Relax check on default data MimeType

If there is more than one non-basic codec (e.g. CBOR and JSON)
RSocketRequester.Builder takes the mime type of the first one rather
than giving up. It is a valid scenario (JSON for server responding to
browser, and CBOR for client talking to server) and it is the default
situation in Boot, and after all the point here is to pick some default
as best as we can with the worst possible outcome being a server
refusing the connection if it doesn't support the mime type. Beyond
that applications can set the dataMimeType on the builder explicitly.

To match that change this commit also ensures RSocketMessageHandler
rejects proactively data mime types it does not support at the point
of accepting a connection.

spring-messaging/src/main/java/org/springframework/messaging/rsocket/DefaultRSocketRequesterBuilder.java

@@ -145,21 +145,14 @@ final class DefaultRSocketRequesterBuilder implements RSocketRequester.Builder {
 		if (this.dataMimeType != null) {
 			return this.dataMimeType;
 		}
-		// Look for non-basic Decoder (e.g. CBOR, Protobuf)
-		MimeType selected = null;
-		List<Decoder<?>> decoders = strategies.decoders();
-		for (Decoder<?> candidate : decoders) {
+		// First non-basic Decoder (e.g. CBOR, Protobuf)
+		for (Decoder<?> candidate : strategies.decoders()) {
 			if (!isCoreCodec(candidate) && !candidate.getDecodableMimeTypes().isEmpty()) {
-				Assert.state(selected == null,
-						() -> "Cannot select default data MimeType based on configured decoders: " + decoders);
-				selected = getMimeType(candidate);
+				return getMimeType(candidate);
 			}
 		}
-		if (selected != null) {
-			return selected;
-		}
-		// Fall back on 1st decoder (e.g. String)
-		for (Decoder<?> decoder : decoders) {
+		// First core decoder (e.g. String)
+		for (Decoder<?> decoder : strategies.decoders()) {
 			if (!decoder.getDecodableMimeTypes().isEmpty()) {
 				return getMimeType(decoder);
 			}

spring-messaging/src/main/java/org/springframework/messaging/rsocket/RSocketRequester.java

@@ -128,11 +128,10 @@ public interface RSocketRequester {
 		/**
 		 * Configure the payload data MimeType to specify on the {@code SETUP}
 		 * frame that applies to the whole connection.
-		 * <p>If this is not set, the builder will try to select the mime type
-		 * based on the presence of a single
+		 * <p>If this is not set, it will be set to the MimeType of the first
 		 * {@link RSocketStrategies.Builder#decoder(Decoder[])  non-default}
-		 * {@code Decoder}, or the first default decoder otherwise
-		 * (i.e. {@code String}) if no others are configured.
+		 * {@code Decoder}, or otherwise fall back on the MimeType of the first
+		 * (default) decoder.
 		 */
 		RSocketRequester.Builder dataMimeType(@Nullable MimeType mimeType);
 

spring-messaging/src/main/java/org/springframework/messaging/rsocket/annotation/support/RSocketMessageHandler.java

@@ -304,7 +304,13 @@ public class RSocketMessageHandler extends MessageMappingMessageHandler {
 	 */
 	public SocketAcceptor serverResponder() {
 		return (setupPayload, sendingRSocket) -> {
-			MessagingRSocket responder = createResponder(setupPayload, sendingRSocket);
+			MessagingRSocket responder;
+			try {
+				responder = createResponder(setupPayload, sendingRSocket);
+			}
+			catch (Throwable ex) {
+				return Mono.error(ex);
+			}
 			return responder.handleConnectionSetupPayload(setupPayload).then(Mono.just(responder));
 		};
 	}
@@ -335,14 +341,15 @@ public class RSocketMessageHandler extends MessageMappingMessageHandler {
 		String s = setupPayload.dataMimeType();
 		MimeType dataMimeType = StringUtils.hasText(s) ? MimeTypeUtils.parseMimeType(s) : this.defaultDataMimeType;
 		Assert.notNull(dataMimeType, "No `dataMimeType` in ConnectionSetupPayload and no default value");
+		Assert.isTrue(isDataMimeTypeSupported(dataMimeType), "Data MimeType '" + dataMimeType + "' not supported");
 
 		s = setupPayload.metadataMimeType();
-		MimeType metadataMimeType = StringUtils.hasText(s) ? MimeTypeUtils.parseMimeType(s) : this.defaultMetadataMimeType;
-		Assert.notNull(metadataMimeType, "No `metadataMimeType` in ConnectionSetupPayload and no default value");
+		MimeType metaMimeType = StringUtils.hasText(s) ? MimeTypeUtils.parseMimeType(s) : this.defaultMetadataMimeType;
+		Assert.notNull(metaMimeType, "No `metadataMimeType` in ConnectionSetupPayload and no default value");
 
 		RSocketStrategies strategies = this.rsocketStrategies;
 		Assert.notNull(strategies, "No RSocketStrategies. Was afterPropertiesSet not called?");
-		RSocketRequester requester = RSocketRequester.wrap(rsocket, dataMimeType, metadataMimeType, strategies);
+		RSocketRequester requester = RSocketRequester.wrap(rsocket, dataMimeType, metaMimeType, strategies);
 
 		Assert.state(this.metadataExtractor != null,
 				() -> "No MetadataExtractor. Was afterPropertiesSet not called?");
@@ -350,10 +357,20 @@ public class RSocketMessageHandler extends MessageMappingMessageHandler {
 		Assert.state(getRouteMatcher() != null,
 				() -> "No RouteMatcher. Was afterPropertiesSet not called?");
 
-		return new MessagingRSocket(dataMimeType, metadataMimeType, this.metadataExtractor, requester,
+		return new MessagingRSocket(dataMimeType, metaMimeType, this.metadataExtractor, requester,
 				this, getRouteMatcher(), strategies);
 	}
 
+	private boolean isDataMimeTypeSupported(MimeType dataMimeType) {
+		for (Encoder<?> encoder : getEncoders()) {
+			for (MimeType encodable : encoder.getEncodableMimeTypes()) {
+				if (encodable.isCompatibleWith(dataMimeType)) {
+					return true;
+				}
+			}
+		}
+		return false;
+	}
 
 	public static ClientRSocketFactoryConfigurer clientResponder(Object... handlers) {
 		return new ResponderConfigurer(handlers);

spring-messaging/src/test/java/org/springframework/messaging/rsocket/DefaultRSocketRequesterBuilderTests.java

@@ -46,7 +46,6 @@ import org.springframework.util.MimeTypeUtils;
 import org.springframework.util.ReflectionUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
-import static org.assertj.core.api.Assertions.assertThatThrownBy;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.ArgumentMatchers.anyInt;
 import static org.mockito.BDDMockito.given;
@@ -131,22 +130,6 @@ public class DefaultRSocketRequesterBuilderTests {
 				.isEqualTo(MimeTypeUtils.APPLICATION_JSON);
 	}
 
-	@Test
-	public void defaultDataMimeTypeWithMultipleCustomDecoderRegitered() {
-		RSocketStrategies strategies = RSocketStrategies.builder()
-				.decoder(new TestJsonDecoder(MimeTypeUtils.APPLICATION_JSON))
-				.decoder(new TestJsonDecoder(MimeTypeUtils.APPLICATION_XML))
-				.build();
-
-		assertThatThrownBy(() ->
-				RSocketRequester
-						.builder()
-						.rsocketStrategies(strategies)
-						.connect(this.transport)
-						.block())
-				.hasMessageContaining("Cannot select default data MimeType");
-	}
-
 	@Test
 	public void dataMimeTypeSet() {
 		RSocketRequester requester = RSocketRequester.builder()