GH-864 Add support for carying HTTP request parameters in Message headers

Resolves #864

docs/src/main/asciidoc/spring-cloud-function.adoc

@@ -650,6 +650,13 @@ When POSTing text the response format might be different with Spring Boot 2.0 an
 
 See <<Testing Functional Applications>> to see the details and example on how to test such application.
 
+==== HTTP Request Parameters
+As you have noticed from the previous table, you can pass an argument to a function as path variable (i.e., `/{function}/{item}`).
+For example, `http://localhost:8080/uppercase/foo` will result in calling `uppercase` function with its input parameter being `foo`.
+
+While this is the recommended approach and the one that fits most use cases cases, there are times when you have to deal with HTTP request parameters. 
+The framework will treat HTTP request parameters similar to the HTTP headers by storing them in `Message` headers under the header key `http_request_param` 
+with its value being a `Map` of request parameters, so in order to access them your function input signature should accept `Message` type (e.g., `Function<Message<String>, String>`). For convenience we provide `HeaderUtils.HTTP_REQUEST_PARAM` constant.
 
 === Function Mapping rules 
 

spring-cloud-function-web/src/main/java/org/springframework/cloud/function/web/util/FunctionWebRequestProcessingHelper.java

@@ -89,7 +89,15 @@ public final class FunctionWebRequestProcessingHelper {
 
 		HttpHeaders headers = wrapper.getHeaders();
 
-		Message<?> inputMessage = argument == null ? null : MessageBuilder.withPayload(argument).copyHeaders(headers.toSingleValueMap()).build();
+		Message<?> inputMessage = null;
+
+		if (argument != null) {
+			MessageBuilder builder = MessageBuilder.withPayload(argument);
+			if (!CollectionUtils.isEmpty(wrapper.getParams())) {
+				builder = builder.setHeader(HeaderUtils.HTTP_REQUEST_PARAM, wrapper.getParams().toSingleValueMap());
+			}
+			inputMessage = builder.copyHeaders(headers.toSingleValueMap()).build();
+		}
 
 		if (function.isRoutingFunction()) {
 			function.setSkipOutputConversion(true);

spring-cloud-function-web/src/main/java/org/springframework/cloud/function/web/util/HeaderUtils.java

@@ -31,6 +31,11 @@ import org.springframework.messaging.MessageHeaders;
  */
 public final class HeaderUtils {
 
+	/**
+	 * Message Header name which contains HTTP request parameters.
+	 */
+	public static final String HTTP_REQUEST_PARAM = "http_request_param";
+
 	private static HttpHeaders IGNORED = new HttpHeaders();
 
 	private static HttpHeaders REQUEST_ONLY = new HttpHeaders();

spring-cloud-function-web/src/test/java/org/springframework/cloud/function/web/mvc/PrefixTests.java

@@ -17,6 +17,8 @@
 package org.springframework.cloud.function.web.mvc;
 
 import java.net.URI;
+import java.util.Map;
+import java.util.function.Function;
 import java.util.function.Supplier;
 
 import org.junit.jupiter.api.Test;
@@ -29,10 +31,12 @@ import org.springframework.boot.test.context.SpringBootTest.WebEnvironment;
 import org.springframework.boot.test.web.client.TestRestTemplate;
 import org.springframework.cloud.function.web.RestApplication;
 import org.springframework.cloud.function.web.mvc.PrefixTests.TestConfiguration;
+import org.springframework.cloud.function.web.util.HeaderUtils;
 import org.springframework.context.annotation.Bean;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.RequestEntity;
 import org.springframework.http.ResponseEntity;
+import org.springframework.messaging.Message;
 import org.springframework.test.context.ContextConfiguration;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -65,6 +69,15 @@ public class PrefixTests {
 		assertThat(result.getStatusCode()).isEqualTo(HttpStatus.NOT_FOUND);
 	}
 
+
+	@Test
+	public void uppercase() throws Exception {
+		ResponseEntity<String> result = this.rest.exchange(
+				RequestEntity.get(new URI("/functions/uppercase/foo?nome=Doe&prenome=John")).build(), String.class);
+		assertThat(result.getStatusCode()).isEqualTo(HttpStatus.OK);
+		assertThat(result.getBody()).isEqualTo("[\"foo\",\"bar\"]");
+	}
+
 	@EnableAutoConfiguration
 	@org.springframework.boot.test.context.TestConfiguration
 	protected static class TestConfiguration {
@@ -74,6 +87,17 @@ public class PrefixTests {
 			return () -> Flux.fromArray(new String[] { "foo", "bar" });
 		}
 
+		@Bean
+		public Function<Message<String>, String[]> uppercase() {
+			return message -> {
+				assertThat(message.getPayload().equals("foo"));
+				Map<String, String> httpParam = (Map<String, String>) message.getHeaders().get(HeaderUtils.HTTP_REQUEST_PARAM);
+				assertThat(httpParam.get("nome")).isEqualTo("Doe");
+				assertThat(httpParam.get("prenome")).isEqualTo("John");
+				return new String[] { "foo", "bar" };
+			};
+		}
+
 	}
 
 }