Restore use of Flux-based encode method in HTTP

Closes gh-24441

spring-core/src/main/java/org/springframework/core/codec/Decoder.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.
@@ -78,9 +78,9 @@ public interface Decoder<T> {
 			@Nullable MimeType mimeType, @Nullable Map<String, Object> hints);
 
 	/**
-	 * Decode a data buffer to an Object of type T. This is useful when the input
-	 * stream consists of discrete messages (or events) and the content for each
-	 * can be decoded on its own.
+	 * Decode a data buffer to an Object of type T. This is useful for scenarios,
+	 * that distinct messages (or events) are decoded and handled individually,
+	 * in fully aggregated form.
 	 * @param buffer the {@code DataBuffer} to decode
 	 * @param targetType the expected output type
 	 * @param mimeType the MIME type associated with the data

spring-core/src/main/java/org/springframework/core/codec/Encoder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 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.
@@ -68,9 +68,9 @@ public interface Encoder<T> {
 			ResolvableType elementType, @Nullable MimeType mimeType, @Nullable Map<String, Object> hints);
 
 	/**
-	 * Encode an Object of type T to a data buffer. This is useful for scenarios
-	 * that produce a stream of discrete messages (or events) and the
-	 * content for each is encoded individually.
+	 * Encode an Object of type T to a data buffer. This is useful for scenarios,
+	 * that distinct messages (or events) are encoded and handled individually,
+	 * in fully aggregated form.
 	 * <p>By default this method raises {@link UnsupportedOperationException}
 	 * and it is expected that some encoders cannot produce a single buffer or
 	 * cannot do so synchronously (e.g. encoding a {@code Resource}).

spring-web/src/main/java/org/springframework/http/codec/EncoderHttpMessageWriter.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.
@@ -29,7 +29,6 @@ import org.springframework.core.codec.AbstractEncoder;
 import org.springframework.core.codec.Encoder;
 import org.springframework.core.codec.Hints;
 import org.springframework.core.io.buffer.DataBuffer;
-import org.springframework.core.io.buffer.DataBufferFactory;
 import org.springframework.core.io.buffer.PooledDataBuffer;
 import org.springframework.http.HttpLogging;
 import org.springframework.http.MediaType;
@@ -114,24 +113,23 @@ public class EncoderHttpMessageWriter<T> implements HttpMessageWriter<T> {
 
 		MediaType contentType = updateContentType(message, mediaType);
 
+		Flux<DataBuffer> body = this.encoder.encode(
+				inputStream, message.bufferFactory(), elementType, contentType, hints);
+
 		if (inputStream instanceof Mono) {
-			return Mono.from(inputStream)
+			return body
+					.singleOrEmpty()
 					.switchIfEmpty(Mono.defer(() -> {
 						message.getHeaders().setContentLength(0);
 						return message.setComplete().then(Mono.empty());
 					}))
-					.flatMap(value -> {
-						DataBufferFactory factory = message.bufferFactory();
-						DataBuffer buffer = this.encoder.encodeValue(value, factory, elementType, contentType, hints);
+					.flatMap(buffer -> {
 						message.getHeaders().setContentLength(buffer.readableByteCount());
 						return message.writeWith(Mono.just(buffer)
 								.doOnDiscard(PooledDataBuffer.class, PooledDataBuffer::release));
 					});
 		}
 
-		Flux<DataBuffer> body = this.encoder.encode(
-				inputStream, message.bufferFactory(), elementType, contentType, hints);
-
 		if (isStreamingMediaType(contentType)) {
 			return message.writeAndFlushWith(body.map(buffer ->
 					Mono.just(buffer).doOnDiscard(PooledDataBuffer.class, PooledDataBuffer::release)));

spring-web/src/main/java/org/springframework/http/codec/ServerSentEventHttpMessageWriter.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.
@@ -18,7 +18,7 @@ package org.springframework.http.codec;
 
 import java.nio.charset.StandardCharsets;
 import java.time.Duration;
-import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
 import java.util.Map;
@@ -112,7 +112,7 @@ public class ServerSentEventHttpMessageWriter implements HttpMessageWriter<Objec
 	}
 
 	private Flux<Publisher<DataBuffer>> encode(Publisher<?> input, ResolvableType elementType,
-			MediaType mediaType, DataBufferFactory bufferFactory, Map<String, Object> hints) {
+			MediaType mediaType, DataBufferFactory factory, Map<String, Object> hints) {
 
 		ResolvableType dataType = (ServerSentEvent.class.isAssignableFrom(elementType.toClass()) ?
 				elementType.getGeneric() : elementType);
@@ -144,13 +144,35 @@ public class ServerSentEventHttpMessageWriter implements HttpMessageWriter<Objec
 				sb.append("data:");
 			}
 
-			Mono<DataBuffer> bufferMono = Mono.fromCallable(() ->
-					bufferFactory.join(encodeEvent(sb, data, dataType, mediaType, bufferFactory, hints)));
+			Flux<DataBuffer> result;
+			if (data == null) {
+				result = Flux.just(encodeText(sb + "\n", mediaType, factory));
+			}
+			else if (data instanceof String) {
+				data = StringUtils.replace((String) data, "\n", "\ndata:");
+				result = Flux.just(encodeText(sb + (String) data + "\n\n", mediaType, factory));
+			}
+			else {
+				result = encodeEvent(sb.toString(), data, dataType, mediaType, factory, hints);
+			}
 
-			return bufferMono.doOnDiscard(PooledDataBuffer.class, DataBufferUtils::release);
+			return result.doOnDiscard(PooledDataBuffer.class, DataBufferUtils::release);
 		});
 	}
 
+	@SuppressWarnings("unchecked")
+	private <T> Flux<DataBuffer> encodeEvent(String eventContent, T data, ResolvableType dataType,
+			MediaType mediaType, DataBufferFactory factory, Map<String, Object> hints) {
+
+		if (this.encoder == null) {
+			throw new CodecException("No SSE encoder configured and the data is not String.");
+		}
+		return Flux.just(factory.join(Arrays.asList(
+				encodeText(eventContent, mediaType, factory),
+				((Encoder<T>) this.encoder).encodeValue(data, factory, dataType, mediaType, hints),
+				encodeText("\n\n", mediaType, factory))));
+	}
+
 	private void writeField(String fieldName, Object fieldValue, StringBuilder sb) {
 		sb.append(fieldName);
 		sb.append(':');
@@ -158,29 +180,6 @@ public class ServerSentEventHttpMessageWriter implements HttpMessageWriter<Objec
 		sb.append("\n");
 	}
 
-	@SuppressWarnings("unchecked")
-	private <T> List<DataBuffer> encodeEvent(CharSequence markup, @Nullable T data, ResolvableType dataType,
-			MediaType mediaType, DataBufferFactory factory, Map<String, Object> hints) {
-
-		List<DataBuffer> result = new ArrayList<>(4);
-		result.add(encodeText(markup, mediaType, factory));
-		if (data != null) {
-			if (data instanceof String) {
-				String dataLine = StringUtils.replace((String) data, "\n", "\ndata:") + "\n";
-				result.add(encodeText(dataLine, mediaType, factory));
-			}
-			else if (this.encoder == null) {
-				throw new CodecException("No SSE encoder configured and the data is not String.");
-			}
-			else {
-				result.add(((Encoder<T>) this.encoder).encodeValue(data, factory, dataType, mediaType, hints));
-				result.add(encodeText("\n", mediaType, factory));
-			}
-		}
-		result.add(encodeText("\n", mediaType, factory));
-		return result;
-	}
-
 	private DataBuffer encodeText(CharSequence text, MediaType mediaType, DataBufferFactory bufferFactory) {
 		Assert.notNull(mediaType.getCharset(), "Expected MediaType with charset");
 		byte[] bytes = text.toString().getBytes(mediaType.getCharset());

spring-web/src/test/java/org/springframework/http/codec/EncoderHttpMessageWriterTests.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.
@@ -157,7 +157,7 @@ class EncoderHttpMessageWriterTests {
 	void setContentLengthForMonoBody() {
 		DefaultDataBufferFactory factory = new DefaultDataBufferFactory();
 		DataBuffer buffer = factory.wrap("body".getBytes(StandardCharsets.UTF_8));
-		configureEncoder(buffer, MimeTypeUtils.TEXT_PLAIN);
+		configureEncoder(Flux.just(buffer), MimeTypeUtils.TEXT_PLAIN);
 		HttpMessageWriter<String> writer = new EncoderHttpMessageWriter<>(this.encoder);
 		writer.write(Mono.just("body"), forClass(String.class), TEXT_PLAIN, this.response, NO_HINTS).block();