Fix handling of ServerSentEvent with Jackson encoder

This commit fixes ServerSentEvent handling with Jackson encoder
when no Accept header is specified.

It also moves the String checks to the JSON codec level, as they do not
make sense for binary formats.

Closes gh-35872

Author: sdeleuze

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

@@ -102,14 +102,7 @@ public int getMaxInMemorySize() {
 
 	@Override
 	public boolean canDecode(ResolvableType elementType, @Nullable MimeType mimeType) {
-		if (!supportsMimeType(mimeType)) {
-			return false;
-		}
-		T mapper = selectMapper(elementType, mimeType);
-		if (mapper == null) {
-			return false;
-		}
-		return !CharSequence.class.isAssignableFrom(elementType.toClass());
+		return supportsMimeType(mimeType) && (selectMapper(elementType, mimeType) != null);
 	}
 
 	@Override

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

@@ -126,14 +126,11 @@ public boolean canEncode(ResolvableType elementType, @Nullable MimeType mimeType
 		if (this.mapperRegistrations != null && selectMapper(elementType, mimeType) == null) {
 			return false;
 		}
-		Class<?> clazz = elementType.resolve();
-		if (clazz == null) {
-			return true;
-		}
-		if (MappingJacksonValue.class.isAssignableFrom(clazz)) {
+		Class<?> elementClass = elementType.toClass();
+		if (MappingJacksonValue.class.isAssignableFrom(elementClass)) {
 			throw new UnsupportedOperationException("MappingJacksonValue is not supported, use hints instead");
 		}
-		return !String.class.isAssignableFrom(clazz);
+		return !ServerSentEvent.class.isAssignableFrom(elementClass);
 	}
 
 	@Override

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

@@ -104,6 +104,11 @@ public JacksonJsonDecoder(JsonMapper mapper, MimeType... mimeTypes) {
 		super(mapper, mimeTypes);
 	}
 
+	@Override
+	public boolean canDecode(ResolvableType elementType, @Nullable MimeType mimeType) {
+		return super.canDecode(elementType, mimeType) && !CharSequence.class.isAssignableFrom(elementType.toClass());
+	}
+
 	@Override
 	protected Flux<DataBuffer> processInput(Publisher<DataBuffer> input, ResolvableType elementType,
 			@Nullable MimeType mimeType, @Nullable Map<String, Object> hints) {

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

@@ -121,6 +121,11 @@ private static PrettyPrinter initSsePrettyPrinter() {
 	}
 
 
+	@Override
+	public boolean canEncode(ResolvableType elementType, @Nullable MimeType mimeType) {
+		return super.canEncode(elementType, mimeType) && !String.class.isAssignableFrom(elementType.toClass());
+	}
+
 	@Override
 	protected List<MimeType> getMediaTypesForProblemDetail() {
 		return problemDetailMimeTypes;

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

@@ -57,7 +57,7 @@ protected void canDecode() {
 		assertThat(decoder.canDecode(ResolvableType.forClass(Pojo.class), MediaType.APPLICATION_CBOR)).isTrue();
 		assertThat(decoder.canDecode(ResolvableType.forClass(Pojo.class), null)).isTrue();
 
-		assertThat(decoder.canDecode(ResolvableType.forClass(String.class), null)).isFalse();
+		assertThat(decoder.canDecode(ResolvableType.forClass(String.class), null)).isTrue();
 		assertThat(decoder.canDecode(ResolvableType.forClass(Pojo.class), APPLICATION_JSON)).isFalse();
 	}
 

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

@@ -59,14 +59,14 @@ void canEncode() {
 		ResolvableType pojoType = ResolvableType.forClass(Pojo.class);
 		assertThat(this.encoder.canEncode(pojoType, MediaType.APPLICATION_CBOR)).isTrue();
 		assertThat(this.encoder.canEncode(pojoType, null)).isTrue();
+		assertThat(this.encoder.canEncode(ResolvableType.forClass(String.class), null)).isTrue();
 
 		// SPR-15464
 		assertThat(this.encoder.canEncode(ResolvableType.NONE, null)).isTrue();
 	}
 
 	@Test
 	void canNotEncode() {
-		assertThat(this.encoder.canEncode(ResolvableType.forClass(String.class), null)).isFalse();
 		assertThat(this.encoder.canEncode(ResolvableType.forClass(Pojo.class), APPLICATION_XML)).isFalse();
 	}
 

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

@@ -62,7 +62,7 @@ protected void canDecode() {
 		assertThat(decoder.canDecode(ResolvableType.forClass(Pojo.class), STREAM_SMILE_MIME_TYPE)).isTrue();
 		assertThat(decoder.canDecode(ResolvableType.forClass(Pojo.class), null)).isTrue();
 
-		assertThat(decoder.canDecode(ResolvableType.forClass(String.class), null)).isFalse();
+		assertThat(decoder.canDecode(ResolvableType.forClass(String.class), null)).isTrue();
 		assertThat(decoder.canDecode(ResolvableType.forClass(Pojo.class), APPLICATION_JSON)).isFalse();
 	}
 

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

@@ -65,14 +65,15 @@ protected void canEncode() {
 		assertThat(this.encoder.canEncode(pojoType, SMILE_MIME_TYPE)).isTrue();
 		assertThat(this.encoder.canEncode(pojoType, STREAM_SMILE_MIME_TYPE)).isTrue();
 		assertThat(this.encoder.canEncode(pojoType, null)).isTrue();
+		assertThat(this.encoder.canEncode(ResolvableType.forClass(String.class), null)).isTrue();
 
 		// SPR-15464
 		assertThat(this.encoder.canEncode(ResolvableType.NONE, null)).isTrue();
 	}
 
 	@Test
 	void cannotEncode() {
-		assertThat(this.encoder.canEncode(ResolvableType.forClass(String.class), null)).isFalse();
+
 		assertThat(this.encoder.canEncode(ResolvableType.forClass(Pojo.class), APPLICATION_XML)).isFalse();
 	}
 

spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/SseIntegrationTests.java

@@ -146,7 +146,6 @@ void sseAsEventWithoutAcceptHeader(HttpServer httpServer, ClientHttpConnector co
 
 		Flux<ServerSentEvent<Person>> result = this.webClient.get()
 				.uri("/event")
-				.accept(TEXT_EVENT_STREAM)
 				.retrieve()
 				.bodyToFlux(new ParameterizedTypeReference<>() {});