Avoid reflection for instantiating MockServerContainerContextCustomizer

Since MockServerContainerContextCustomizerFactory already has a
`webSocketPresent` check, there should be no need to instantiate
MockServerContainerContextCustomizer via reflection.

See spring-projectsgh-29028
Closes spring-projectsgh-29035

Author: sbrannen

spring-test/src/main/java/org/springframework/test/context/web/socket/MockServerContainerContextCustomizerFactory.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2022 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.
@@ -18,7 +18,6 @@
 
 import java.util.List;
 
-import org.springframework.beans.BeanUtils;
 import org.springframework.core.annotation.AnnotatedElementUtils;
 import org.springframework.lang.Nullable;
 import org.springframework.test.context.ContextConfigurationAttributes;
@@ -39,9 +38,6 @@ class MockServerContainerContextCustomizerFactory implements ContextCustomizerFa
 	private static final String WEB_APP_CONFIGURATION_ANNOTATION_CLASS_NAME =
 			"org.springframework.test.context.web.WebAppConfiguration";
 
-	private static final String MOCK_SERVER_CONTAINER_CONTEXT_CUSTOMIZER_CLASS_NAME =
-			"org.springframework.test.context.web.socket.MockServerContainerContextCustomizer";
-
 	private static final boolean webSocketPresent = ClassUtils.isPresent("jakarta.websocket.server.ServerContainer",
 			MockServerContainerContextCustomizerFactory.class.getClassLoader());
 
@@ -52,17 +48,8 @@ public ContextCustomizer createContextCustomizer(Class<?> testClass,
 			List<ContextConfigurationAttributes> configAttributes) {
 
 		if (webSocketPresent && isAnnotatedWithWebAppConfiguration(testClass)) {
-			try {
-				Class<?> clazz = ClassUtils.forName(MOCK_SERVER_CONTAINER_CONTEXT_CUSTOMIZER_CLASS_NAME,
-						getClass().getClassLoader());
-				return (ContextCustomizer) BeanUtils.instantiateClass(clazz);
-			}
-			catch (Throwable ex) {
-				throw new IllegalStateException("Failed to enable WebSocket test support; could not load class: " +
-						MOCK_SERVER_CONTAINER_CONTEXT_CUSTOMIZER_CLASS_NAME, ex);
-			}
+			return new MockServerContainerContextCustomizer();
 		}
-
 		// Else, nothing to customize
 		return null;
 	}