Add support for MonoSupplier

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

@@ -37,6 +37,7 @@ import org.springframework.cloud.function.core.FluxSupplier;
 import org.springframework.cloud.function.core.FluxToMonoFunction;
 import org.springframework.cloud.function.core.FluxedConsumer;
 import org.springframework.cloud.function.core.FluxedFunction;
+import org.springframework.cloud.function.core.MonoSupplier;
 import org.springframework.cloud.function.core.MonoToFluxFunction;
 import org.springframework.util.Assert;
 import org.springframework.util.CollectionUtils;
@@ -155,17 +156,19 @@ public class FunctionRegistration<T> implements BeanNameAware {
 			result = new FunctionRegistration<S>(target);
 			result.type(this.type.getType());
 
-			if (!type.isWrapper()) {
+			if (!this.type.isWrapper()) {
 				target = target instanceof Supplier
 						? (S) new FluxSupplier((Supplier<?>) target)
 						: target instanceof Function
 								? (S) new FluxFunction((Function<?, ?>) target)
 								: (S) new FluxConsumer((Consumer<?>) target);
 			}
-			else if (Mono.class.isAssignableFrom(type.getOutputWrapper())) {
+			else if (Mono.class.isAssignableFrom(this.type.getOutputWrapper())) {
-				target = (S) new FluxToMonoFunction((Function) target);
+				target = target instanceof Supplier
+						? (S) new MonoSupplier((Supplier<?>) target)
+						: (S) new FluxToMonoFunction((Function<?, ?>) target);
 			}
-			else if (Mono.class.isAssignableFrom(type.getInputWrapper())) {
+			else if (Mono.class.isAssignableFrom(this.type.getInputWrapper())) {
 				target = (S) new MonoToFluxFunction((Function) target);
 			}
 			else if (target instanceof Consumer) {

spring-cloud-function-core/src/main/java/org/springframework/cloud/function/core/MonoSupplier.java

@@ -0,0 +1,50 @@
+/*
+ * Copyright 2012-2019 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.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.cloud.function.core;
+
+import java.util.function.Supplier;
+
+import reactor.core.publisher.Mono;
+
+/**
+ * {@link Supplier} implementation that wraps a target Supplier so that the target's
+ * simple output type will be wrapped in a {@link Mono} instance.
+ *
+ * @param <T> output type of target supplier
+ * @author Mark Fisher
+ */
+public class MonoSupplier<T> implements Supplier<Mono<T>>, FluxWrapper<Supplier<T>> {
+
+	private final Supplier<T> supplier;
+
+	public MonoSupplier(Supplier<T> supplier) {
+		this.supplier = supplier;
+	}
+
+	@Override
+	public Supplier<T> getTarget() {
+		return this.supplier;
+	}
+
+	@Override
+	@SuppressWarnings({ "unchecked", "rawtypes" })
+	public Mono<T> get() {
+		Object result = this.supplier.get();
+		return Mono.just((T) result);
+	}
+
+}