Support @ModelAttribute with suspending function in WebFlux

Closes gh-30894

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/ModelInitializer.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.
@@ -16,6 +16,7 @@
 
 package org.springframework.web.reactive.result.method.annotation;
 
+import java.lang.reflect.Method;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
@@ -25,6 +26,7 @@ import java.util.Optional;
 import reactor.core.publisher.Mono;
 
 import org.springframework.core.Conventions;
+import org.springframework.core.KotlinDetector;
 import org.springframework.core.MethodParameter;
 import org.springframework.core.ReactiveAdapter;
 import org.springframework.core.ReactiveAdapterRegistry;
@@ -45,6 +47,7 @@ import org.springframework.web.server.ServerWebExchange;
  * default model initialization through {@code @ModelAttribute} methods.
  *
  * @author Rossen Stoyanchev
+ * @author Sebastien Deleuze
  * @since 5.0
  */
 class ModelInitializer {
@@ -119,18 +122,22 @@ class ModelInitializer {
 		Object value = handlerResult.getReturnValue();
 		if (value != null) {
 			ResolvableType type = handlerResult.getReturnType();
+			MethodParameter typeSource = handlerResult.getReturnTypeSource();
 			ReactiveAdapter adapter = this.adapterRegistry.getAdapter(type.resolve(), value);
-			if (isAsyncVoidType(type, adapter)) {
+			if (isAsyncVoidType(type, typeSource, adapter)) {
 				return Mono.from(adapter.toPublisher(value));
 			}
-			String name = getAttributeName(handlerResult.getReturnTypeSource());
+			String name = getAttributeName(typeSource);
 			bindingContext.getModel().asMap().putIfAbsent(name, value);
 		}
 		return Mono.empty();
 	}
 
-	private boolean isAsyncVoidType(ResolvableType type, @Nullable  ReactiveAdapter adapter) {
-		return (adapter != null && (adapter.isNoValue() || type.resolveGeneric() == Void.class));
+
+	private boolean isAsyncVoidType(ResolvableType type, MethodParameter typeSource, @Nullable  ReactiveAdapter adapter) {
+		Method method = typeSource.getMethod();
+		return (adapter != null && (adapter.isNoValue() || type.resolveGeneric() == Void.class)) ||
+				(method != null && KotlinDetector.isSuspendingFunction(method) && typeSource.getParameterType() == void.class);
 	}
 
 	private String getAttributeName(MethodParameter param) {

spring-webflux/src/test/kotlin/org/springframework/web/reactive/result/method/annotation/ModelInitializerKotlinTests.kt

@@ -0,0 +1,100 @@
+/*
+ * 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.web.reactive.result.method.annotation
+
+import kotlinx.coroutines.delay
+import org.assertj.core.api.Assertions
+import org.junit.jupiter.api.BeforeEach
+import org.junit.jupiter.api.Test
+import org.springframework.context.support.StaticApplicationContext
+import org.springframework.core.ReactiveAdapterRegistry
+import org.springframework.ui.Model
+import org.springframework.web.bind.annotation.GetMapping
+import org.springframework.web.bind.annotation.ModelAttribute
+import org.springframework.web.bind.support.ConfigurableWebBindingInitializer
+import org.springframework.web.method.HandlerMethod
+import org.springframework.web.server.ServerWebExchange
+import org.springframework.web.testfixture.http.server.reactive.MockServerHttpRequest
+import org.springframework.web.testfixture.method.ResolvableMethod
+import org.springframework.web.testfixture.server.MockServerWebExchange
+import reactor.core.publisher.Mono
+import java.time.Duration
+
+/**
+ * Kotlin test fixture for [ModelInitializer].
+ *
+ * @author Sebastien Deleuze
+ */
+class ModelInitializerKotlinTests {
+
+	private val timeout = Duration.ofMillis(5000)
+
+	private lateinit var modelInitializer: ModelInitializer
+
+	private val exchange: ServerWebExchange = MockServerWebExchange.from(MockServerHttpRequest.get("/path"))
+
+	@BeforeEach
+	fun setup() {
+		val adapterRegistry = ReactiveAdapterRegistry.getSharedInstance()
+		val resolverConfigurer = ArgumentResolverConfigurer()
+		resolverConfigurer.addCustomResolver(ModelMethodArgumentResolver(adapterRegistry))
+		val methodResolver = ControllerMethodResolver(resolverConfigurer, adapterRegistry, StaticApplicationContext(),
+			emptyList())
+		modelInitializer = ModelInitializer(methodResolver, adapterRegistry)
+	}
+
+	@Test
+	@Suppress("UNCHECKED_CAST")
+	fun modelAttributeMethods() {
+		val controller = TestController()
+		val method = ResolvableMethod.on(TestController::class.java).annotPresent(GetMapping::class.java)
+			.resolveMethod()
+		val handlerMethod = HandlerMethod(controller, method)
+		val context  = InitBinderBindingContext(ConfigurableWebBindingInitializer(), emptyList())
+		this.modelInitializer.initModel(handlerMethod, context, this.exchange).block(timeout)
+		val model = context.model.asMap()
+		Assertions.assertThat(model).hasSize(2)
+		val monoValue = model["suspendingReturnValue"] as Mono<TestBean>
+		Assertions.assertThat(monoValue.block(timeout)!!.name).isEqualTo("Suspending return value")
+		val value = model["suspendingModelParameter"] as TestBean
+		Assertions.assertThat(value.name).isEqualTo("Suspending model parameter")
+	}
+
+
+	private data class TestBean(val name: String)
+
+	private class TestController {
+
+		@ModelAttribute("suspendingReturnValue")
+		suspend fun suspendingReturnValue(): TestBean {
+			delay(1)
+			return TestBean("Suspending return value")
+		}
+
+		@ModelAttribute
+		suspend fun suspendingModelParameter(model: Model) {
+			delay(1)
+			model.addAttribute("suspendingModelParameter", TestBean("Suspending model parameter"))
+		}
+
+		@GetMapping
+		fun handleGet() {
+		}
+
+	}
+
+}