Optional WebSocket engine presence in WebFlux config

Closes gh-22587

spring-webflux/src/main/java/org/springframework/web/reactive/config/WebFluxConfigurationSupport.java

@@ -65,7 +65,9 @@ import org.springframework.web.reactive.result.method.annotation.ResponseBodyRes
 import org.springframework.web.reactive.result.method.annotation.ResponseEntityResultHandler;
 import org.springframework.web.reactive.result.view.ViewResolutionResultHandler;
 import org.springframework.web.reactive.result.view.ViewResolver;
+import org.springframework.web.reactive.socket.WebSocketHandler;
 import org.springframework.web.reactive.socket.server.WebSocketService;
+import org.springframework.web.reactive.socket.server.support.HandshakeWebSocketService;
 import org.springframework.web.reactive.socket.server.support.WebSocketHandlerAdapter;
 import org.springframework.web.server.ServerWebExchange;
 import org.springframework.web.server.WebExceptionHandler;
@@ -435,17 +437,29 @@ public class WebFluxConfigurationSupport implements ApplicationContextAware {
 
 	@Bean
 	public WebSocketHandlerAdapter webFluxWebSocketHandlerAdapter() {
-		WebSocketService service = getWebSocketService();
-		WebSocketHandlerAdapter adapter = (service != null ?
-				new WebSocketHandlerAdapter(service) : new WebSocketHandlerAdapter());
+		WebSocketHandlerAdapter adapter = new WebSocketHandlerAdapter(initWebSocketService());
 
-		// For backwards compatibility, lower the (default) priority
+		// Lower the (default) priority for now, for backwards compatibility
 		int defaultOrder = adapter.getOrder();
 		adapter.setOrder(defaultOrder + 1);
 
 		return adapter;
 	}
 
+	private WebSocketService initWebSocketService() {
+		WebSocketService service = getWebSocketService();
+		if (service == null) {
+			try {
+				service = new HandshakeWebSocketService();
+			}
+			catch (IllegalStateException ex) {
+				// Don't fail, test environment perhaps
+				service = new NoUpgradeStrategyWebSocketService();
+			}
+		}
+		return service;
+	}
+
 	@Nullable
 	protected WebSocketService getWebSocketService() {
 		return null;
@@ -535,4 +549,13 @@ public class WebFluxConfigurationSupport implements ApplicationContextAware {
 		}
 	}
 
+
+	private static final class NoUpgradeStrategyWebSocketService implements WebSocketService {
+
+		@Override
+		public Mono<Void> handleRequest(ServerWebExchange exchange, WebSocketHandler webSocketHandler) {
+			return Mono.error(new IllegalStateException("No suitable RequestUpgradeStrategy"));
+		}
+	}
+
 }