GH-587 Add support for inferring 'accept' content type for simple types

This fix also introduces new Function property 'accept' with no default value which implicitely would default to application/json unless the output type of the function is String at which point it would default to text/plain. However, if it was explicitely set in FunctionProperties it will be used regardless of the function output type.
Resolves #587

spring-cloud-function-rsocket/src/main/java/org/springframework/cloud/function/rsocket/FunctionRSocketMessageHandler.java

@@ -41,7 +41,6 @@ import org.springframework.core.codec.ByteArrayEncoder;
 import org.springframework.core.codec.Decoder;
 import org.springframework.core.codec.Encoder;
 import org.springframework.core.io.buffer.DataBuffer;
-import org.springframework.http.server.PathContainer;
 import org.springframework.lang.Nullable;
 import org.springframework.messaging.Message;
 import org.springframework.messaging.MessageHeaders;
@@ -57,11 +56,8 @@ import org.springframework.messaging.rsocket.annotation.support.RSocketFrameType
 import org.springframework.messaging.rsocket.annotation.support.RSocketMessageHandler;
 import org.springframework.messaging.rsocket.annotation.support.RSocketPayloadReturnValueHandler;
 import org.springframework.messaging.support.MessageBuilder;
-import org.springframework.util.AntPathMatcher;
 import org.springframework.util.MimeTypeUtils;
 import org.springframework.util.ReflectionUtils;
-import org.springframework.util.RouteMatcher;
-import org.springframework.util.SimpleRouteMatcher;
 import org.springframework.util.StringUtils;
 import org.springframework.web.util.pattern.PathPatternRouteMatcher;
 
@@ -116,25 +112,18 @@ class FunctionRSocketMessageHandler extends RSocketMessageHandler {
 	 * Will check if there is a function handler registered for destination before proceeding.
 	 * This typically happens when user avoids using 'spring.cloud.function.definition' property.
 	 */
-	@SuppressWarnings("unchecked")
 	@Override
 	public Mono<Void> handleMessage(Message<?> message) throws MessagingException {
-
 		if (!FrameType.SETUP.equals(message.getHeaders().get("rsocketFrameType"))) {
 			String destination = this.getDestination(message).value();
 			if (!StringUtils.hasText(destination)) {
-				destination = this.functionProperties.getDefinition();
-				Map<String, Object> headersMap = (Map<String, Object>) ReflectionUtils
-						.getField(this.headersField, message.getHeaders());
-
-				PathPatternRouteMatcher matcher = new PathPatternRouteMatcher();
-
-				headersMap.put(DestinationPatternsMessageCondition.LOOKUP_DESTINATION_HEADER, matcher.parseRoute(destination));
+				destination = this.discoverAndInjectDestinationHeader(message);
 			}
+
 			Set<String> mappings = this.getDestinationLookup().keySet();
 			if (!mappings.contains(destination)) {
 				FunctionInvocationWrapper function = FunctionRSocketUtils
-						.registerFunctionForDestination(destination, functionCatalog, this.getApplicationContext());
+						.registerFunctionForDestination(destination, this.functionCatalog, this.getApplicationContext());
 				this.registerFunctionHandler(new RSocketListenerFunction(function), destination);
 			}
 		}
@@ -162,6 +151,18 @@ class FunctionRSocketMessageHandler extends RSocketMessageHandler {
 			getReactiveAdapterRegistry()));
 	}
 
+	@SuppressWarnings("unchecked")
+	private String discoverAndInjectDestinationHeader(Message<?> message) {
+		String destination = this.functionProperties.getDefinition();
+		Map<String, Object> headersMap = (Map<String, Object>) ReflectionUtils
+				.getField(this.headersField, message.getHeaders());
+
+		PathPatternRouteMatcher matcher = new PathPatternRouteMatcher();
+
+		headersMap.put(DestinationPatternsMessageCondition.LOOKUP_DESTINATION_HEADER, matcher.parseRoute(destination));
+		return destination;
+	}
+
 	protected static final class MessageHandlerMethodArgumentResolver implements SyncHandlerMethodArgumentResolver {
 
 		private final Decoder<byte[]> decoder = new ByteArrayDecoder();

spring-cloud-function-rsocket/src/main/java/org/springframework/cloud/function/rsocket/FunctionRSocketUtils.java

@@ -16,6 +16,7 @@
 
 package org.springframework.cloud.function.rsocket;
 
+import java.lang.reflect.Type;
 import java.net.URI;
 import java.util.regex.Pattern;
 
@@ -23,6 +24,7 @@ import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
 
 import org.springframework.cloud.function.context.FunctionCatalog;
+import org.springframework.cloud.function.context.FunctionProperties;
 import org.springframework.cloud.function.context.FunctionRegistration;
 import org.springframework.cloud.function.context.FunctionRegistry;
 import org.springframework.cloud.function.context.catalog.FunctionTypeUtils;
@@ -50,10 +52,25 @@ final class FunctionRSocketUtils {
 
 	}
 
-	static FunctionInvocationWrapper registerFunctionForDestination(String destination, FunctionCatalog functionCatalog,
+	static FunctionInvocationWrapper registerFunctionForDestination(String functionDefinition, FunctionCatalog functionCatalog,
 			ApplicationContext applicationContext) {
-		registerRSocketForwardingFunctionIfNecessary(destination, functionCatalog, applicationContext);
-		FunctionInvocationWrapper function = functionCatalog.lookup(destination, "application/json");
+
+		registerRSocketForwardingFunctionIfNecessary(functionDefinition, functionCatalog, applicationContext);
+		FunctionProperties functionProperties = applicationContext.getBean(FunctionProperties.class);
+		String acceptContentType = functionProperties.getAccept();
+		if (!StringUtils.hasText(acceptContentType)) {
+			FunctionInvocationWrapper function = functionCatalog.lookup(functionDefinition);
+			Type functionType = function.getFunctionType();
+			Type outputType = FunctionTypeUtils.getOutputType(functionType, 0);
+			if (outputType instanceof Class && String.class.isAssignableFrom((Class<?>) outputType)) {
+				acceptContentType = "text/plain";
+			}
+			else {
+				acceptContentType = "application/json";
+			}
+		}
+
+		FunctionInvocationWrapper function = functionCatalog.lookup(functionDefinition, acceptContentType);
 		return function;
 	}
 
@@ -73,6 +90,7 @@ final class FunctionRSocketUtils {
 
 				String forwardingUrl = functionToRSocketDefinition[1];
 				Builder rsocketRequesterBuilder = applicationContext.getBean(Builder.class);
+
 				RSocketRequester rsocketRequester = (WS_URI_PATTERN.matcher(forwardingUrl).matches())
 						? rsocketRequesterBuilder.websocket(URI.create(forwardingUrl))
 						: rsocketRequesterBuilder.tcp(hostPort[0], Integer.parseInt(hostPort[1]));