Nullability fine-tuning based on IntelliJ IDEA 2018.3 inspection

Issue: SPR-15540

(cherry picked from commit bf272b0b21)

spring-context/src/main/java/org/springframework/context/annotation/ConfigurationClassEnhancer.java

@@ -395,6 +395,7 @@ class ConfigurationClassEnhancer {
 				Object beanInstance = (useArgs ? beanFactory.getBean(beanName, beanMethodArgs) :
 						beanFactory.getBean(beanName));
 				if (!ClassUtils.isAssignableValue(beanMethod.getReturnType(), beanInstance)) {
+					// Detect package-protected NullBean instance through equals(null) check
 					if (beanInstance.equals(null)) {
 						if (logger.isDebugEnabled()) {
 							logger.debug(String.format("@Bean method %s.%s called as bean reference " +

spring-context/src/main/java/org/springframework/jmx/support/ConnectorServerFactoryBean.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -170,11 +170,12 @@ public class ConnectorServerFactoryBean extends MBeanRegistrationSupport
 		try {
 			if (this.threaded) {
 				// Start the connector server asynchronously (in a separate thread).
+				final JMXConnectorServer serverToStart = this.connectorServer;
 				Thread connectorThread = new Thread() {
 					@Override
 					public void run() {
 						try {
-							connectorServer.start();
+							serverToStart.start();
 						}
 						catch (IOException ex) {
 							throw new JmxException("Could not start JMX connector server after delay", ex);