GH-585 Add initial support for passing/parsing Message headers

This woudl allow Message headers to be passed as JSON string via rsocket metadata

Resolves #585

spring-cloud-function-rsocket/src/main/java/org/springframework/cloud/function/rsocket/FunctionRSocketMessageHandler.java

@@ -16,9 +16,11 @@
 
 package org.springframework.cloud.function.rsocket;
 
+import java.lang.reflect.Field;
 import java.lang.reflect.Method;
 import java.util.Collections;
 import java.util.List;
+import java.util.Map;
 import java.util.Set;
 import java.util.function.Function;
 
@@ -30,6 +32,7 @@ import reactor.core.publisher.Mono;
 import org.springframework.cloud.function.context.FunctionCatalog;
 import org.springframework.cloud.function.context.catalog.SimpleFunctionRegistry.FunctionInvocationWrapper;
 import org.springframework.core.MethodParameter;
+import org.springframework.core.ParameterizedTypeReference;
 import org.springframework.core.ReactiveAdapterRegistry;
 import org.springframework.core.ResolvableType;
 import org.springframework.core.codec.ByteArrayDecoder;
@@ -39,16 +42,20 @@ import org.springframework.core.codec.Encoder;
 import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.lang.Nullable;
 import org.springframework.messaging.Message;
+import org.springframework.messaging.MessageHeaders;
 import org.springframework.messaging.MessagingException;
 import org.springframework.messaging.handler.CompositeMessageCondition;
 import org.springframework.messaging.handler.DestinationPatternsMessageCondition;
 import org.springframework.messaging.handler.invocation.reactive.HandlerMethodArgumentResolver;
 import org.springframework.messaging.handler.invocation.reactive.HandlerMethodReturnValueHandler;
 import org.springframework.messaging.handler.invocation.reactive.SyncHandlerMethodArgumentResolver;
+import org.springframework.messaging.rsocket.DefaultMetadataExtractor;
+import org.springframework.messaging.rsocket.MetadataExtractor;
 import org.springframework.messaging.rsocket.annotation.support.RSocketFrameTypeMessageCondition;
 import org.springframework.messaging.rsocket.annotation.support.RSocketMessageHandler;
 import org.springframework.messaging.rsocket.annotation.support.RSocketPayloadReturnValueHandler;
 import org.springframework.messaging.support.MessageBuilder;
+import org.springframework.util.MimeTypeUtils;
 import org.springframework.util.ReflectionUtils;
 
 /**
@@ -63,6 +70,8 @@ class FunctionRSocketMessageHandler extends RSocketMessageHandler {
 
 	private final FunctionCatalog functionCatalog;
 
+	private final Field headersField;
+
 	private static final Method FUNCTION_APPLY_METHOD =
 		ReflectionUtils.findMethod(Function.class, "apply", (Class<?>[]) null);
 
@@ -76,6 +85,8 @@ class FunctionRSocketMessageHandler extends RSocketMessageHandler {
 	FunctionRSocketMessageHandler(FunctionCatalog functionCatalog) {
 		setHandlerPredicate((clazz) -> false);
 		this.functionCatalog = functionCatalog;
+		this.headersField = ReflectionUtils.findField(MessageHeaders.class, "headers");
+		this.headersField.setAccessible(true);
 	}
 
 
@@ -85,12 +96,19 @@ class FunctionRSocketMessageHandler extends RSocketMessageHandler {
 		super.afterPropertiesSet();
 	}
 
+	@SuppressWarnings("unchecked")
+	@Override
+	public MetadataExtractor getMetadataExtractor() {
+		return new HeadersAwareMetadataExtractor((List<Decoder<?>>) this.getDecoders());
+	}
+
 	/**
 	 * Will check if there is a function handler registered for destination before proceeding.
 	 * This typically happens when user avoids using 'spring.cloud.function.definition' property.
 	 */
 	@Override
 	public Mono<Void> handleMessage(Message<?> message) throws MessagingException {
+
 		if (!FrameType.SETUP.equals(message.getHeaders().get("rsocketFrameType"))) {
 			String destination = this.getDestination(message).value();
 			Set<String> mappings = this.getDestinationLookup().keySet();
@@ -165,7 +183,17 @@ class FunctionRSocketMessageHandler extends RSocketMessageHandler {
 			}
 			return super.handleReturnValue(returnValue, returnType, message);
 		}
+	}
 
+	private static class HeadersAwareMetadataExtractor extends DefaultMetadataExtractor {
+		HeadersAwareMetadataExtractor(List<Decoder<?>> decoders) {
+			super(decoders);
+			super.metadataToExtract(MimeTypeUtils.APPLICATION_JSON,
+					new ParameterizedTypeReference<Map<String, String>>() {
+					}, (jsonMap, outputMap) -> {
+						outputMap.putAll(jsonMap);
+					});
+		}
 	}
 
 }