Create session-based WebSessionIdResolver

Introduces HeaderSessionIdResolver, which reads session ids from a session header and generates a session response header when new session are created.

Related issue: SPR-15917

spring-web/src/main/java/org/springframework/web/server/session/HeaderSessionIdResolver.java

@@ -0,0 +1,72 @@
+/*
+ * Copyright 2017 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.web.server.session;
+
+import java.util.Collections;
+import java.util.List;
+
+import org.springframework.http.HttpHeaders;
+import org.springframework.util.Assert;
+import org.springframework.web.server.ServerWebExchange;
+
+/**
+ * Header-based {@link WebSessionIdResolver}.
+ * 
+ * @author Greg Turnquist
+ * @since 5.0
+ */
+public class HeaderSessionIdResolver implements WebSessionIdResolver {
+
+	private String headerName = "SESSION";
+
+	/**
+	 * Set the name of the session header to use for the session id.
+	 * <p>By default set to "SESSION".
+	 * @param headerName the header name
+	 */
+	public void setHeaderName(String headerName) {
+		Assert.hasText(headerName, "'headerName' must not be empty.");
+		this.headerName = headerName;
+	}
+
+	/**
+	 * Return the configured header name.
+	 */
+	public String getHeaderName() {
+		return this.headerName;
+	}
+
+	@Override
+	public List<String> resolveSessionIds(ServerWebExchange exchange) {
+		HttpHeaders headers = exchange.getRequest().getHeaders();
+		List<String> sessionHeaders = headers.get(this.getHeaderName());
+		if (sessionHeaders == null) {
+			return Collections.emptyList();
+		}
+		return sessionHeaders;
+	}
+
+	@Override
+	public void setSessionId(ServerWebExchange exchange, String id) {
+		Assert.notNull(id, "'id' is required.");
+		exchange.getResponse().getHeaders().set(this.headerName, id);
+	}
+
+	@Override
+	public void expireSession(ServerWebExchange exchange) {
+		this.setSessionId(exchange, "");
+	}
+}