Polish + minor refactoring of SSE reader and writer

Instead of accepting List<Encoder|Decoder> and then look for the first
to support JSON, always expect a single JSON [Encoder|Decoder] and use
that unconditionally.

When writing use the nested ResolvableType instead of the Class of the
actual value which should better support generics.

Remove the SSE hint and pass "text/event-stream" as the media type
instead to serve as a hint. We are expecting a JSON encoder and using
it unconditionally in any case so this should be good enough.

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

@@ -29,7 +29,6 @@ import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 
 import org.springframework.core.ResolvableType;
-import org.springframework.core.codec.CodecException;
 import org.springframework.core.codec.Decoder;
 import org.springframework.core.codec.StringDecoder;
 import org.springframework.core.io.buffer.DataBuffer;
@@ -39,7 +38,6 @@ import org.springframework.core.io.buffer.DefaultDataBufferFactory;
 import org.springframework.http.MediaType;
 import org.springframework.http.ReactiveHttpInputMessage;
 import org.springframework.util.Assert;
-import org.springframework.util.MimeTypeUtils;
 
 import static java.util.stream.Collectors.joining;
 
@@ -61,18 +59,22 @@ public class ServerSentEventHttpMessageReader implements HttpMessageReader<Objec
 	private static final StringDecoder stringDecoder = new StringDecoder(false);
 
 
-	private final List<Decoder<?>> dataDecoders;
+	private final Decoder<?> decoder;
 
 
-	public ServerSentEventHttpMessageReader() {
-		this.dataDecoders = Collections.emptyList();
+	/**
+	 * Constructor with JSON {@code Encoder} for encoding objects.
+	 */
+	public ServerSentEventHttpMessageReader(Decoder<?> decoder) {
+		Assert.notNull(decoder, "Decoder must not be null");
+		this.decoder = decoder;
 	}
 
-	public ServerSentEventHttpMessageReader(List<Decoder<?>> dataDecoders) {
-		Assert.notNull(dataDecoders, "'dataDecoders' must not be null");
-		this.dataDecoders = new ArrayList<>(dataDecoders);
-	}
 
+	@Override
+	public List<MediaType> getReadableMediaTypes() {
+		return Collections.singletonList(MediaType.TEXT_EVENT_STREAM);
+	}
 
 	@Override
 	public boolean canRead(ResolvableType elementType, MediaType mediaType) {
@@ -80,18 +82,13 @@ public class ServerSentEventHttpMessageReader implements HttpMessageReader<Objec
 				ServerSentEvent.class.isAssignableFrom(elementType.getRawClass());
 	}
 
-	@Override
-	public List<MediaType> getReadableMediaTypes() {
-		return Collections.singletonList(MediaType.TEXT_EVENT_STREAM);
-	}
-
 
 	@Override
 	public Flux<Object> read(ResolvableType elementType, ReactiveHttpInputMessage message,
 			Map<String, Object> hints) {
 
-		boolean hasSseWrapper = ServerSentEvent.class.isAssignableFrom(elementType.getRawClass());
-		ResolvableType dataType = (hasSseWrapper ? elementType.getGeneric(0) : elementType);
+		boolean shouldWrap = ServerSentEvent.class.isAssignableFrom(elementType.getRawClass());
+		ResolvableType valueType = shouldWrap ? elementType.getGeneric(0) : elementType;
 
 		return Flux.from(message.getBody())
 				.concatMap(ServerSentEventHttpMessageReader::splitOnNewline)
@@ -103,8 +100,8 @@ public class ServerSentEventHttpMessageReader implements HttpMessageReader<Objec
 				.bufferUntil(line -> line.equals("\n"))
 				.concatMap(rawLines -> {
 					String[] lines = rawLines.stream().collect(joining()).split("\\r?\\n");
-					ServerSentEvent<Object> event = buildEvent(lines, dataType, hints);
-					return (hasSseWrapper ? Mono.just(event) : Mono.justOrEmpty(event.data()));
+					ServerSentEvent<Object> event = buildEvent(lines, valueType, hints);
+					return (shouldWrap ? Mono.just(event) : Mono.justOrEmpty(event.data()));
 				})
 				.cast(Object.class);
 	}
@@ -126,7 +123,8 @@ public class ServerSentEventHttpMessageReader implements HttpMessageReader<Objec
 		return Flux.fromIterable(results);
 	}
 
-	private ServerSentEvent<Object> buildEvent(String[] lines, ResolvableType dataType, Map<String, Object> hints) {
+	private ServerSentEvent<Object> buildEvent(String[] lines, ResolvableType valueType,
+			Map<String, Object> hints) {
 
 		ServerSentEvent.Builder<Object> sseBuilder = ServerSentEvent.builder();
 		StringBuilder mutableData = new StringBuilder();
@@ -151,7 +149,7 @@ public class ServerSentEventHttpMessageReader implements HttpMessageReader<Objec
 		}
 		if (mutableData.length() > 0) {
 			String data = mutableData.toString();
-			sseBuilder.data(decodeData(data, dataType, hints));
+			sseBuilder.data(decodeData(data, valueType, hints));
 		}
 		if (mutableComment.length() > 0) {
 			String comment = mutableComment.toString();
@@ -169,11 +167,8 @@ public class ServerSentEventHttpMessageReader implements HttpMessageReader<Objec
 		byte[] bytes = data.getBytes(StandardCharsets.UTF_8);
 		Mono<DataBuffer> input = Mono.just(bufferFactory.wrap(bytes));
 
-		return this.dataDecoders.stream()
-				.filter(e -> e.canDecode(dataType, MimeTypeUtils.APPLICATION_JSON))
-				.findFirst()
-				.orElseThrow(() -> new CodecException("No suitable decoder found!"))
-				.decodeToMono(input, dataType, MimeTypeUtils.APPLICATION_JSON, hints)
+		return this.decoder
+				.decodeToMono(input, dataType, MediaType.TEXT_EVENT_STREAM, hints)
 				.block(Duration.ZERO);
 	}
 
@@ -181,7 +176,7 @@ public class ServerSentEventHttpMessageReader implements HttpMessageReader<Objec
 	public Mono<Object> readMono(ResolvableType elementType, ReactiveHttpInputMessage message,
 			Map<String, Object> hints) {
 
-		// Let's give StringDecoder a chance since SSE is ordered ahead of it
+		// For single String give StringDecoder a chance which comes after SSE in the order
 
 		if (String.class.equals(elementType.getRawClass())) {
 			Flux<DataBuffer> body = message.getBody();

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

@@ -17,19 +17,16 @@
 package org.springframework.http.codec;
 
 import java.nio.charset.StandardCharsets;
-import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
-import java.util.Optional;
 
 import org.reactivestreams.Publisher;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 
 import org.springframework.core.ResolvableType;
-import org.springframework.core.codec.CodecException;
 import org.springframework.core.codec.Encoder;
 import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.core.io.buffer.DataBufferFactory;
@@ -38,39 +35,37 @@ import org.springframework.http.ReactiveHttpOutputMessage;
 import org.springframework.http.server.reactive.ServerHttpRequest;
 import org.springframework.http.server.reactive.ServerHttpResponse;
 import org.springframework.util.Assert;
-import org.springframework.util.MimeTypeUtils;
 
 /**
- * Writer that supports a stream of {@link ServerSentEvent}s and also plain
- * {@link Object}s which is the same as an {@link ServerSentEvent} with data
- * only.
+ * {@code ServerHttpMessageWriter} for {@code "text/event-stream"} responses.
  *
  * @author Sebastien Deleuze
  * @author Arjen Poutsma
+ * @author Rossen Stoyanchev
  * @since 5.0
  */
 public class ServerSentEventHttpMessageWriter implements ServerHttpMessageWriter<Object> {
 
-	/**
-	 * Server-Sent Events hint key expecting a {@link Boolean} value which when set to true
-	 * will adapt the content in order to comply with Server-Sent Events recommendation.
-	 * For example, it will append "data:" after each line break with data encoders
-	 * supporting it.
-	 * @see <a href="https://www.w3.org/TR/eventsource/">Server-Sent Events W3C recommendation</a>
-	 */
-	public static final String SSE_CONTENT_HINT = ServerSentEventHttpMessageWriter.class.getName() + ".sseContent";
+	private static final List<MediaType> WRITABLE_MEDIA_TYPES =
+			Collections.singletonList(MediaType.TEXT_EVENT_STREAM);
 
 
-	private final List<Encoder<?>> dataEncoders;
+	private final Encoder<?> encoder;
 
 
-	public ServerSentEventHttpMessageWriter() {
-		this.dataEncoders = Collections.emptyList();
+	/**
+	 * Constructor with JSON {@code Encoder} for encoding objects. Support for
+	 * {@code String} event data is built-in.
+	 */
+	public ServerSentEventHttpMessageWriter(Encoder<?> encoder) {
+		Assert.notNull(encoder, "'encoder' must not be null");
+		this.encoder = encoder;
 	}
 
-	public ServerSentEventHttpMessageWriter(List<Encoder<?>> dataEncoders) {
-		Assert.notNull(dataEncoders, "'dataEncoders' must not be null");
-		this.dataEncoders = new ArrayList<>(dataEncoders);
+
+	@Override
+	public List<MediaType> getWritableMediaTypes() {
+		return WRITABLE_MEDIA_TYPES;
 	}
 
 
@@ -81,61 +76,35 @@ public class ServerSentEventHttpMessageWriter implements ServerHttpMessageWriter
 	}
 
 	@Override
-	public List<MediaType> getWritableMediaTypes() {
-		return Collections.singletonList(MediaType.TEXT_EVENT_STREAM);
-	}
-
-	@Override
-	public Mono<Void> write(Publisher<?> inputStream, ResolvableType elementType, MediaType mediaType,
+	public Mono<Void> write(Publisher<?> input, ResolvableType elementType, MediaType mediaType,
 			ReactiveHttpOutputMessage message, Map<String, Object> hints) {
 
 		message.getHeaders().setContentType(MediaType.TEXT_EVENT_STREAM);
+		return message.writeAndFlushWith(encode(input, message.bufferFactory(), elementType, hints));
+	}
 
-		DataBufferFactory bufferFactory = message.bufferFactory();
-		Flux<Publisher<DataBuffer>> body = encode(inputStream, bufferFactory, elementType, hints);
+	private Flux<Publisher<DataBuffer>> encode(Publisher<?> input, DataBufferFactory factory,
+			ResolvableType elementType, Map<String, Object> hints) {
 
-		return message.writeAndFlushWith(body);
-	}
+		ResolvableType valueType = ServerSentEvent.class.isAssignableFrom(elementType.getRawClass()) ?
+				elementType.getGeneric(0) : elementType;
 
-	private Flux<Publisher<DataBuffer>> encode(Publisher<?> inputStream, DataBufferFactory bufferFactory,
-			ResolvableType type, Map<String, Object> hints) {
-
-		Map<String, Object> hintsWithSse = new HashMap<>(hints);
-		hintsWithSse.put(SSE_CONTENT_HINT, true);
-		return Flux.from(inputStream)
-				.map(o -> toSseEvent(o, type))
-				.map(sse -> {
-					StringBuilder sb = new StringBuilder();
-					sse.id().ifPresent(id -> writeField("id", id, sb));
-					sse.event().ifPresent(event -> writeField("event", event, sb));
-					sse.retry().ifPresent(retry -> writeField("retry", retry.toMillis(), sb));
-					sse.comment().ifPresent(comment -> {
-						comment = comment.replaceAll("\\n", "\n:");
-						sb.append(':').append(comment).append("\n");
-					});
-					Flux<DataBuffer> dataBuffer = sse.data()
-							.<Flux<DataBuffer>>map(data -> {
-								sb.append("data:");
-								if (data instanceof String) {
-									String stringData = ((String) data).replaceAll("\\n", "\ndata:");
-									sb.append(stringData).append('\n');
-									return Flux.empty();
-								}
-								else {
-									return applyEncoder(data, bufferFactory, hintsWithSse);
-								}
-							}).orElse(Flux.empty());
-
-					return Flux.concat(encodeString(sb.toString(), bufferFactory), dataBuffer,
-							encodeString("\n", bufferFactory));
-				});
+		return Flux.from(input).map(element -> {
 
-	}
+			ServerSentEvent<?> sse = element instanceof ServerSentEvent ?
+					(ServerSentEvent<?>) element : ServerSentEvent.builder().data(element).build();
+
+			StringBuilder sb = new StringBuilder();
+			sse.id().ifPresent(v -> writeField("id", v, sb));
+			sse.event().ifPresent(v -> writeField("event", v, sb));
+			sse.retry().ifPresent(v -> writeField("retry", v.toMillis(), sb));
+			sse.comment().ifPresent(v -> sb.append(':').append(v.replaceAll("\\n", "\n:")).append("\n"));
+			sse.data().ifPresent(v -> sb.append("data:"));
 
-	private ServerSentEvent<?> toSseEvent(Object data, ResolvableType type) {
-		return ServerSentEvent.class.isAssignableFrom(type.getRawClass())
-				? (ServerSentEvent<?>) data
-				: ServerSentEvent.builder().data(data).build();
+			return Flux.concat(encodeText(sb, factory),
+					encodeData(sse, valueType, factory, hints),
+					encodeText("\n", factory));
+		});
 	}
 
 	private void writeField(String fieldName, Object fieldValue, StringBuilder stringBuilder) {
@@ -146,40 +115,50 @@ public class ServerSentEventHttpMessageWriter implements ServerHttpMessageWriter
 	}
 
 	@SuppressWarnings("unchecked")
-	private <T> Flux<DataBuffer> applyEncoder(Object data, DataBufferFactory bufferFactory, Map<String, Object> hints) {
-		ResolvableType elementType = ResolvableType.forClass(data.getClass());
-		Optional<Encoder<?>> encoder = dataEncoders
-				.stream()
-				.filter(e -> e.canEncode(elementType, MimeTypeUtils.APPLICATION_JSON))
-				.findFirst();
-		return ((Encoder<T>) encoder.orElseThrow(() -> new CodecException("No suitable encoder found!")))
-				.encode(Mono.just((T) data), bufferFactory, elementType, MimeTypeUtils.APPLICATION_JSON, hints)
-				.concatWith(encodeString("\n", bufferFactory));
+	private <T> Flux<DataBuffer> encodeData(ServerSentEvent<?> event, ResolvableType valueType,
+			DataBufferFactory factory, Map<String, Object> hints) {
+
+		Object data = event.data().orElse(null);
+		if (data == null) {
+			return Flux.empty();
+		}
+
+		if (data instanceof String) {
+			String text = (String) data;
+			return Flux.from(encodeText(text.replaceAll("\\n", "\ndata:") + "\n", factory));
+		}
+
+		return ((Encoder<T>) this.encoder)
+				.encode(Mono.just((T) data), factory, valueType, MediaType.TEXT_EVENT_STREAM, hints)
+				.concatWith(encodeText("\n", factory));
 	}
 
-	private Mono<DataBuffer> encodeString(String str, DataBufferFactory bufferFactory) {
-		byte[] bytes = str.getBytes(StandardCharsets.UTF_8);
+	private Mono<DataBuffer> encodeText(CharSequence text, DataBufferFactory bufferFactory) {
+		byte[] bytes = text.toString().getBytes(StandardCharsets.UTF_8);
 		DataBuffer buffer = bufferFactory.allocateBuffer(bytes.length).write(bytes);
 		return Mono.just(buffer);
 	}
 
 	@Override
-	public Mono<Void> write(Publisher<?> inputStream, ResolvableType actualType, ResolvableType elementType,
+	public Mono<Void> write(Publisher<?> input, ResolvableType actualType, ResolvableType elementType,
 			MediaType mediaType, ServerHttpRequest request, ServerHttpResponse response,
 			Map<String, Object> hints) {
 
-		Map<String, Object> allHints = this.dataEncoders.stream()
-				.filter(encoder -> encoder instanceof ServerHttpEncoder)
-				.map(encoder -> (ServerHttpEncoder<?>) encoder)
-				.map(encoder -> encoder.getEncodeHints(actualType, elementType, mediaType, request, response))
-				.reduce(new HashMap<>(), (t, u) -> {
-					t.putAll(u);
-					return t;
-				});
-
+		Map<String, Object> allHints = new HashMap<>();
+		allHints.putAll(getEncodeHints(actualType, elementType, mediaType, request, response));
 		allHints.putAll(hints);
 
-		return write(inputStream, elementType, mediaType, response, allHints);
+		return write(input, elementType, mediaType, response, allHints);
+	}
+
+	private Map<String, Object> getEncodeHints(ResolvableType actualType, ResolvableType elementType,
+			MediaType mediaType, ServerHttpRequest request, ServerHttpResponse response) {
+
+		if (this.encoder instanceof ServerHttpEncoder) {
+			ServerHttpEncoder<?> httpEncoder = (ServerHttpEncoder<?>) this.encoder;
+			return httpEncoder.getEncodeHints(actualType, elementType, mediaType, request, response);
+		}
+		return Collections.emptyMap();
 	}
 
 }

spring-web/src/main/java/org/springframework/http/codec/json/Jackson2JsonEncoder.java

@@ -28,7 +28,6 @@ import com.fasterxml.jackson.core.util.DefaultPrettyPrinter;
 import com.fasterxml.jackson.databind.JavaType;
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.fasterxml.jackson.databind.ObjectWriter;
-import com.fasterxml.jackson.databind.SerializationConfig;
 import com.fasterxml.jackson.databind.SerializationFeature;
 import com.fasterxml.jackson.databind.type.TypeFactory;
 import org.reactivestreams.Publisher;
@@ -42,7 +41,6 @@ import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.core.io.buffer.DataBufferFactory;
 import org.springframework.http.MediaType;
 import org.springframework.http.codec.ServerHttpEncoder;
-import org.springframework.http.codec.ServerSentEventHttpMessageWriter;
 import org.springframework.http.converter.json.Jackson2ObjectMapperBuilder;
 import org.springframework.http.server.reactive.ServerHttpRequest;
 import org.springframework.http.server.reactive.ServerHttpResponse;
@@ -97,20 +95,24 @@ public class Jackson2JsonEncoder extends Jackson2CodecSupport implements ServerH
 		Assert.notNull(elementType, "'elementType' must not be null");
 
 		if (inputStream instanceof Mono) {
-			return Flux.from(inputStream).map(value -> encodeValue(value, bufferFactory, elementType, hints));
+			return Flux.from(inputStream).map(value ->
+					encodeValue(value, mimeType, bufferFactory, elementType, hints));
 		}
 		else if (APPLICATION_STREAM_JSON.isCompatibleWith(mimeType)) {
 			return Flux.from(inputStream).map(value -> {
-				DataBuffer buffer = encodeValue(value, bufferFactory, elementType, hints);
+				DataBuffer buffer = encodeValue(value, mimeType, bufferFactory, elementType, hints);
 				buffer.write(new byte[]{'\n'});
 				return buffer;
 			});
 		}
-		ResolvableType listType = ResolvableType.forClassWithGenerics(List.class, elementType);
-		return Flux.from(inputStream).collectList().map(list -> encodeValue(list, bufferFactory, listType, hints)).flux();
+		else {
+			ResolvableType listType = ResolvableType.forClassWithGenerics(List.class, elementType);
+			return Flux.from(inputStream).collectList().map(list ->
+					encodeValue(list, mimeType, bufferFactory, listType, hints)).flux();
+		}
 	}
 
-	private DataBuffer encodeValue(Object value, DataBufferFactory bufferFactory,
+	private DataBuffer encodeValue(Object value, MimeType mimeType, DataBufferFactory bufferFactory,
 			ResolvableType elementType, Map<String, Object> hints) {
 
 		TypeFactory typeFactory = this.mapper.getTypeFactory();
@@ -126,9 +128,9 @@ public class Jackson2JsonEncoder extends Jackson2CodecSupport implements ServerH
 			writer = writer.forType(javaType);
 		}
 
-		SerializationConfig config = writer.getConfig();
-		Boolean sseHint = (Boolean) hints.get(ServerSentEventHttpMessageWriter.SSE_CONTENT_HINT);
-		if (Boolean.TRUE.equals(sseHint) && config.isEnabled(SerializationFeature.INDENT_OUTPUT)) {
+		if (MediaType.TEXT_EVENT_STREAM.isCompatibleWith(mimeType) &&
+				writer.getConfig().isEnabled(SerializationFeature.INDENT_OUTPUT)) {
+
 			writer = writer.with(this.ssePrettyPrinter);
 		}
 

spring-web/src/test/java/org/springframework/http/codec/ServerSentEventHttpMessageReaderTests.java

@@ -21,7 +21,6 @@ import java.util.Collections;
 
 import org.junit.Test;
 import reactor.core.publisher.Flux;
-import reactor.core.publisher.Mono;
 import reactor.test.StepVerifier;
 
 import org.springframework.core.ResolvableType;
@@ -39,8 +38,8 @@ import static org.junit.Assert.assertTrue;
  */
 public class ServerSentEventHttpMessageReaderTests extends AbstractDataBufferAllocatingTestCase {
 
-	private ServerSentEventHttpMessageReader messageReader = new ServerSentEventHttpMessageReader(
-			Collections.singletonList(new Jackson2JsonDecoder()));
+	private ServerSentEventHttpMessageReader messageReader =
+			new ServerSentEventHttpMessageReader(new Jackson2JsonDecoder());
 
 	@Test
 	public void cantRead() {

spring-web/src/test/java/org/springframework/http/codec/ServerSentEventHttpMessageWriterTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -18,14 +18,15 @@ package org.springframework.http.codec;
 
 import java.time.Duration;
 import java.util.Collections;
+import java.util.Map;
 
 import com.fasterxml.jackson.databind.ObjectMapper;
 import org.junit.Test;
+import org.reactivestreams.Publisher;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 import reactor.test.StepVerifier;
 
-import org.springframework.core.ResolvableType;
 import org.springframework.core.io.buffer.AbstractDataBufferAllocatingTestCase;
 import org.springframework.http.MediaType;
 import org.springframework.http.codec.json.Jackson2JsonEncoder;
@@ -34,49 +35,45 @@ import org.springframework.mock.http.server.reactive.test.MockServerHttpResponse
 
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertTrue;
+import static org.springframework.core.ResolvableType.forClass;
 
 /**
+ * Unit tests for {@link ServerSentEventHttpMessageWriter}.
  * @author Sebastien Deleuze
+ * @author Rossen Stoyanchev
  */
 public class ServerSentEventHttpMessageWriterTests extends AbstractDataBufferAllocatingTestCase {
 
-	private ServerSentEventHttpMessageWriter messageWriter = new ServerSentEventHttpMessageWriter(
-			Collections.singletonList(new Jackson2JsonEncoder()));
+	public static final Map<String, Object> HINTS = Collections.emptyMap();
+
+	private ServerSentEventHttpMessageWriter messageWriter =
+			new ServerSentEventHttpMessageWriter(new Jackson2JsonEncoder());
+
 
 	@Test
-	public void cantRead() {
-		assertFalse(messageWriter.canWrite(ResolvableType.forClass(Object.class),
-				new MediaType("foo", "bar")));
+	public void canWrite() {
+		assertTrue(this.messageWriter.canWrite(forClass(Object.class), null));
+		assertTrue(this.messageWriter.canWrite(null, MediaType.TEXT_EVENT_STREAM));
+		assertTrue(this.messageWriter.canWrite(forClass(ServerSentEvent.class), new MediaType("foo", "bar")));
 	}
 
 	@Test
-	public void canRead() {
-		assertTrue(messageWriter.canWrite(ResolvableType.forClass(Object.class), null));
-		assertTrue(messageWriter.canWrite(ResolvableType.forClass(Object.class),
-				new MediaType("text", "event-stream")));
-		assertTrue(messageWriter.canWrite(ResolvableType.forClass(ServerSentEvent.class),
-				new MediaType("bar", "bar")));
+	public void canNotWrite() {
+		assertFalse(this.messageWriter.canWrite(forClass(Object.class), new MediaType("foo", "bar")));
 	}
 
 	@Test
 	public void writeServerSentEvent() {
-		ServerSentEvent<String> event = ServerSentEvent.<String>builder().
-				data("bar").id("c42").event("foo").comment("bla\nbla bla\nbla bla bla")
-				.retry(Duration.ofMillis(123L)).build();
 
-		Mono<ServerSentEvent<String>> source = Mono.just(event);
+		ServerSentEvent<?> event = ServerSentEvent.builder().data("bar").id("c42").event("foo")
+				.comment("bla\nbla bla\nbla bla bla").retry(Duration.ofMillis(123L)).build();
+
+		Mono<ServerSentEvent> source = Mono.just(event);
 		MockServerHttpResponse outputMessage = new MockServerHttpResponse();
-		messageWriter.write(source, ResolvableType.forClass(ServerSentEvent.class),
-				new MediaType("text", "event-stream"), outputMessage, Collections.emptyMap()).block(Duration.ofMillis(5000));
+		testWrite(source, outputMessage, ServerSentEvent.class);
 
 		StepVerifier.create(outputMessage.getBodyAsString())
-				.expectNext("id:c42\n" +
-						"event:foo\n" +
-						"retry:123\n" +
-						":bla\n" +
-						":bla bla\n" +
-						":bla bla bla\n" +
-						"data:bar\n\n")
+				.expectNext("id:c42\nevent:foo\nretry:123\n:bla\n:bla bla\n:bla bla bla\ndata:bar\n\n")
 				.expectComplete()
 				.verify();
 	}
@@ -85,8 +82,7 @@ public class ServerSentEventHttpMessageWriterTests extends AbstractDataBufferAll
 	public void writeString() {
 		Flux<String> source = Flux.just("foo", "bar");
 		MockServerHttpResponse outputMessage = new MockServerHttpResponse();
-		messageWriter.write(source, ResolvableType.forClass(String.class),
-				new MediaType("text", "event-stream"), outputMessage, Collections.emptyMap()).block(Duration.ofMillis(5000));
+		testWrite(source, outputMessage, String.class);
 
 		StepVerifier.create(outputMessage.getBodyAsString())
 				.expectNext("data:foo\n\ndata:bar\n\n")
@@ -98,25 +94,19 @@ public class ServerSentEventHttpMessageWriterTests extends AbstractDataBufferAll
 	public void writeMultiLineString() {
 		Flux<String> source = Flux.just("foo\nbar", "foo\nbaz");
 		MockServerHttpResponse outputMessage = new MockServerHttpResponse();
-		messageWriter.write(source, ResolvableType.forClass(String.class),
-				new MediaType("text", "event-stream"), outputMessage, Collections.emptyMap()).block(Duration.ofMillis(5000));
+		testWrite(source, outputMessage, String.class);
 
 		StepVerifier.create(outputMessage.getBodyAsString())
-				.expectNext("data:foo\n" +
-						"data:bar\n\n" +
-						"data:foo\n" +
-						"data:baz\n\n")
+				.expectNext("data:foo\ndata:bar\n\ndata:foo\ndata:baz\n\n")
 				.expectComplete()
 				.verify();
 	}
 
 	@Test
 	public void writePojo() {
-		Flux<Pojo> source = Flux.just(new Pojo("foofoo", "barbar"),
-				new Pojo("foofoofoo", "barbarbar"));
+		Flux<Pojo> source = Flux.just(new Pojo("foofoo", "barbar"), new Pojo("foofoofoo", "barbarbar"));
 		MockServerHttpResponse outputMessage = new MockServerHttpResponse();
-		messageWriter.write(source, ResolvableType.forClass(Pojo.class),
-				MediaType.TEXT_EVENT_STREAM, outputMessage, Collections.emptyMap()).block(Duration.ofMillis(5000));
+		testWrite(source, outputMessage, Pojo.class);
 
 		StepVerifier.create(outputMessage.getBodyAsString())
 				.expectNext("data:{\"foo\":\"foofoo\",\"bar\":\"barbar\"}\n\n" +
@@ -127,15 +117,13 @@ public class ServerSentEventHttpMessageWriterTests extends AbstractDataBufferAll
 
 	@Test  // SPR-14899
 	public void writePojoWithPrettyPrint() {
+
 		ObjectMapper mapper = Jackson2ObjectMapperBuilder.json().indentOutput(true).build();
-		this.messageWriter = new ServerSentEventHttpMessageWriter(
-				Collections.singletonList(new Jackson2JsonEncoder(mapper)));
+		this.messageWriter = new ServerSentEventHttpMessageWriter(new Jackson2JsonEncoder(mapper));
 
-		Flux<Pojo> source = Flux.just(new Pojo("foofoo", "barbar"),
-				new Pojo("foofoofoo", "barbarbar"));
+		Flux<Pojo> source = Flux.just(new Pojo("foofoo", "barbar"), new Pojo("foofoofoo", "barbarbar"));
 		MockServerHttpResponse outputMessage = new MockServerHttpResponse();
-		messageWriter.write(source, ResolvableType.forClass(Pojo.class),
-				MediaType.TEXT_EVENT_STREAM, outputMessage, Collections.emptyMap()).block(Duration.ofMillis(5000));
+		testWrite(source, outputMessage, Pojo.class);
 
 		StepVerifier.create(outputMessage.getBodyAsString())
 				.expectNext("data:{\n" +
@@ -148,4 +136,9 @@ public class ServerSentEventHttpMessageWriterTests extends AbstractDataBufferAll
 				.verify();
 	}
 
+	private <T> void testWrite(Publisher<T> source, MockServerHttpResponse outputMessage, Class<T> clazz) {
+		this.messageWriter.write(source, forClass(clazz),
+				MediaType.TEXT_EVENT_STREAM, outputMessage, HINTS).block(Duration.ofMillis(5000));
+	}
+
 }

spring-webflux/src/main/java/org/springframework/web/reactive/config/WebFluxConfigurationSupport.java

@@ -37,7 +37,6 @@ import org.springframework.core.codec.ByteBufferEncoder;
 import org.springframework.core.codec.CharSequenceEncoder;
 import org.springframework.core.codec.DataBufferDecoder;
 import org.springframework.core.codec.DataBufferEncoder;
-import org.springframework.core.codec.Encoder;
 import org.springframework.core.codec.ResourceDecoder;
 import org.springframework.core.codec.StringDecoder;
 import org.springframework.core.convert.converter.Converter;
@@ -475,7 +474,6 @@ public class WebFluxConfigurationSupport implements ApplicationContextAware {
 	 * {@link #configureMessageWriters(List)}.
 	 */
 	protected final void addDefaultHttpMessageWriters(List<ServerHttpMessageWriter<?>> writers) {
-		List<Encoder<?>> sseDataEncoders = new ArrayList<>();
 		writers.add(new EncoderHttpMessageWriter<>(new ByteArrayEncoder()));
 		writers.add(new EncoderHttpMessageWriter<>(new ByteBufferEncoder()));
 		writers.add(new EncoderHttpMessageWriter<>(new DataBufferEncoder()));
@@ -485,11 +483,10 @@ public class WebFluxConfigurationSupport implements ApplicationContextAware {
 			writers.add(new EncoderHttpMessageWriter<>(new Jaxb2XmlEncoder()));
 		}
 		if (jackson2Present) {
-			Jackson2JsonEncoder encoder = new Jackson2JsonEncoder();
-			writers.add(new EncoderHttpMessageWriter<>(encoder));
-			sseDataEncoders.add(encoder);
+			Jackson2JsonEncoder jacksonEncoder = new Jackson2JsonEncoder();
+			writers.add(new EncoderHttpMessageWriter<>(jacksonEncoder));
+			writers.add(new ServerSentEventHttpMessageWriter(jacksonEncoder));
 		}
-		writers.add(new ServerSentEventHttpMessageWriter(sseDataEncoders));
 	}
 
 	/**

spring-webflux/src/main/java/org/springframework/web/reactive/function/client/DefaultExchangeStrategiesBuilder.java

@@ -77,7 +77,10 @@ class DefaultExchangeStrategiesBuilder implements ExchangeStrategies.Builder {
 	private void defaultReaders() {
 		messageReader(new DecoderHttpMessageReader<>(new ByteArrayDecoder()));
 		messageReader(new DecoderHttpMessageReader<>(new ByteBufferDecoder()));
-		messageReader(new ServerSentEventHttpMessageReader(sseDecoders()));
+		if (jackson2Present) {
+			// SSE ahead of String e.g. "test/event-stream" + Flux<String>
+			messageReader(new ServerSentEventHttpMessageReader(new Jackson2JsonDecoder()));
+		}
 		messageReader(new DecoderHttpMessageReader<>(new StringDecoder(false)));
 		if (jaxb2Present) {
 			messageReader(new DecoderHttpMessageReader<>(new Jaxb2XmlDecoder()));
@@ -87,11 +90,6 @@ class DefaultExchangeStrategiesBuilder implements ExchangeStrategies.Builder {
 		}
 	}
 
-	private List<Decoder<?>> sseDecoders() {
-		return jackson2Present ? Collections.singletonList(new Jackson2JsonDecoder()) :
-				Collections.emptyList();
-	}
-
 	private void defaultWriters() {
 		messageWriter(new EncoderHttpMessageWriter<>(new ByteArrayEncoder()));
 		messageWriter(new EncoderHttpMessageWriter<>(new ByteBufferEncoder()));

spring-webflux/src/main/java/org/springframework/web/reactive/function/server/DefaultHandlerStrategiesBuilder.java

@@ -99,11 +99,7 @@ class DefaultHandlerStrategiesBuilder implements HandlerStrategies.Builder {
 			messageReader(new DecoderHttpMessageReader<>(new Jackson2JsonDecoder()));
 			Jackson2JsonEncoder jsonEncoder = new Jackson2JsonEncoder();
 			messageWriter(new EncoderHttpMessageWriter<>(jsonEncoder));
-			messageWriter(
-					new ServerSentEventHttpMessageWriter(Collections.singletonList(jsonEncoder)));
-		}
-		else {
-			messageWriter(new ServerSentEventHttpMessageWriter());
+			messageWriter(new ServerSentEventHttpMessageWriter(jsonEncoder));
 		}
 		localeResolver(DEFAULT_LOCALE_RESOLVER);
 	}

spring-webflux/src/test/java/org/springframework/web/reactive/function/BodyInsertersTests.java

@@ -21,7 +21,6 @@ import java.nio.ByteBuffer;
 import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
 import java.util.ArrayList;
-import java.util.Collections;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
@@ -59,9 +58,9 @@ import org.springframework.mock.http.server.reactive.test.MockServerHttpResponse
 import org.springframework.util.LinkedMultiValueMap;
 import org.springframework.util.MultiValueMap;
 
-import static java.nio.charset.StandardCharsets.*;
-import static org.junit.Assert.*;
-import static org.springframework.http.codec.json.Jackson2CodecSupport.*;
+import static java.nio.charset.StandardCharsets.UTF_8;
+import static org.junit.Assert.assertArrayEquals;
+import static org.springframework.http.codec.json.Jackson2CodecSupport.JSON_VIEW_HINT;
 
 /**
  * @author Arjen Poutsma
@@ -83,8 +82,7 @@ public class BodyInsertersTests {
 		messageWriters.add(new EncoderHttpMessageWriter<>(new Jaxb2XmlEncoder()));
 		Jackson2JsonEncoder jsonEncoder = new Jackson2JsonEncoder();
 		messageWriters.add(new EncoderHttpMessageWriter<>(jsonEncoder));
-		messageWriters
-				.add(new ServerSentEventHttpMessageWriter(Collections.singletonList(jsonEncoder)));
+		messageWriters.add(new ServerSentEventHttpMessageWriter(jsonEncoder));
 		messageWriters.add(new FormHttpMessageWriter());
 
 		this.context = new BodyInserter.Context() {