String encoding for any MIME type

CharSequenceEncoder now supports all MIME types, however since encoding
Flux<String> can overlap with other encoders (e.g. SSE) there are now
two ways to create a CharSequenceEncoder -- with support for text/plain
only or with support for any MIME type.

In WebFlux configuration we insert one CharSequenceEncoder for
text/plain (as we have so far) and a second instance with support for
any MIME type at the very end.

Issue: SPR-15374

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

@@ -37,7 +37,6 @@ import org.springframework.core.io.buffer.DataBufferUtils;
 import org.springframework.core.io.buffer.DefaultDataBufferFactory;
 import org.springframework.http.MediaType;
 import org.springframework.http.ReactiveHttpInputMessage;
-import org.springframework.util.Assert;
 
 import static java.util.stream.Collectors.joining;
 
@@ -63,10 +62,18 @@ public class ServerSentEventHttpMessageReader implements HttpMessageReader<Objec
 
 
 	/**
-	 * Constructor with JSON {@code Encoder} for encoding objects.
+	 * Constructor without a {@code Decoder}. In this mode only {@code String}
+	 * is supported as the data of an event.
+	 */
+	public ServerSentEventHttpMessageReader() {
+		this(null);
+	}
+
+	/**
+	 * Constructor with JSON {@code Decoder} for decoding to Objects. Support
+	 * for decoding to {@code String} event data is built-in.
 	 */
 	public ServerSentEventHttpMessageReader(Decoder<?> decoder) {
-		Assert.notNull(decoder, "Decoder must not be null");
 		this.decoder = decoder;
 	}
 
@@ -85,7 +92,7 @@ public class ServerSentEventHttpMessageReader implements HttpMessageReader<Objec
 
 	@Override
 	public boolean canRead(ResolvableType elementType, MediaType mediaType) {
-		return MediaType.TEXT_EVENT_STREAM.isCompatibleWith(mediaType) ||
+		return MediaType.TEXT_EVENT_STREAM.includes(mediaType) ||
 				ServerSentEvent.class.isAssignableFrom(elementType.getRawClass());
 	}
 
@@ -183,8 +190,6 @@ public class ServerSentEventHttpMessageReader implements HttpMessageReader<Objec
 	public Mono<Object> readMono(ResolvableType elementType, ReactiveHttpInputMessage message,
 			Map<String, Object> hints) {
 
-		// 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();
 			return stringDecoder.decodeToMono(body, elementType, null, null).cast(Object.class);

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

@@ -27,6 +27,7 @@ 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;
@@ -34,7 +35,6 @@ import org.springframework.http.MediaType;
 import org.springframework.http.ReactiveHttpOutputMessage;
 import org.springframework.http.server.reactive.ServerHttpRequest;
 import org.springframework.http.server.reactive.ServerHttpResponse;
-import org.springframework.util.Assert;
 
 /**
  * {@code ServerHttpMessageWriter} for {@code "text/event-stream"} responses.
@@ -53,18 +53,25 @@ public class ServerSentEventHttpMessageWriter implements ServerHttpMessageWriter
 	private final Encoder<?> encoder;
 
 
+	/**
+	 * Constructor without an {@code Encoder}. In this mode only {@code String}
+	 * is supported for event data to be encoded.
+	 */
+	public ServerSentEventHttpMessageWriter() {
+		this(null);
+	}
+
 	/**
 	 * 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;
 	}
 
 
 	/**
-	 * Return the configured {@code Encoder}.
+	 * Return the configured {@code Encoder}, possibly {@code null}.
 	 */
 	public Encoder<?> getEncoder() {
 		return this.encoder;
@@ -78,7 +85,7 @@ public class ServerSentEventHttpMessageWriter implements ServerHttpMessageWriter
 
 	@Override
 	public boolean canWrite(ResolvableType elementType, MediaType mediaType) {
-		return mediaType == null || MediaType.TEXT_EVENT_STREAM.isCompatibleWith(mediaType) ||
+		return mediaType == null || MediaType.TEXT_EVENT_STREAM.includes(mediaType) ||
 				ServerSentEvent.class.isAssignableFrom(elementType.getRawClass());
 	}
 
@@ -135,6 +142,10 @@ public class ServerSentEventHttpMessageWriter implements ServerHttpMessageWriter
 			return Flux.from(encodeText(text.replaceAll("\\n", "\ndata:") + "\n", factory));
 		}
 
+		if (this.encoder == null) {
+			return Flux.error(new CodecException("No SSE encoder configured and the data is not String."));
+		}
+
 		return ((Encoder<T>) this.encoder)
 				.encode(Mono.just((T) data), factory, valueType, MediaType.TEXT_EVENT_STREAM, hints)
 				.concatWith(encodeText("\n", factory));