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

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

@@ -102,14 +102,7 @@ public abstract class AbstractJacksonDecoder<T extends ObjectMapper> extends Jac
 
 	@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 abstract class AbstractJacksonEncoder<T extends ObjectMapper> extends Jac
 		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 class JacksonJsonDecoder extends AbstractJacksonDecoder<JsonMapper> {
 		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 @@ public class JacksonJsonEncoder extends AbstractJacksonEncoder<JsonMapper> {
 	}
 
 
+	@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 @@ class JacksonCborDecoderTests extends AbstractDecoderTests<JacksonCborDecoder> {
 		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

@@ -59,6 +59,7 @@ class JacksonCborEncoderTests extends AbstractLeakCheckingTests {
 		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();
@@ -66,7 +67,6 @@ class JacksonCborEncoderTests extends AbstractLeakCheckingTests {
 
 	@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 @@ class JacksonSmileDecoderTests extends AbstractDecoderTests<JacksonSmileDecoder>
 		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

@@ -65,6 +65,7 @@ class JacksonSmileEncoderTests extends AbstractEncoderTests<JacksonSmileEncoder>
 		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();
@@ -72,7 +73,7 @@ class JacksonSmileEncoderTests extends AbstractEncoderTests<JacksonSmileEncoder>
 
 	@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 @@ class SseIntegrationTests extends AbstractHttpHandlerIntegrationTests {
 
 		Flux<ServerSentEvent<Person>> result = this.webClient.get()
 				.uri("/event")
-				.accept(TEXT_EVENT_STREAM)
 				.retrieve()
 				.bodyToFlux(new ParameterizedTypeReference<>() {});