Insert StringMessageConverter for SSE

Closes gh-24465

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -18,6 +18,8 @@ package org.springframework.web.servlet.mvc.method.annotation;
 
 import java.io.IOException;
 import java.io.OutputStream;
+import java.nio.charset.StandardCharsets;
+import java.util.ArrayList;
 import java.util.List;
 import java.util.function.Consumer;
 
@@ -33,6 +35,7 @@ import org.springframework.http.HttpStatus;
 import org.springframework.http.MediaType;
 import org.springframework.http.ResponseEntity;
 import org.springframework.http.converter.HttpMessageConverter;
+import org.springframework.http.converter.StringHttpMessageConverter;
 import org.springframework.http.server.ServerHttpResponse;
 import org.springframework.http.server.ServletServerHttpResponse;
 import org.springframework.lang.Nullable;
@@ -60,6 +63,8 @@ public class ResponseBodyEmitterReturnValueHandler implements HandlerMethodRetur
 
 	private final List<HttpMessageConverter<?>> messageConverters;
 
+	private final List<HttpMessageConverter<?>> sseMessageConverters;
+
 	private final ReactiveTypeHandler reactiveHandler;
 
 
@@ -72,6 +77,7 @@ public class ResponseBodyEmitterReturnValueHandler implements HandlerMethodRetur
 	public ResponseBodyEmitterReturnValueHandler(List<HttpMessageConverter<?>> messageConverters) {
 		Assert.notEmpty(messageConverters, "HttpMessageConverter List must not be empty");
 		this.messageConverters = messageConverters;
+		this.sseMessageConverters = initSseConverters(messageConverters);
 		this.reactiveHandler = new ReactiveTypeHandler();
 	}
 
@@ -88,9 +94,22 @@ public class ResponseBodyEmitterReturnValueHandler implements HandlerMethodRetur
 
 		Assert.notEmpty(messageConverters, "HttpMessageConverter List must not be empty");
 		this.messageConverters = messageConverters;
+		this.sseMessageConverters = initSseConverters(messageConverters);
 		this.reactiveHandler = new ReactiveTypeHandler(registry, executor, manager);
 	}
 
+	private static List<HttpMessageConverter<?>> initSseConverters(List<HttpMessageConverter<?>> converters) {
+		for (HttpMessageConverter<?> converter : converters) {
+			if (converter.canWrite(String.class, MediaType.TEXT_PLAIN)) {
+				return converters;
+			}
+		}
+		List<HttpMessageConverter<?>> result = new ArrayList<>(converters.size() + 1);
+		result.add(new StringHttpMessageConverter(StandardCharsets.UTF_8));
+		result.addAll(converters);
+		return result;
+	}
+
 
 	@Override
 	public boolean supportsReturnType(MethodParameter returnType) {
@@ -186,7 +205,7 @@ public class ResponseBodyEmitterReturnValueHandler implements HandlerMethodRetur
 
 		@SuppressWarnings("unchecked")
 		private <T> void sendInternal(T data, @Nullable MediaType mediaType) throws IOException {
-			for (HttpMessageConverter<?> converter : ResponseBodyEmitterReturnValueHandler.this.messageConverters) {
+			for (HttpMessageConverter<?> converter : ResponseBodyEmitterReturnValueHandler.this.sseMessageConverters) {
 				if (converter.canWrite(data.getClass(), mediaType)) {
 					((HttpMessageConverter<T>) converter).write(data, mediaType, this.outputMessage);
 					this.outputMessage.flush();

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -16,7 +16,6 @@
 
 package org.springframework.web.servlet.mvc.method.annotation;
 
-import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
 import java.util.concurrent.atomic.AtomicReference;
@@ -31,7 +30,6 @@ import org.springframework.core.MethodParameter;
 import org.springframework.core.ResolvableType;
 import org.springframework.http.ResponseEntity;
 import org.springframework.http.converter.HttpMessageConverter;
-import org.springframework.http.converter.StringHttpMessageConverter;
 import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
 import org.springframework.web.context.request.NativeWebRequest;
 import org.springframework.web.context.request.ServletWebRequest;
@@ -72,8 +70,8 @@ public class ResponseBodyEmitterReturnValueHandlerTests {
 	@BeforeEach
 	public void setup() throws Exception {
 
-		List<HttpMessageConverter<?>> converters = Arrays.asList(
+		List<HttpMessageConverter<?>> converters =
-				new StringHttpMessageConverter(), new MappingJackson2HttpMessageConverter());
+				Collections.singletonList(new MappingJackson2HttpMessageConverter());
 
 		this.handler = new ResponseBodyEmitterReturnValueHandler(converters);
 		this.request = new MockHttpServletRequest();