Detect unsupported suspending handler methods in Spring MVC

closes gh-23585

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

@@ -34,9 +34,13 @@ import java.util.stream.Collectors;
 import javax.servlet.ServletException;
 import javax.servlet.http.HttpServletRequest;
 
+import kotlin.reflect.KFunction;
+import kotlin.reflect.jvm.ReflectJvmMapping;
+
 import org.springframework.aop.support.AopUtils;
 import org.springframework.beans.factory.BeanFactoryUtils;
 import org.springframework.beans.factory.InitializingBean;
+import org.springframework.core.KotlinDetector;
 import org.springframework.core.MethodIntrospector;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
@@ -585,6 +589,10 @@ public abstract class AbstractHandlerMethodMapping<T> extends AbstractHandlerMap
 		}
 
 		public void register(T mapping, Object handler, Method method) {
+			// Assert that the handler method is not a suspending one.
+			if (KotlinDetector.isKotlinType(method.getDeclaringClass()) && KotlinDelegate.isSuspend(method)) {
+				throw new IllegalStateException("Unsupported suspending handler method detected: " + method);
+			}
 			this.readWriteLock.writeLock().lock();
 			try {
 				HandlerMethod handlerMethod = createHandlerMethod(handler, method);
@@ -793,4 +801,15 @@ public abstract class AbstractHandlerMethodMapping<T> extends AbstractHandlerMap
 		}
 	}
 
+	/**
+	 * Inner class to avoid a hard dependency on Kotlin at runtime.
+	 */
+	private static class KotlinDelegate {
+
+		static private boolean isSuspend(Method method) {
+			KFunction<?> function = ReflectJvmMapping.getKotlinFunction(method);
+			return function != null && function.isSuspend();
+		}
+	}
+
 }