Fix for change in Jetty 9.4.20.v20190813

Closes gh-23500

spring-websocket/src/main/java/org/springframework/web/socket/server/jetty/JettyRequestUpgradeStrategy.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * 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.
@@ -17,6 +17,7 @@
 package org.springframework.web.socket.server.jetty;
 
 import java.io.IOException;
+import java.lang.reflect.Method;
 import java.security.Principal;
 import java.util.ArrayList;
 import java.util.List;
@@ -28,6 +29,7 @@ import javax.servlet.http.HttpServletResponse;
 
 import org.eclipse.jetty.websocket.api.WebSocketPolicy;
 import org.eclipse.jetty.websocket.api.extensions.ExtensionConfig;
+import org.eclipse.jetty.websocket.api.extensions.ExtensionFactory;
 import org.eclipse.jetty.websocket.server.HandshakeRFC6455;
 import org.eclipse.jetty.websocket.server.WebSocketServerFactory;
 
@@ -38,7 +40,9 @@ import org.springframework.http.server.ServerHttpResponse;
 import org.springframework.http.server.ServletServerHttpRequest;
 import org.springframework.http.server.ServletServerHttpResponse;
 import org.springframework.util.Assert;
+import org.springframework.util.ClassUtils;
 import org.springframework.util.CollectionUtils;
+import org.springframework.util.ReflectionUtils;
 import org.springframework.web.context.ServletContextAware;
 import org.springframework.web.socket.WebSocketExtension;
 import org.springframework.web.socket.WebSocketHandler;
@@ -167,7 +171,7 @@ public class JettyRequestUpgradeStrategy implements RequestUpgradeStrategy, Serv
 	}
 
 	private List<WebSocketExtension> buildWebSocketExtensions() {
-		Set<String> names = this.factory.getExtensionFactory().getExtensionNames();
+		Set<String> names = getExtensionNames();
 		List<WebSocketExtension> result = new ArrayList<>(names.size());
 		for (String name : names) {
 			result.add(new WebSocketExtension(name));
@@ -175,6 +179,18 @@ public class JettyRequestUpgradeStrategy implements RequestUpgradeStrategy, Serv
 		return result;
 	}
 
+	@SuppressWarnings({"unchecked"})
+	private Set<String> getExtensionNames() {
+		try {
+			return this.factory.getExtensionFactory().getExtensionNames();
+		}
+		catch (IncompatibleClassChangeError ex) {
+			// 9.4.20.v20190813: ExtensionFactory (abstract class -> interface)
+			Method method = ClassUtils.getMethod(ExtensionFactory.class, "getExtensionNames");
+			return (Set<String>) ReflectionUtils.invokeMethod(method, this.factory.getExtensionFactory());
+		}
+	}
+
 	@Override
 	public void upgrade(ServerHttpRequest request, ServerHttpResponse response,
 			String selectedProtocol, List<WebSocketExtension> selectedExtensions, Principal user,