Treat kotlin.Unit as void

spring-core/src/main/java/org/springframework/core/KotlinDetector.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 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.
@@ -27,6 +27,7 @@
  *
  * @author Juergen Hoeller
  * @author Sebastien Deleuze
+ * @author Yanming Zhou
  * @since 5.0
  */
 @SuppressWarnings("unchecked")
@@ -93,4 +94,19 @@ public static boolean isSuspendingFunction(Method method) {
 		return false;
 	}
 
+	/**
+	 * Return {@code true} if the class is {@code kotlin.Unit}.
+	 * @since 6.1.1
+	 */
+	public static boolean isKotlinUnit(Class<?> clazz) {
+		return "kotlin.Unit".equals(clazz.getName());
+	}
+
+	/**
+	 * Return {@code true} if the object is a {@code kotlin.Unit}.
+	 * @since 6.1.1
+	 */
+	public static boolean isKotlinUnit(@Nullable Object object) {
+		return (object != null && isKotlinUnit(object.getClass()));
+	}
 }

spring-web/src/main/java/org/springframework/web/method/support/InvocableHandlerMethod.java

@@ -51,6 +51,7 @@
  * @author Rossen Stoyanchev
  * @author Juergen Hoeller
  * @author Sebastien Deleuze
+ * @author Yanming Zhou
  * @since 3.1
  */
 public class InvocableHandlerMethod extends HandlerMethod {
@@ -315,7 +316,11 @@ public static Object invokeFunction(Method method, Object target, Object[] args)
 					}
 				}
 			}
-			return function.callBy(argMap);
+			Object returnValue = function.callBy(argMap);
+			if (KotlinDetector.isKotlinUnit(returnValue)) {
+				returnValue = null;
+			}
+			return returnValue;
 		}
 	}
 

spring-web/src/test/kotlin/org/springframework/web/method/support/InvocableHandlerMethodKotlinTests.kt

@@ -28,6 +28,7 @@ import org.springframework.web.testfixture.servlet.MockHttpServletResponse
  * Kotlin unit tests for {@link InvocableHandlerMethod}.
  *
  * @author Sebastien Deleuze
+ * @author Yanming Zhou
  */
 class InvocableHandlerMethodKotlinTests {
 
@@ -71,6 +72,11 @@ class InvocableHandlerMethodKotlinTests {
 		Assertions.assertThat(value).isEqualTo("true")
 	}
 
+	@Test
+	fun shouldTreatUnitAsVoid() {
+		Assertions.assertThat(getInvocable().invokeForRequest(request, null)).isNull()
+	}
+
 	private fun getInvocable(vararg argTypes: Class<*>): InvocableHandlerMethod {
 		val method = ResolvableMethod.on(Handler::class.java).argTypes(*argTypes).resolveMethod()
 		val handlerMethod = InvocableHandlerMethod(Handler(), method)
@@ -95,6 +101,8 @@ class InvocableHandlerMethodKotlinTests {
 
 		fun nullableBooleanDefaultValue(status: Boolean? = true) =
 			status.toString()
+
+		fun returnUnit() {}
 	}
 
 }