Correct log level

Issue: SPR-16528

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/ResponseBodyEmitter.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.
@@ -190,6 +190,10 @@ private void sendInternal(Object object, @Nullable MediaType mediaType) throws I
 	 * Complete request processing.
 	 * <p>A dispatch is made into the app server where Spring MVC completes
 	 * asynchronous request processing.
+	 * <p><strong>Note:</strong> you do not need to call this method after an
+	 * {@link IOException} from any of the {@code send} methods. The Servlet
+	 * container will generate an error notification that Spring MVC will process
+	 * and handle through the exception resolver mechanism and then complete.
 	 */
 	public synchronized void complete() {
 		this.complete = true;

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/ResponseEntityExceptionHandler.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.
@@ -457,8 +457,8 @@ protected ResponseEntity<Object> handleAsyncRequestTimeoutException(
 			HttpServletRequest request = servletWebRequest.getRequest();
 			HttpServletResponse response = servletWebRequest.getResponse();
 			if (response != null && response.isCommitted()) {
-				if (logger.isErrorEnabled()) {
-					logger.error("Async timeout for " + request.getMethod() + " [" + request.getRequestURI() + "]");
+				if (logger.isDebugEnabled()) {
+					logger.debug("Async timeout for " + request.getMethod() + " [" + request.getRequestURI() + "]");
 				}
 				return null;
 			}

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/support/DefaultHandlerExceptionResolver.java

@@ -536,8 +536,8 @@ protected ModelAndView handleAsyncRequestTimeoutException(AsyncRequestTimeoutExc
 		if (!response.isCommitted()) {
 			response.sendError(HttpServletResponse.SC_SERVICE_UNAVAILABLE);
 		}
-		else if (logger.isErrorEnabled()) {
-			logger.error("Async timeout for " + request.getMethod() + " [" + request.getRequestURI() + "]");
+		else if (logger.isDebugEnabled()) {
+			logger.debug("Async timeout for " + request.getMethod() + " [" + request.getRequestURI() + "]");
 		}
 		return new ModelAndView();
 	}