Initial commit of Google Function Adapter

implement HttpFunction
add headers processing
Some refactoring [WIP]
Add invoker integration test
Make test classes nested within test.
Add sample and refdoc

Resolves #468

spring-cloud-function-adapters/spring-cloud-function-adapter-gcp/src/main/java/org/springframework/cloud/function/adapter/gcloud/GcfSpringBootHttpRequestHandler.java

@@ -0,0 +1,73 @@
+/*
+ * Copyright 2020-2020 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
+ *
+ *      https://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.adapter.gcloud;
+
+import com.google.cloud.functions.HttpFunction;
+import com.google.cloud.functions.HttpRequest;
+import com.google.cloud.functions.HttpResponse;
+import com.google.gson.Gson;
+import org.reactivestreams.Publisher;
+import reactor.core.publisher.Mono;
+
+import org.springframework.cloud.function.context.AbstractSpringFunctionAdapterInitializer;
+
+/**
+ * Implementation of {@link HttpFunction} for Google Cloud Function (GCF).
+ * This is the Spring Cloud Function adapter for GCF HTTP function.
+ *
+ * @author Dmitry Solomakha
+ * @author Mike Eltsufin
+ */
+public class GcfSpringBootHttpRequestHandler
+	extends AbstractSpringFunctionAdapterInitializer<HttpRequest> implements HttpFunction {
+
+	private final Gson gson = new Gson();
+
+	public GcfSpringBootHttpRequestHandler() {
+		super();
+	}
+
+	public GcfSpringBootHttpRequestHandler(Class<?> configurationClass) {
+		super(configurationClass);
+	}
+
+	/**
+	 * The implementation of a GCF {@link HttpFunction} that will be used as the entrypoint from GCF.
+	 */
+	@Override
+	public void service(HttpRequest httpRequest, HttpResponse httpResponse) throws Exception {
+		Thread.currentThread()
+			.setContextClassLoader(GcfSpringBootHttpRequestHandler.class.getClassLoader());
+
+		initialize(httpRequest);
+
+		Publisher<?> input;
+		if (getInputType() == Void.class) {
+			input = Mono.empty();
+		}
+		else {
+			input = Mono.just(gson.fromJson(httpRequest.getReader(), getInputType()));
+		}
+
+		Publisher<?> output = this.apply(input);
+
+		Object result = this.result(input, output);
+
+		httpResponse.getWriter().write(gson.toJson(result));
+		httpResponse.getWriter().close();
+	}
+}