Add filter to add exchange to Reactor Context

Closes gh-21746

spring-web/src/main/java/org/springframework/web/filter/reactive/ServerWebExchangeContextFilter.java

@@ -0,0 +1,64 @@
+/*
+ * Copyright 2002-2019 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
+ *
+ *      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.web.filter.reactive;
+
+import java.util.Optional;
+
+import reactor.core.publisher.Mono;
+import reactor.util.context.Context;
+
+import org.springframework.web.server.ServerWebExchange;
+import org.springframework.web.server.WebFilter;
+import org.springframework.web.server.WebFilterChain;
+
+/**
+ * Inserts an attribute in the Reactor {@link Context} that makes the current
+ * {@link ServerWebExchange} available under the attribute name
+ * {@link #EXCHANGE_CONTEXT_ATTRIBUTE}. This is useful for access to the
+ * exchange without explicitly passing it to components that participate in
+ * request processing.
+ *
+ * <p>The convenience method {@link #get(Context)} looks up the exchange.
+ *
+ * @author Rossen Stoyanchev
+ * @since 5.2
+ */
+public class ServerWebExchangeContextFilter implements WebFilter {
+
+	/** Attribute name under which the exchange is saved in the context. */
+	public static final String EXCHANGE_CONTEXT_ATTRIBUTE =
+			ServerWebExchangeContextFilter.class.getName() + ".EXCHANGE_CONTEXT";
+
+
+	@Override
+	public Mono<Void> filter(ServerWebExchange exchange, WebFilterChain chain) {
+		return chain.filter(exchange)
+				.subscriberContext(cxt -> cxt.put(EXCHANGE_CONTEXT_ATTRIBUTE, exchange));
+	}
+
+
+	/**
+	 * Access the {@link ServerWebExchange} from the Reactor Context, if available,
+	 * which is if {@link ServerWebExchangeContextFilter} is configured for use
+	 * and the give context was obtained from a request processing chain.
+	 * @param context the context in which to access the exchange
+	 * @return the exchange
+	 */
+	public static Optional<ServerWebExchange> get(Context context) {
+		return context.getOrEmpty(EXCHANGE_CONTEXT_ATTRIBUTE);
+	}
+
+}