Simplify initialization of WebSession Mono

The DefaultWebSessionManager now uses Mono.defer to protect the call
to getSession from parsing session cookies immediately. This allows
pre-initializing the Mono<WebSession> upfront vs using a lock.

spring-web-reactive/src/main/java/org/springframework/web/server/adapter/DefaultServerWebExchange.java

@@ -39,15 +39,9 @@ public class DefaultServerWebExchange implements ServerWebExchange {
 
 	private final ServerHttpResponse response;
 
-	private final WebSessionManager sessionManager;
-
-
 	private final Map<String, Object> attributes = new ConcurrentHashMap<>();
 
-	private final Object createSessionLock = new Object();
-
-	private Mono<WebSession> sessionMono;
-
+	private final Mono<WebSession> sessionMono;
 
 
 	public DefaultServerWebExchange(ServerHttpRequest request, ServerHttpResponse response,
@@ -58,7 +52,7 @@ public class DefaultServerWebExchange implements ServerWebExchange {
 		Assert.notNull(response, "'sessionManager' is required.");
 		this.request = request;
 		this.response = response;
-		this.sessionManager = sessionManager;
+		this.sessionMono = sessionManager.getSession(this).cache();
 	}
 
 
@@ -84,13 +78,6 @@ public class DefaultServerWebExchange implements ServerWebExchange {
 
 	@Override
 	public Mono<WebSession> getSession() {
-		if (this.sessionMono == null) {
-			synchronized (this.createSessionLock) {
-				if (this.sessionMono == null) {
-					this.sessionMono = this.sessionManager.getSession(this).cache();
-				}
-			}
-		}
 		return this.sessionMono;
 	}
 

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

@@ -99,12 +99,13 @@ public class DefaultWebSessionManager implements WebSessionManager {
 
 	@Override
 	public Mono<WebSession> getSession(ServerWebExchange exchange) {
-		return Flux.fromIterable(getSessionIdResolver().resolveSessionIds(exchange))
+		return Mono.defer(() ->
+				Flux.fromIterable(getSessionIdResolver().resolveSessionIds(exchange))
 						.concatMap(this.sessionStore::retrieveSession)
 						.next()
 						.then(session -> validateSession(exchange, session))
 						.otherwiseIfEmpty(createSession(exchange))
-				.map(session -> extendSession(exchange, session));
+						.map(session -> extendSession(exchange, session)));
 	}
 
 	protected Mono<WebSession> validateSession(ServerWebExchange exchange, WebSession session) {