Consider Void.class a primitive wrapper in ClassUtils

Prior to this commit, ClassUtils.isPrimitiveOrWrapper() and
ClassUtils.isPrimitiveWrapper() did not return true for Void.class.
However, ClassUtils.isPrimitiveOrWrapper() did return true for
void.class. This lacking symmetry is inconsistent and can lead to bugs
in reflective code.

See: https://github.com/spring-projects/spring-data-r2dbc/issues/159

This commit addresses this by adding an entry for Void.class -> void.class
in the internal primitiveWrapperTypeMap in ClassUtils.

Closes gh-23572

spring-core/src/main/java/org/springframework/util/ClassUtils.java

@@ -120,6 +120,7 @@ public abstract class ClassUtils {
 		primitiveWrapperTypeMap.put(Integer.class, int.class);
 		primitiveWrapperTypeMap.put(Long.class, long.class);
 		primitiveWrapperTypeMap.put(Short.class, short.class);
+		primitiveWrapperTypeMap.put(Void.class, void.class);
 
 		// Map entry iteration is less expensive to initialize than forEach with lambdas
 		for (Map.Entry<Class<?>, Class<?>> entry : primitiveWrapperTypeMap.entrySet()) {
@@ -462,7 +463,8 @@ public abstract class ClassUtils {
 
 	/**
 	 * Check if the given class represents a primitive wrapper,
-	 * i.e. Boolean, Byte, Character, Short, Integer, Long, Float, or Double.
+	 * i.e. Boolean, Byte, Character, Short, Integer, Long, Float, Double, or
+	 * Void.
 	 * @param clazz the class to check
 	 * @return whether the given class is a primitive wrapper class
 	 */
@@ -473,10 +475,12 @@ public abstract class ClassUtils {
 
 	/**
 	 * Check if the given class represents a primitive (i.e. boolean, byte,
-	 * char, short, int, long, float, or double) or a primitive wrapper
-	 * (i.e. Boolean, Byte, Character, Short, Integer, Long, Float, or Double).
+	 * char, short, int, long, float, or double), {@code void}, or a wrapper for
+	 * those types (i.e. Boolean, Byte, Character, Short, Integer, Long, Float,
+	 * Double, or Void).
 	 * @param clazz the class to check
-	 * @return whether the given class is a primitive or primitive wrapper class
+	 * @return {@code true} if the given class represents a primitive, void, or
+	 * a wrapper class
 	 */
 	public static boolean isPrimitiveOrWrapper(Class<?> clazz) {
 		Assert.notNull(clazz, "Class must not be null");

spring-core/src/test/java/org/springframework/util/ClassUtilsTests.java

 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 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.
@@ -40,14 +40,17 @@ import org.springframework.tests.sample.objects.TestObject;
 import static org.junit.Assert.*;
 
 /**
+ * Unit tests for {@link ClassUtils}.
+ *
  * @author Colin Sampaleanu
  * @author Juergen Hoeller
  * @author Rob Harrop
  * @author Rick Evans
+ * @author Sam Brannen
  */
 public class ClassUtilsTests {
 
-	private ClassLoader classLoader = getClass().getClassLoader();
+	private final ClassLoader classLoader = getClass().getClassLoader();
 
 
 	@Before
@@ -384,6 +387,42 @@ public class ClassUtilsTests {
 		assertNull(ClassUtils.determineCommonAncestor(String.class, List.class));
 	}
 
+	@Test
+	public void isPrimitiveWrapper() {
+		assertTrue(ClassUtils.isPrimitiveWrapper(Boolean.class));
+		assertTrue(ClassUtils.isPrimitiveWrapper(Character.class));
+		assertTrue(ClassUtils.isPrimitiveWrapper(Byte.class));
+		assertTrue(ClassUtils.isPrimitiveWrapper(Short.class));
+		assertTrue(ClassUtils.isPrimitiveWrapper(Integer.class));
+		assertTrue(ClassUtils.isPrimitiveWrapper(Long.class));
+		assertTrue(ClassUtils.isPrimitiveWrapper(Float.class));
+		assertTrue(ClassUtils.isPrimitiveWrapper(Double.class));
+		assertTrue(ClassUtils.isPrimitiveWrapper(Void.class));
+	}
+
+	@Test
+	public void isPrimitiveOrWrapper() {
+		assertTrue(ClassUtils.isPrimitiveOrWrapper(boolean.class));
+		assertTrue(ClassUtils.isPrimitiveOrWrapper(char.class));
+		assertTrue(ClassUtils.isPrimitiveOrWrapper(byte.class));
+		assertTrue(ClassUtils.isPrimitiveOrWrapper(short.class));
+		assertTrue(ClassUtils.isPrimitiveOrWrapper(int.class));
+		assertTrue(ClassUtils.isPrimitiveOrWrapper(long.class));
+		assertTrue(ClassUtils.isPrimitiveOrWrapper(float.class));
+		assertTrue(ClassUtils.isPrimitiveOrWrapper(double.class));
+		assertTrue(ClassUtils.isPrimitiveOrWrapper(void.class));
+
+		assertTrue(ClassUtils.isPrimitiveOrWrapper(Boolean.class));
+		assertTrue(ClassUtils.isPrimitiveOrWrapper(Character.class));
+		assertTrue(ClassUtils.isPrimitiveOrWrapper(Byte.class));
+		assertTrue(ClassUtils.isPrimitiveOrWrapper(Short.class));
+		assertTrue(ClassUtils.isPrimitiveOrWrapper(Integer.class));
+		assertTrue(ClassUtils.isPrimitiveOrWrapper(Long.class));
+		assertTrue(ClassUtils.isPrimitiveOrWrapper(Float.class));
+		assertTrue(ClassUtils.isPrimitiveOrWrapper(Double.class));
+		assertTrue(ClassUtils.isPrimitiveOrWrapper(Void.class));
+	}
+
 
 	public static class InnerClass {
 

spring-webflux/src/main/java/org/springframework/web/reactive/result/view/ViewResolutionResultHandler.java

@@ -162,7 +162,7 @@ public class ViewResolutionResultHandler extends HandlerResultHandlerSupport
 
 		return (CharSequence.class.isAssignableFrom(type) || Rendering.class.isAssignableFrom(type) ||
 				Model.class.isAssignableFrom(type) || Map.class.isAssignableFrom(type) ||
-				void.class.equals(type) || View.class.isAssignableFrom(type) ||
+				Void.class.equals(type) || void.class.equals(type) || View.class.isAssignableFrom(type) ||
 				!BeanUtils.isSimpleProperty(type));
 	}
 

spring-webflux/src/test/java/org/springframework/web/reactive/result/view/ViewResolutionResultHandlerTests.java

@@ -114,7 +114,12 @@ public class ViewResolutionResultHandlerTests {
 	private void testSupports(MethodParameter returnType, boolean supports) {
 		ViewResolutionResultHandler resultHandler = resultHandler(mock(ViewResolver.class));
 		HandlerResult handlerResult = new HandlerResult(new Object(), null, returnType, this.bindingContext);
-		assertEquals(supports, resultHandler.supports(handlerResult));
+		if (supports) {
+			assertTrue("return type [" + returnType + "] should be supported", resultHandler.supports(handlerResult));
+		}
+		else {
+			assertFalse("return type [" + returnType + "] should not be supported", resultHandler.supports(handlerResult));
+		}
 	}
 
 	@Test