Deferred buffer allocation in SSE message writer

See gh-26746

spring-web/src/main/java/org/springframework/http/codec/ServerSentEventHttpMessageWriter.java

 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -18,7 +18,6 @@ package org.springframework.http.codec;
 
 import java.nio.charset.StandardCharsets;
 import java.time.Duration;
-import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
 import java.util.Map;
@@ -171,12 +170,14 @@ public class ServerSentEventHttpMessageWriter implements HttpMessageWriter<Objec
 		if (this.encoder == null) {
 			throw new CodecException("No SSE encoder configured and the data is not String.");
 		}
-		DataBuffer buffer = ((Encoder<T>) this.encoder).encodeValue(data, factory, dataType, mediaType, hints);
-		Hints.touchDataBuffer(buffer, hints, logger);
-		return Flux.just(factory.join(Arrays.asList(
-				encodeText(eventContent, mediaType, factory),
-				buffer,
-				encodeText("\n\n", mediaType, factory))));
+
+		return Flux.defer(() -> {
+			DataBuffer startBuffer = encodeText(eventContent, mediaType, factory);
+			DataBuffer endBuffer = encodeText("\n\n", mediaType, factory);
+			DataBuffer dataBuffer = ((Encoder<T>) this.encoder).encodeValue(data, factory, dataType, mediaType, hints);
+			Hints.touchDataBuffer(dataBuffer, hints, logger);
+			return Flux.just(startBuffer, dataBuffer, endBuffer);
+		});
 	}
 
 	private void writeField(String fieldName, Object fieldValue, StringBuilder sb) {