SubProtocolWebSocketHandler closes sessions outside of lifecycle monitor

Issue: SPR-16488

spring-websocket/src/main/java/org/springframework/web/socket/messaging/SubProtocolWebSocketHandler.java

@@ -245,6 +245,7 @@ public class SubProtocolWebSocketHandler
 	@Override
 	public final void start() {
 		Assert.isTrue(this.defaultProtocolHandler != null || !this.protocolHandlers.isEmpty(), "No handlers");
+
 		synchronized (this.lifecycleMonitor) {
 			this.clientOutboundChannel.subscribe(this);
 			this.running = true;
@@ -256,6 +257,9 @@ public class SubProtocolWebSocketHandler
 		synchronized (this.lifecycleMonitor) {
 			this.running = false;
 			this.clientOutboundChannel.unsubscribe(this);
+		}
+
+		// Proactively notify all active WebSocket sessions
 		for (WebSocketSessionHolder holder : this.sessions.values()) {
 			try {
 				holder.getSession().close(CloseStatus.GOING_AWAY);
@@ -267,7 +271,6 @@ public class SubProtocolWebSocketHandler
 			}
 		}
 	}
-	}
 
 	@Override
 	public final void stop(Runnable callback) {
@@ -291,6 +294,7 @@ public class SubProtocolWebSocketHandler
 		if (!session.isOpen()) {
 			return;
 		}
+
 		this.stats.incrementSessionCount(session);
 		session = decorateSession(session);
 		this.sessions.put(session.getId(), new WebSocketSessionHolder(session));