joining(".", "", "." + userType.getSimpleName())) :
+				userType.getSimpleName());
 		Function<Method, String> methodFormatter = method -> Arrays.stream(method.getParameterTypes())
 				.map(Class::getSimpleName)
 				.collect(Collectors.joining(",", "(", ")"));