Support List and Publisher<Fragment> return values

See gh-33162

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

@@ -17,6 +17,7 @@
 package org.springframework.web.reactive.result.view;
 
 import java.util.ArrayList;
+import java.util.Collection;
 import java.util.Collections;
 import java.util.List;
 import java.util.Locale;
@@ -154,13 +155,21 @@ public class ViewResolutionResultHandler extends HandlerResultHandlerSupport imp
 			return true;
 		}
 
-		Class<?> type = result.getReturnType().toClass();
+		ResolvableType returnType = result.getReturnType();
+		Class<?> type = returnType.toClass();
+
 		ReactiveAdapter adapter = getAdapter(result);
 		if (adapter != null) {
 			if (adapter.isNoValue()) {
 				return true;
 			}
-			type = result.getReturnType().getGeneric().toClass();
+
+			type = returnType.getGeneric().toClass();
+			returnType = returnType.getNested(2);
+
+			if (adapter.isMultiValue()) {
+				return Fragment.class.isAssignableFrom(type);
+			}
 		}
 
 		return (CharSequence.class.isAssignableFrom(type) ||
@@ -169,9 +178,19 @@ public class ViewResolutionResultHandler extends HandlerResultHandlerSupport imp
 				Model.class.isAssignableFrom(type) ||
 				Map.class.isAssignableFrom(type) ||
 				View.class.isAssignableFrom(type) ||
+				isFragmentCollection(returnType.getNested(2)) ||
 				!BeanUtils.isSimpleProperty(type));
 	}
 
+	private boolean hasModelAnnotation(MethodParameter parameter) {
+		return parameter.hasMethodAnnotation(ModelAttribute.class);
+	}
+
+	private static boolean isFragmentCollection(ResolvableType returnType) {
+		Class<?> clazz = returnType.resolve(Object.class);
+		return (Collection.class.isAssignableFrom(clazz) && Fragment.class.equals(returnType.getNested(2).resolve()));
+	}
+
 	@Override
 	@SuppressWarnings("unchecked")
 	public Mono<Void> handleResult(ServerWebExchange exchange, HandlerResult result) {
@@ -181,14 +200,19 @@ public class ViewResolutionResultHandler extends HandlerResultHandlerSupport imp
 
 		if (adapter != null) {
 			if (adapter.isMultiValue()) {
-				throw new IllegalArgumentException("Multi-value producer: " + result.getReturnType());
-			}
+				valueMono = (result.getReturnValue() != null ?
+						Mono.just(FragmentRendering.fromPublisher(adapter.toPublisher(result.getReturnValue())).build()) :
+						Mono.empty());
 
-			valueMono = (result.getReturnValue() != null ?
-					Mono.from(adapter.toPublisher(result.getReturnValue())) : Mono.empty());
+				valueType = ResolvableType.forClass(FragmentRendering.class);
+			}
+			else {
+				valueMono = (result.getReturnValue() != null ?
+						Mono.from(adapter.toPublisher(result.getReturnValue())) : Mono.empty());
 
-			valueType = (adapter.isNoValue() ? ResolvableType.forClass(Void.class) :
-					result.getReturnType().getGeneric());
+				valueType = (adapter.isNoValue() ? ResolvableType.forClass(Void.class) :
+						result.getReturnType().getGeneric());
+			}
 		}
 		else {
 			valueMono = Mono.justOrEmpty(result.getReturnValue());
@@ -210,6 +234,11 @@ public class ViewResolutionResultHandler extends HandlerResultHandlerSupport imp
 						clazz = returnValue.getClass();
 					}
 
+					if (Collection.class.isAssignableFrom(clazz)) {
+						returnValue = FragmentRendering.fromCollection((Collection<Fragment>) returnValue).build();
+						clazz = FragmentRendering.class;
+					}
+
 					if (returnValue == NO_VALUE || ClassUtils.isVoidType(clazz)) {
 						viewsMono = resolveViews(getDefaultViewName(exchange), locale);
 					}
@@ -266,10 +295,6 @@ public class ViewResolutionResultHandler extends HandlerResultHandlerSupport imp
 				});
 	}
 
-	private boolean hasModelAnnotation(MethodParameter parameter) {
-		return parameter.hasMethodAnnotation(ModelAttribute.class);
-	}
-
 	/**
 	 * Select a default view name when a controller did not specify it.
 	 * Use the request path the leading and trailing slash stripped.

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

@@ -33,6 +33,7 @@ import org.springframework.context.annotation.AnnotationConfigApplicationContext
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.context.support.ResourceBundleMessageSource;
+import org.springframework.core.MethodParameter;
 import org.springframework.http.MediaType;
 import org.springframework.web.reactive.BindingContext;
 import org.springframework.web.reactive.HandlerResult;
@@ -44,42 +45,46 @@ import org.springframework.web.testfixture.http.server.reactive.MockServerHttpRe
 import org.springframework.web.testfixture.server.MockServerWebExchange;
 
 import static org.assertj.core.api.AssertionsForClassTypes.assertThat;
-import static org.junit.jupiter.api.Named.named;
 import static org.springframework.web.testfixture.method.ResolvableMethod.on;
 
 /**
- * Tests for multi-view rendering through {@link ViewResolutionResultHandler}.
+ * Tests for {@link Fragment} rendering through {@link ViewResolutionResultHandler}.
  *
  * @author Rossen Stoyanchev
  */
-public class FragmentResolutionResultHandlerTests {
-
-		static Stream<Arguments> arguments() {
-			Fragment f1 = Fragment.create("fragment1", Map.of("foo", "Foo"));
-			Fragment f2 = Fragment.create("fragment2", Map.of("bar", "Bar"));
-			return Stream.of(
-					Arguments.of(named("Flux",
-							FragmentRendering.fromPublisher(Flux.just(f1, f2).subscribeOn(Schedulers.boundedElastic()))
-									.headers(headers -> headers.setContentType(MediaType.TEXT_HTML))
-									.build())),
-					Arguments.of(named("List",
-							FragmentRendering.fromCollection(List.of(f1, f2))
-									.headers(headers -> headers.setContentType(MediaType.TEXT_HTML))
-									.build()))
-			);}
+public class FragmentViewResolutionResultHandlerTests {
+
+	static Stream<Arguments> arguments() {
+		Fragment f1 = Fragment.create("fragment1", Map.of("foo", "Foo"));
+		Fragment f2 = Fragment.create("fragment2", Map.of("bar", "Bar"));
+		return Stream.of(
+				Arguments.of(
+						FragmentRendering.fromPublisher(Flux.just(f1, f2).subscribeOn(Schedulers.boundedElastic()))
+								.headers(headers -> headers.setContentType(MediaType.TEXT_HTML))
+								.build(),
+						on(Handler.class).resolveReturnType(FragmentRendering.class)),
+				Arguments.of(
+						FragmentRendering.fromCollection(List.of(f1, f2))
+								.headers(headers -> headers.setContentType(MediaType.TEXT_HTML))
+								.build(),
+						on(Handler.class).resolveReturnType(FragmentRendering.class)),
+				Arguments.of(
+						Flux.just(f1, f2).subscribeOn(Schedulers.boundedElastic()),
+						on(Handler.class).resolveReturnType(Flux.class, Fragment.class)),
+				Arguments.of(
+						List.of(f1, f2),
+						on(Handler.class).resolveReturnType(List.class, Fragment.class)));
+	}
 
 
 	@ParameterizedTest
 	@MethodSource("arguments")
-	void render(FragmentRendering rendering) {
-
+	void render(Object returnValue, MethodParameter parameter) {
 		Locale locale = Locale.ENGLISH;
 		MockServerHttpRequest request = MockServerHttpRequest.get("/").acceptLanguageAsLocales(locale).build();
 		MockServerWebExchange exchange = MockServerWebExchange.from(request);
 
-		HandlerResult result = new HandlerResult(
-				new Handler(), rendering, on(Handler.class).resolveReturnType(FragmentRendering.class),
-				new BindingContext());
+		HandlerResult result = new HandlerResult(new Handler(), returnValue, parameter, new BindingContext());
 
 		String body = initHandler().handleResult(exchange, result)
 				.then(Mono.defer(() -> exchange.getResponse().getBodyAsString()))
@@ -102,10 +107,15 @@ public class FragmentResolutionResultHandlerTests {
 	}
 
 
+	@SuppressWarnings("unused")
 	private static class Handler {
 
 		FragmentRendering rendering() { return null; }
 
+		Flux<Fragment> fragmentFlux() { return null; }
+
+		List<Fragment> fragmentList() { return null; }
+
 	}
 
 

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

@@ -79,9 +79,14 @@ class ViewResolutionResultHandlerTests {
 		testSupports(on(Handler.class).resolveReturnType(Mono.class, String.class));
 
 		testSupports(on(Handler.class).resolveReturnType(Rendering.class));
-		testSupports(on(Handler.class).resolveReturnType(FragmentRendering.class));
 		testSupports(on(Handler.class).resolveReturnType(Mono.class, Rendering.class));
 
+		testSupports(on(Handler.class).resolveReturnType(FragmentRendering.class));
+		testSupports(on(Handler.class).resolveReturnType(Flux.class, Fragment.class));
+		testSupports(on(Handler.class).resolveReturnType(List.class, Fragment.class));
+		testSupports(on(Handler.class).resolveReturnType(
+				Mono.class, ResolvableType.forClassWithGenerics(List.class, Fragment.class)));
+
 		testSupports(on(Handler.class).resolveReturnType(View.class));
 		testSupports(on(Handler.class).resolveReturnType(Mono.class, View.class));
 
@@ -436,6 +441,9 @@ class ViewResolutionResultHandlerTests {
 		Mono<Rendering> monoRendering() { return null; }
 
 		FragmentRendering fragmentRendering() { return null; }
+		Flux<Fragment> fragmentFlux() { return null; }
+		Mono<List<Fragment>> monoFragmentList() { return null; }
+		List<Fragment> fragmentList() { return null; }
 
 		View view() { return null; }
 		Mono<View> monoView() { return null; }