String decoding for text only vs any MIME type

Follow-up to:
https://github.com/spring-projects/spring-framework/commit/3d68c496f15c80a75711480a569cda1f7c64dc91

StringDecoder can be created in text-only vs "*/*" mode which in turn
allows a more intuitive order of client side decoders, e.g. SSE does
not have to be ahead of StringDecoder.

The commit also explicitly disables String from the supported types in
Jackson2Decoder leaving it to the StringDecoder in "*/*" mode which
comes after. This does not change the current arrangement since the
the StringDecoder ahead having "*/*" picks up JSON content just the
same.

From a broader perspective this change allows any decoder to deal with
String if it wants to after examining the content type be it the SSE
or another, custom decoder. For Jackson there is very little value in
decoding to String which works only if the output contains a single
JSON string but will fail to parse anything else (JSON object/array)
while StringDecoder in "*/*" mode will not fail.

Issue: SPR-15374

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

@@ -55,7 +55,7 @@ public class ServerSentEventHttpMessageReader implements HttpMessageReader<Objec
 
 	private static final DataBufferFactory bufferFactory = new DefaultDataBufferFactory();
 
-	private static final StringDecoder stringDecoder = new StringDecoder(false);
+	private static final StringDecoder stringDecoder = StringDecoder.textPlainOnly(false);
 
 
 	private final Decoder<?> decoder;
@@ -190,6 +190,9 @@ public class ServerSentEventHttpMessageReader implements HttpMessageReader<Objec
 	public Mono<Object> readMono(ResolvableType elementType, ReactiveHttpInputMessage message,
 			Map<String, Object> hints) {
 
+		// We're ahead of String + "*/*"
+		// Let's see if we can aggregate the output (lest we time out)...
+
 		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/json/Jackson2CodecSupport.java

@@ -74,6 +74,11 @@ public abstract class Jackson2CodecSupport {
 	}
 
 
+	protected boolean supportsMimeType(MimeType mimeType) {
+		return mimeType == null ||
+				JSON_MIME_TYPES.stream().anyMatch(m -> m.isCompatibleWith(mimeType));
+	}
+
 	/**
 	 * Return the Jackson {@link JavaType} for the specified type and context class.
 	 * <p>The default implementation returns {@code typeFactory.constructType(type, contextClass)},

spring-web/src/main/java/org/springframework/http/codec/json/Jackson2JsonDecoder.java

@@ -67,10 +67,12 @@ public class Jackson2JsonDecoder extends Jackson2CodecSupport implements HttpDec
 	@Override
 	public boolean canDecode(ResolvableType elementType, MimeType mimeType) {
 		JavaType javaType = this.mapper.getTypeFactory().constructType(elementType.getType());
-		return this.mapper.canDeserialize(javaType) &&
-				(mimeType == null || JSON_MIME_TYPES.stream().anyMatch(m -> m.isCompatibleWith(mimeType)));
+		// Skip String (CharSequenceDecoder + "*/*" comes after)
+		return !CharSequence.class.isAssignableFrom(elementType.resolve(Object.class)) &&
+				this.mapper.canDeserialize(javaType) && supportsMimeType(mimeType);
 	}
 
+
 	@Override
 	public List<MimeType> getDecodableMimeTypes() {
 		return JSON_MIME_TYPES;

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

@@ -104,8 +104,8 @@ public class Jackson2JsonEncoder extends Jackson2CodecSupport implements HttpEnc
 
 	@Override
 	public boolean canEncode(ResolvableType elementType, MimeType mimeType) {
-		return this.mapper.canSerialize(elementType.getRawClass()) &&
-				(mimeType == null || JSON_MIME_TYPES.stream().anyMatch(m -> m.isCompatibleWith(mimeType)));
+		Class<?> clazz = elementType.getRawClass();
+		return this.mapper.canSerialize(clazz) && supportsMimeType(mimeType);
 	}
 
 	@Override