GH-1300 Add support for post processing Kotlin functions

Resolves #1300

Author: olegz

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

@@ -56,6 +56,7 @@
 import org.springframework.cloud.function.context.FunctionRegistration;
 import org.springframework.cloud.function.context.FunctionRegistry;
 import org.springframework.cloud.function.context.PostProcessingFunction;
+import org.springframework.cloud.function.context.config.KotlinLambdaToFunctionAutoConfiguration;
 import org.springframework.cloud.function.context.config.RoutingFunction;
 import org.springframework.cloud.function.core.FunctionInvocationHelper;
 import org.springframework.cloud.function.json.JsonMapper;
@@ -72,12 +73,15 @@
 import org.springframework.messaging.converter.MessageConverter;
 import org.springframework.messaging.support.MessageBuilder;
 import org.springframework.util.Assert;
+import org.springframework.util.ClassUtils;
 import org.springframework.util.CollectionUtils;
 import org.springframework.util.MimeTypeUtils;
 import org.springframework.util.ObjectUtils;
 import org.springframework.util.StringUtils;
 
 
+
+
 /**
  * Implementation of {@link FunctionCatalog} and {@link FunctionRegistry} which
  * does not depend on Spring's {@link BeanFactory}.
@@ -443,6 +447,11 @@ public class FunctionInvocationWrapper implements Function<Object, Object>, Cons
 			if (target instanceof PostProcessingFunction) {
 				this.postProcessor = (PostProcessingFunction) target;
 			}
+			if (ClassUtils.isPresent("kotlin.jvm.functions.Function0", ClassUtils.getDefaultClassLoader())
+				&& target instanceof KotlinLambdaToFunctionAutoConfiguration.KotlinFunctionWrapper kotlinFunction
+				&& kotlinFunction.getKotlinLambdaTarget() instanceof PostProcessingFunction) {
+				this.postProcessor = (PostProcessingFunction) kotlinFunction.getKotlinLambdaTarget();
+			}
 			this.target = target;
 			this.inputType = this.normalizeType(inputType);
 			this.outputType = this.normalizeType(outputType);
@@ -770,6 +779,9 @@ else if (this.isConsumer()) {
 			}
 
 			if (this.postProcessor != null) {
+				if (!(result instanceof Message)) {
+					result = MessageBuilder.withPayload(result).build();
+				}
 				this.unconvertedResult.set((Message<Object>) result);
 			}
 

spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/config/KotlinLambdaToFunctionAutoConfiguration.java

@@ -62,6 +62,8 @@ public class KotlinLambdaToFunctionAutoConfiguration {
 	public static final class KotlinFunctionWrapper implements Function<Object, Object>, Supplier<Object>, Consumer<Object>,
 			Function0<Object>, Function1<Object, Object>, Function2<Object, Object, Object>,
 			Function3<Object, Object, Object, Object>, Function4<Object, Object, Object, Object, Object> {
+
+
 		private final Object kotlinLambdaTarget;
 
 		private String name;
@@ -72,6 +74,10 @@ public KotlinFunctionWrapper(Object kotlinLambdaTarget) {
 			this.kotlinLambdaTarget = kotlinLambdaTarget;
 		}
 
+		public Object getKotlinLambdaTarget() {
+			return kotlinLambdaTarget;
+		}
+
 		@Override
 		public Object apply(Object input) {
 			if (ObjectUtils.isEmpty(input)) {