Improve output type conversion handling

- Re-enable, clean and improve  special handling for collection/array output type
- Add tests to validate and demonstrate the differences in  special handling of collection of Messages ve collection of non-Messages

Resolves #464

spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/catalog/BeanFactoryAwareFunctionRegistry.java

@@ -17,9 +17,11 @@
 package org.springframework.cloud.function.context.catalog;
 
 import java.lang.reflect.Field;
+import java.lang.reflect.GenericArrayType;
 import java.lang.reflect.Method;
 import java.lang.reflect.ParameterizedType;
 import java.lang.reflect.Type;
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.HashMap;
@@ -28,6 +30,7 @@ import java.util.List;
 import java.util.Map;
 import java.util.Optional;
 import java.util.Set;
+import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.Consumer;
 import java.util.function.Function;
 import java.util.function.Supplier;
@@ -688,34 +691,32 @@ public class BeanFactoryAwareFunctionRegistry
 				else {
 					for (int i = 0; i < acceptedContentTypes.size() && convertedValue == null; i++) {
 						MimeType acceptedContentType = acceptedContentTypes.get(i);
-						if (value instanceof Message) {
+						/*
-							Message<?> message = (Message<?>) value;
+						 * We need to treat Iterables differently since they may represent collection of Messages
-							if (message.getPayload() instanceof byte[]) {
+						 * which should be converted individually
-								convertedValue = message;
+						 */
+						boolean convertIndividualItem = false;
+						if (value instanceof Iterable || (ObjectUtils.isArray(value) && !(value instanceof byte[]))) {
+							Type outputType = FunctionTypeUtils.getOutputType(functionType, 0);
+							if (outputType instanceof ParameterizedType) {
+								convertIndividualItem = FunctionTypeUtils.isMessage(FunctionTypeUtils.getImmediateGenericType(outputType, 0));
 							}
-							else {
+							else if (outputType instanceof GenericArrayType) {
-								convertedValue = this.convertValueToMessage(message, enricher, acceptedContentType);
+								convertIndividualItem = FunctionTypeUtils.isMessage(((GenericArrayType) outputType).getGenericComponentType());
 							}
 						}
-//<<<<<<< HEAD
+
-//						else if (value instanceof byte[]) {
+						if (convertIndividualItem) {
-//							convertedValue = MessageBuilder.withPayload(value)
+							if (ObjectUtils.isArray(value)) {
-//								.setHeader(MessageHeaders.CONTENT_TYPE, acceptedContentType).build();
+								value = Arrays.asList((Object[]) value);
-//						}
+							}
-//						else if (value instanceof Iterable || ObjectUtils.isArray(value)) {
+							AtomicReference<List<Message>> messages = new AtomicReference<List<Message>>(new ArrayList<>());
-//							boolean isArray = ObjectUtils.isArray(value);
+							((Iterable) value).forEach(element ->
-//							if (isArray) {
+								messages.get()
-//								value = Arrays.asList((Object[]) value);
+									.add((Message) convertOutputValueIfNecessary(element, enricher, acceptedContentType
-//							}
+										.toString())));
-//							AtomicReference<List<Message>> messages = new AtomicReference<List<Message>>(new ArrayList<>());
+							convertedValue = messages.get();
-//							((Iterable) value).forEach(element ->
+						}
-//								messages.get()
-//									.add((Message) convertOutputValueIfNecessary(element, enricher, acceptedContentType
-//										.toString())));
-//							convertedValue = messages.get();
-//						}
-//=======
-//>>>>>>> Don't treat byte[] or collections in a special way.
 						else {
 							convertedValue = this.convertValueToMessage(value, enricher, acceptedContentType);
 						}

spring-cloud-function-context/src/test/java/org/springframework/cloud/function/context/catalog/BeanFactoryAwareFunctionRegistryTests.java

@@ -26,6 +26,7 @@ import java.util.Map;
 import java.util.function.Consumer;
 import java.util.function.Function;
 import java.util.function.Supplier;
+import java.util.stream.Collectors;
 
 import org.junit.Ignore;
 import org.junit.Test;
@@ -50,6 +51,7 @@ import org.springframework.messaging.converter.MessageConverter;
 import org.springframework.messaging.support.GenericMessage;
 import org.springframework.messaging.support.MessageBuilder;
 import org.springframework.stereotype.Component;
+import org.springframework.util.CollectionUtils;
 import org.springframework.util.MimeType;
 import org.springframework.util.ReflectionUtils;
 
@@ -336,6 +338,7 @@ public class BeanFactoryAwareFunctionRegistryTests {
 		assertThat(composed).isFalse();
 	}
 
+	@SuppressWarnings("unchecked")
 	@Test
 	public void byteArrayNoSpecialHandling() throws Exception {
 		FunctionCatalog catalog = this.configureCatalog(ByteArrayFunction.class);
@@ -345,6 +348,21 @@ public class BeanFactoryAwareFunctionRegistryTests {
 		assertThat(result.getPayload()).isEqualTo("\"b2xsZWg=\"".getBytes());
 	}
 
+	@SuppressWarnings("rawtypes")
+	@Test
+	public void testMultipleValuesInOutputHandling() throws Exception {
+		FunctionCatalog catalog = this.configureCatalog(CollectionOutConfiguration.class);
+		FunctionInvocationWrapper function = catalog.lookup("parseToList", "application/json");
+		assertThat(function).isNotNull();
+		Object result = (Message) function.apply(MessageBuilder.withPayload("1, 2, 3".getBytes()).setHeader(MessageHeaders.CONTENT_TYPE, "text/plain").build());
+		assertThat(result instanceof Message).isTrue();
+
+		function = catalog.lookup("parseToListOfMessages", "application/json");
+		assertThat(function).isNotNull();
+		result = function.apply(MessageBuilder.withPayload("1, 2, 3".getBytes()).setHeader(MessageHeaders.CONTENT_TYPE, "text/plain").build());
+		assertThat(result instanceof Message).isFalse();
+	}
+
 	@SuppressWarnings("unchecked")
 	@Test
 	public void testSerializationWithCompatibleWildcardSubtypeAcceptHeader() {
@@ -370,6 +388,25 @@ public class BeanFactoryAwareFunctionRegistryTests {
 		assertThat(dateResult.getHeaders().get("accept")).isNull();
 	}
 
+	@SuppressWarnings("unchecked")
+	@EnableAutoConfiguration
+	public static class CollectionOutConfiguration {
+
+		@Bean
+		public Function<String, List<String>> parseToList() {
+			return v -> CollectionUtils.arrayToList(v.split(","));
+		}
+
+		@Bean
+		public Function<String, List<Message<String>>> parseToListOfMessages() {
+			return v -> {
+				List<Message<String>> list = (List<Message<String>>) CollectionUtils.arrayToList(v.split(",")).stream()
+						.map(value -> MessageBuilder.withPayload(value).build()).collect(Collectors.toList());
+				return list;
+			};
+		}
+	}
+
 	@EnableAutoConfiguration
 	public static class NegotiatingMessageConverterConfiguration {