Make the logger static final

See gh-18575

Author: 王洋

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/event/EventPublishingRunListener.java

@@ -127,7 +127,7 @@ public void failed(ConfigurableApplicationContext context, Throwable exception)
 
 	private static class LoggingErrorHandler implements ErrorHandler {
 
-		private static Log logger = LogFactory.getLog(EventPublishingRunListener.class);
+		private static final Log logger = LogFactory.getLog(EventPublishingRunListener.class);
 
 		@Override
 		public void handleError(Throwable throwable) {