WebFlux support for SSE with multiline fragments

See gh-33194

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

@@ -41,6 +41,7 @@ import org.springframework.core.ResolvableType;
 import org.springframework.core.annotation.AnnotationAwareOrderComparator;
 import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.core.io.buffer.DataBufferFactory;
+import org.springframework.core.io.buffer.DataBufferUtils;
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpStatusCode;
 import org.springframework.http.MediaType;
@@ -538,21 +539,25 @@ public class ViewResolutionResultHandler extends HandlerResultHandlerSupport imp
 
 		@Override
 		public Flux<DataBuffer> format(
-				Flux<DataBuffer> fragmentContent, Fragment fragment, ServerWebExchange exchange) {
+				Flux<DataBuffer> fragmentFlux, Fragment fragment, ServerWebExchange exchange) {
 
-			Charset charset = StandardCharsets.UTF_8;
+			MediaType mediaType = exchange.getResponse().getHeaders().getContentType();
-			MediaType contentType = exchange.getResponse().getHeaders().getContentType();
+			Charset charset = (mediaType != null && mediaType.getCharset() != null ?
-			if (contentType != null && contentType.getCharset() != null) {
+					mediaType.getCharset() : StandardCharsets.UTF_8);
-				charset = contentType.getCharset();
-			}
 
 			DataBufferFactory bufferFactory = exchange.getResponse().bufferFactory();
 
-			String eventLine = fragment.viewName() != null ? "event:" + fragment.viewName() + "\n" : "";
+			String eventLine = (fragment.viewName() != null ? "event:" + fragment.viewName() + "\n" : "");
 			DataBuffer prefix = encodeText(eventLine + "data:", charset, bufferFactory);
 			DataBuffer suffix = encodeText("\n\n", charset, bufferFactory);
 
-			return Flux.concat(Flux.just(prefix), fragmentContent, Flux.just(suffix));
+			Mono<DataBuffer> content = DataBufferUtils.join(fragmentFlux)
+					.map(dataBuffer -> {
+						String s = dataBuffer.toString(charset).replace("\n", "\ndata:");
+						return bufferFactory.wrap(s.getBytes(charset));
+					});
+
+			return Flux.concat(Flux.just(prefix), content, Flux.just(suffix));
 		}
 
 		private DataBuffer encodeText(String text, Charset charset, DataBufferFactory bufferFactory) {

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

@@ -43,6 +43,7 @@ import org.springframework.web.reactive.accept.RequestedContentTypeResolver;
 import org.springframework.web.reactive.result.view.script.ScriptTemplateConfigurer;
 import org.springframework.web.reactive.result.view.script.ScriptTemplateViewResolver;
 import org.springframework.web.testfixture.http.server.reactive.MockServerHttpRequest;
+import org.springframework.web.testfixture.http.server.reactive.MockServerHttpResponse;
 import org.springframework.web.testfixture.server.MockServerWebExchange;
 
 import static org.assertj.core.api.AssertionsForClassTypes.assertThat;
@@ -87,7 +88,14 @@ public class FragmentViewResolutionResultHandlerTests {
 				.then(Mono.defer(() -> exchange.getResponse().getBodyAsString()))
 				.block(Duration.ofSeconds(60));
 
-		assertThat(body).isEqualTo("<p>Hello Foo</p><p>Hello Bar</p>");
+		assertThat(exchange.getResponse().getHeaders().getContentType()).isEqualTo(MediaType.TEXT_HTML);
+		assertThat(body).isEqualTo("""
+				<p>
+					Hello Foo
+				</p>\
+				<p>
+					Hello Bar
+				</p>""");
 	}
 
 	@Test
@@ -98,6 +106,7 @@ public class FragmentViewResolutionResultHandlerTests {
 				.build();
 
 		MockServerWebExchange exchange = MockServerWebExchange.from(request);
+		MockServerHttpResponse response = exchange.getResponse();
 
 		HandlerResult result = new HandlerResult(
 				new Handler(),
@@ -106,15 +115,20 @@ public class FragmentViewResolutionResultHandlerTests {
 				new BindingContext());
 
 		String body = initHandler().handleResult(exchange, result)
-				.then(Mono.defer(() -> exchange.getResponse().getBodyAsString()))
+				.then(Mono.defer(response::getBodyAsString))
 				.block(Duration.ofSeconds(60));
 
+		assertThat(response.getHeaders().getContentType()).isEqualTo(MediaType.TEXT_EVENT_STREAM);
 		assertThat(body).isEqualTo("""
 				event:fragment1
-				data:<p>Hello Foo</p>
+				data:<p>
+				data:	Hello Foo
+				data:</p>
 
 				event:fragment2
-				data:<p>Hello Bar</p>
+				data:<p>
+				data:	Hello Bar
+				data:</p>
 
 				""");
 	}

spring-webflux/src/test/resources/org/springframework/web/reactive/result/view/script/kotlin/fragment1.kts

@@ -1,3 +1,6 @@
 import org.springframework.web.reactive.result.view.script.*
 
-"""<p>${i18n("hello")} $foo</p>"""
+"""
+	|<p>
+	|	${i18n("hello")} $foo
+	|</p>""".trimMargin()

spring-webflux/src/test/resources/org/springframework/web/reactive/result/view/script/kotlin/fragment2.kts

@@ -1,3 +1,6 @@
 import org.springframework.web.reactive.result.view.script.*
 
-"""<p>${i18n("hello")} $bar</p>"""
+"""
+	|<p>
+	|	${i18n("hello")} $bar
+	|</p>""".trimMargin()

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/SseEmitter.java

@@ -30,6 +30,7 @@ import org.springframework.http.server.ServerHttpResponse;
 import org.springframework.lang.Nullable;
 import org.springframework.util.ObjectUtils;
 import org.springframework.util.StringUtils;
+import org.springframework.web.servlet.ModelAndView;
 
 /**
  * A specialization of {@link ResponseBodyEmitter} for sending
@@ -203,6 +204,8 @@ public class SseEmitter extends ResponseBodyEmitter {
 		@Nullable
 		private StringBuilder sb;
 
+		private boolean hasName;
+
 		@Override
 		public SseEventBuilder id(String id) {
 			append("id:").append(id).append('\n');
@@ -211,6 +214,7 @@ public class SseEmitter extends ResponseBodyEmitter {
 
 		@Override
 		public SseEventBuilder name(String name) {
+			this.hasName = true;
 			append("event:").append(name).append('\n');
 			return this;
 		}
@@ -234,6 +238,9 @@ public class SseEmitter extends ResponseBodyEmitter {
 
 		@Override
 		public SseEventBuilder data(Object object, @Nullable MediaType mediaType) {
+			if (object instanceof ModelAndView mav && !this.hasName && mav.getViewName() != null) {
+				name(mav.getViewName());
+			}
 			append("data:");
 			saveAppendedText();
 			if (object instanceof String text) {

spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/FragmentRenderingStreamTests.java

@@ -19,7 +19,9 @@ package org.springframework.web.servlet.mvc.method.annotation;
 import java.util.List;
 import java.util.Map;
 
+import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
+import reactor.core.publisher.Flux;
 
 import org.springframework.context.annotation.AnnotationConfigApplicationContext;
 import org.springframework.context.annotation.Bean;
@@ -28,7 +30,8 @@ import org.springframework.context.support.ResourceBundleMessageSource;
 import org.springframework.core.MethodParameter;
 import org.springframework.core.ReactiveAdapterRegistry;
 import org.springframework.core.task.SyncTaskExecutor;
-import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
+import org.springframework.http.HttpHeaders;
+import org.springframework.http.converter.StringHttpMessageConverter;
 import org.springframework.web.accept.ContentNegotiationManager;
 import org.springframework.web.context.request.NativeWebRequest;
 import org.springframework.web.context.request.ServletWebRequest;
@@ -51,8 +54,20 @@ import static org.springframework.web.testfixture.method.ResolvableMethod.on;
  */
 public class FragmentRenderingStreamTests {
 
-	@Test
+	private final MockHttpServletRequest request = new MockHttpServletRequest();
-	void streamFragments() throws Exception {
+
+	private final MockHttpServletResponse response = new MockHttpServletResponse();
+
+	private final NativeWebRequest webRequest = new ServletWebRequest(request, response);
+
+	private ResponseBodyEmitterReturnValueHandler handler;
+
+
+	@BeforeEach
+	void setUp() {
+		AsyncWebRequest asyncWebRequest = new StandardServletAsyncWebRequest(this.request, this.response);
+		WebAsyncUtils.getAsyncManager(this.webRequest).setAsyncWebRequest(asyncWebRequest);
+		this.request.setAsyncSupported(true);
 
 		AnnotationConfigApplicationContext context =
 				new AnnotationConfigApplicationContext(ScriptTemplatingConfiguration.class);
@@ -61,44 +76,84 @@ public class FragmentRenderingStreamTests {
 		ScriptTemplateViewResolver viewResolver = new ScriptTemplateViewResolver(prefix, ".kts");
 		viewResolver.setApplicationContext(context);
 
-		ResponseBodyEmitterReturnValueHandler handler = new ResponseBodyEmitterReturnValueHandler(
+		this.handler = new ResponseBodyEmitterReturnValueHandler(
-				List.of(new MappingJackson2HttpMessageConverter()),
+				List.of(new StringHttpMessageConverter()),
 				ReactiveAdapterRegistry.getSharedInstance(), new SyncTaskExecutor(),
 				new ContentNegotiationManager(),
 				List.of(viewResolver), null);
+	}
 
-		MockHttpServletRequest request = new MockHttpServletRequest();
-		MockHttpServletResponse response = new MockHttpServletResponse();
-		NativeWebRequest webRequest = new ServletWebRequest(request, response);
-
-		AsyncWebRequest asyncWebRequest = new StandardServletAsyncWebRequest(request, response);
-		WebAsyncUtils.getAsyncManager(webRequest).setAsyncWebRequest(asyncWebRequest);
-		request.setAsyncSupported(true);
 
+	@Test
+	void streamWithSseEmitter() throws Exception {
 		MethodParameter type = on(TestController.class).resolveReturnType(SseEmitter.class);
-		SseEmitter emitter = new SseEmitter();
-		handler.handleReturnValue(emitter, type, new ModelAndViewContainer(), webRequest);
 
-		assertThat(request.isAsyncStarted()).isTrue();
+		SseEmitter emitter = new SseEmitter();
-		assertThat(response.getStatus()).isEqualTo(200);
+		this.handler.handleReturnValue(emitter, type, new ModelAndViewContainer(), webRequest);
+
+		assertThat(this.request.isAsyncStarted()).isTrue();
+		assertThat(this.response.getStatus()).isEqualTo(200);
 
 		ModelAndView mav1 = new ModelAndView("fragment1", Map.of("foo", "Foo"));
 		ModelAndView mav2 = new ModelAndView("fragment2", Map.of("bar", "Bar"));
 
-		emitter.send(SseEmitter.event().data(mav1).data(mav2));
+		emitter.send(SseEmitter.event().data(mav1));
+		emitter.send(SseEmitter.event().data(mav2));
 
-		assertThat(response.getContentType()).isEqualTo("text/event-stream");
+		assertThat(this.response.getContentType()).isEqualTo("text/event-stream");
-		assertThat(response.getContentAsString()).isEqualTo(("""
+		assertThat(this.response.getContentAsString()).isEqualTo(("""
-				data:<p>Hello Foo</p>
+				event:fragment1
-				data:<p>Hello Bar</p>
+				data:<p>
+				data:	Hello Foo
+				data:</p>
+
+				event:fragment2
+				data:<p>
+				data:	Hello Bar
+				data:</p>
+
+				"""));
+	}
+
+	@Test
+	void streamWithFlux() throws Exception {
+		MethodParameter type = on(TestController.class).resolveReturnType(Flux.class, ModelAndView.class);
+
+		this.request.addHeader(HttpHeaders.ACCEPT, "text/event-stream");
+
+		Flux<ModelAndView> flux = Flux.just(
+				new ModelAndView("fragment1", Map.of("foo", "Foo")),
+				new ModelAndView("fragment2", Map.of("bar", "Bar")));
+
+		this.handler.handleReturnValue(flux, type, new ModelAndViewContainer(), webRequest);
+
+		assertThat(this.request.isAsyncStarted()).isTrue();
+		assertThat(this.response.getStatus()).isEqualTo(200);
+
+		assertThat(this.response.getContentType()).isEqualTo("text/event-stream");
+		assertThat(this.response.getContentAsString()).isEqualTo(("""
+				event:fragment1
+				data:<p>
+				data:	Hello Foo
+				data:</p>
+
+				event:fragment2
+				data:<p>
+				data:	Hello Bar
+				data:</p>
 
 				"""));
 	}
 
 
+	@SuppressWarnings({"unused", "DataFlowIssue"})
 	private static class TestController {
 
-		SseEmitter handle() {
+		SseEmitter handleWithSseEmitter() {
+			return null;
+		}
+
+		Flux<ModelAndView> handleWithFlux() {
 			return null;
 		}
 	}

spring-webmvc/src/test/java/org/springframework/web/servlet/view/DefaultFragmentsRenderingTests.java

@@ -64,7 +64,13 @@ public class DefaultFragmentsRenderingTests {
 		view.resolveNestedViews(viewResolver, Locale.ENGLISH);
 		view.render(Collections.emptyMap(), request, response);
 
-		assertThat(response.getContentAsString()).isEqualTo("<p>Hello Foo</p><p>Hello Bar</p>");
+		assertThat(response.getContentAsString()).isEqualTo("""
+				<p>
+					Hello Foo
+				</p>\
+				<p>
+					Hello Bar
+				</p>""");
 	}
 
 

spring-webmvc/src/test/resources/org/springframework/web/servlet/view/script/kotlin/fragment1.kts

@@ -1,3 +1,6 @@
 import org.springframework.web.servlet.view.script.*
 
-"""<p>${i18n("hello")} $foo</p>"""
+"""
+	|<p>
+	|	${i18n("hello")} $foo
+	|</p>""".trimMargin()

spring-webmvc/src/test/resources/org/springframework/web/servlet/view/script/kotlin/fragment2.kts

@@ -1,3 +1,6 @@
 import org.springframework.web.servlet.view.script.*
 
-"""<p>${i18n("hello")} $bar</p>"""
+"""
+	|<p>
+	|	${i18n("hello")} $bar
+	|</p>""".trimMargin()