GH-635 Fix support for returning empty Collections

Resolves #635

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

@@ -900,7 +900,7 @@ public class SimpleFunctionRegistry implements FunctionRegistry, FunctionInspect
 			if (ObjectUtils.isArray(payload)) {
 				payload = CollectionUtils.arrayToList(payload);
 			}
-			if (payload instanceof Collection
+			if (payload instanceof Collection && !CollectionUtils.isEmpty((Collection<?>) payload)
 					&& Message.class.isAssignableFrom(CollectionUtils.findCommonElementType((Collection<?>) payload))) {
 				return true;
 			}

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2019-2020 the original author or authors.
+ * Copyright 2019-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -21,6 +21,7 @@ import java.io.Serializable;
 import java.lang.reflect.Field;
 import java.nio.charset.StandardCharsets;
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.Date;
 import java.util.List;
 import java.util.Map;
@@ -47,6 +48,7 @@ import org.springframework.cloud.function.context.FunctionRegistration;
 import org.springframework.cloud.function.context.FunctionRegistry;
 import org.springframework.cloud.function.context.FunctionType;
 import org.springframework.cloud.function.context.catalog.SimpleFunctionRegistry.FunctionInvocationWrapper;
+import org.springframework.cloud.function.json.JsonMapper;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
@@ -73,12 +75,14 @@ import static org.junit.jupiter.api.Assertions.fail;
  */
 public class BeanFactoryAwareFunctionRegistryTests {
 
+	private ApplicationContext context;
+
 	private FunctionCatalog configureCatalog() {
 		return this.configureCatalog(SampleFunctionConfiguration.class);
 	}
 
 	private FunctionCatalog configureCatalog(Class<?>... configClass) {
-		ApplicationContext context = new SpringApplicationBuilder(configClass)
+		this.context = new SpringApplicationBuilder(configClass)
 				.run("--logging.level.org.springframework.cloud.function=DEBUG",
 						"--spring.main.lazy-initialization=true");
 		FunctionCatalog catalog = context.getBean(FunctionCatalog.class);
@@ -550,6 +554,28 @@ public class BeanFactoryAwareFunctionRegistryTests {
 		assertThat(result.block()).isEqualTo("hello");
 	}
 
+	@SuppressWarnings({ "rawtypes", "unchecked" })
+	@Test
+	public void testGH_635() throws Exception {
+		FunctionCatalog catalog = this.configureCatalog(SCF_GH_635ConfigurationAsFunction.class);
+		Function lmFunction = catalog.lookup("emptyMessageList", "application/json");
+		List<Message<?>> emptyListOfMessages = (List<Message<?>>) lmFunction.apply(MessageBuilder.withPayload("hello").build());
+		assertThat(emptyListOfMessages).isEmpty();
+		emptyListOfMessages = (List<Message<?>>) lmFunction.apply("hello");
+		assertThat(emptyListOfMessages).isEmpty();
+
+		JsonMapper mapper = this.context.getBean(JsonMapper.class);
+		Function lsFunction = catalog.lookup("emptyStringList", "application/json");
+		Message<byte[]> emptyListOfString = (Message<byte[]>) lsFunction.apply(MessageBuilder.withPayload("hello").build());
+		List resultList = mapper.fromJson(emptyListOfString.getPayload(), List.class);
+		assertThat(resultList).isEmpty();
+		emptyListOfString = (Message<byte[]>) lsFunction.apply("hello");
+		resultList = mapper.fromJson(emptyListOfString.getPayload(), List.class);
+		assertThat(resultList).isEmpty();
+	}
+
+
+
 	@EnableAutoConfiguration
 	public static class PojoToMessageFunctionCompositionConfiguration {
 
@@ -967,6 +993,20 @@ public class BeanFactoryAwareFunctionRegistryTests {
 		}
 	}
 
+	@EnableAutoConfiguration
+	public static class SCF_GH_635ConfigurationAsFunction {
+
+		@Bean
+		public Function<String, List<Message<?>>> emptyMessageList() {
+			return input -> Collections.emptyList();
+		}
+
+		@Bean
+		public Function<String, List<String>> emptyStringList() {
+			return input -> Collections.emptyList();
+		}
+	}
+
 	public static class Person {
 		private String name;
 		private int id;