Refine Jackson canEncode() / canDecode() implementation

This commit refine Jackson canEncode() / canDecode()
implementation by using ObjectMapper canSerialize() /
canDeserialize() methods.

Issue: SPR-14748

spring-core/src/main/java/org/springframework/core/codec/Decoder.java

@@ -42,7 +42,7 @@ public interface Decoder<T> {
 	 * Whether the decoder supports the given target element type and the MIME
 	 * type of the source stream.
 	 * @param elementType the target element type for the output stream
-	 * @param mimeType the mime type associated with the stream to decode
+	 * @param mimeType the mime type associated with the stream to decode, can be {@code null} if not specified.
 	 * @return {@code true} if supported, {@code false} otherwise
 	 */
 	boolean canDecode(ResolvableType elementType, MimeType mimeType);

spring-core/src/main/java/org/springframework/core/codec/Encoder.java

@@ -43,7 +43,7 @@ public interface Encoder<T> {
 	 * Whether the encoder supports the given source element type and the MIME
 	 * type for the output stream.
 	 * @param elementType the type of elements in the source stream
-	 * @param mimeType the MIME type for the output stream
+	 * @param mimeType the MIME type for the output stream, can be {@code null} if not specified.
 	 * @return {@code true} if supported, {@code false} otherwise
 	 */
 	boolean canEncode(ResolvableType elementType, MimeType mimeType);

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

@@ -23,6 +23,7 @@ import java.util.Map;
 import com.fasterxml.jackson.databind.JavaType;
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.fasterxml.jackson.databind.ObjectReader;
+import com.fasterxml.jackson.databind.type.TypeFactory;
 import org.reactivestreams.Publisher;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
@@ -63,10 +64,9 @@ public class Jackson2JsonDecoder extends AbstractJackson2Codec implements Decode
 
 	@Override
 	public boolean canDecode(ResolvableType elementType, MimeType mimeType) {
-		if (mimeType == null) {
+		JavaType javaType = this.mapper.getTypeFactory().constructType(elementType.getType());
-			return true;
+		return this.mapper.canDeserialize(javaType) &&
-		}
+				(mimeType == null || JSON_MIME_TYPES.stream().anyMatch(m -> m.isCompatibleWith(mimeType)));
-		return JSON_MIME_TYPES.stream().anyMatch(m -> m.isCompatibleWith(mimeType));
 	}
 
 	@Override

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

@@ -68,10 +68,8 @@ public class Jackson2JsonEncoder extends AbstractJackson2Codec implements Encode
 
 	@Override
 	public boolean canEncode(ResolvableType elementType, MimeType mimeType) {
-		if (mimeType == null) {
+		return this.mapper.canSerialize(elementType.getRawClass()) &&
-			return true;
+				(mimeType == null || JSON_MIME_TYPES.stream().anyMatch(m -> m.isCompatibleWith(mimeType)));
-		}
-		return JSON_MIME_TYPES.stream().anyMatch(m -> m.isCompatibleWith(mimeType));
 	}
 
 	@Override

spring-web/src/test/java/org/springframework/http/codec/json/Jackson2JsonDecoderTests.java

@@ -46,9 +46,10 @@ public class Jackson2JsonDecoderTests extends AbstractDataBufferAllocatingTestCa
 	@Test
 	public void canDecode() {
 		Jackson2JsonDecoder decoder = new Jackson2JsonDecoder();
-
+		ResolvableType type = ResolvableType.forClass(Pojo.class);
-		assertTrue(decoder.canDecode(null, MediaType.APPLICATION_JSON));
+		assertTrue(decoder.canDecode(type, MediaType.APPLICATION_JSON));
-		assertFalse(decoder.canDecode(null, MediaType.APPLICATION_XML));
+		assertTrue(decoder.canDecode(type, null));
+		assertFalse(decoder.canDecode(type, MediaType.APPLICATION_XML));
 	}
 
 	@Test

spring-web/src/test/java/org/springframework/http/codec/json/Jackson2JsonEncoderTests.java

@@ -31,6 +31,7 @@ import org.springframework.core.io.buffer.AbstractDataBufferAllocatingTestCase;
 import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.http.MediaType;
 import org.springframework.http.codec.Pojo;
+import org.springframework.http.codec.ServerSentEvent;
 import org.springframework.tests.TestSubscriber;
 
 import static org.junit.Assert.*;
@@ -45,8 +46,12 @@ public class Jackson2JsonEncoderTests extends AbstractDataBufferAllocatingTestCa
 
 	@Test
 	public void canEncode() {
-		assertTrue(this.encoder.canEncode(null, MediaType.APPLICATION_JSON));
+		ResolvableType pojoType = ResolvableType.forClass(Pojo.class);
-		assertFalse(this.encoder.canEncode(null, MediaType.APPLICATION_XML));
+		assertTrue(this.encoder.canEncode(pojoType, MediaType.APPLICATION_JSON));
+		assertTrue(this.encoder.canEncode(pojoType, null));
+		assertFalse(this.encoder.canEncode(pojoType, MediaType.APPLICATION_XML));
+		ResolvableType sseType = ResolvableType.forClass(ServerSentEvent.class);
+		assertFalse(this.encoder.canEncode(sseType, MediaType.APPLICATION_JSON));
 	}
 
 	@Test