Improve WebFlux error logging

This commit avoids printing the stacktrace for ResponseStatusException
in order to be consistent with WebFlux ResponseStatusExceptionHandler
and because this stacktrace is usually not very useful in Reactive
world and mainly pollutes the logs, only the message is logged in that
case.

It also logs a WARN message for Bad Request (400) HTTP responses in
order to have a feedback when an exception is thrown due to client error
(unable to deserialize request body for example).

See related SPR-15083 issue on Spring Framework side.

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/reactive/error/DefaultErrorWebExceptionHandler.java

@@ -39,6 +39,7 @@ import org.springframework.web.reactive.function.server.RouterFunction;
 import org.springframework.web.reactive.function.server.RouterFunctions;
 import org.springframework.web.reactive.function.server.ServerRequest;
 import org.springframework.web.reactive.function.server.ServerResponse;
+import org.springframework.web.server.ResponseStatusException;
 
 /**
  * Basic global {@link org.springframework.web.server.WebExceptionHandler}, rendering
@@ -188,16 +189,43 @@ public class DefaultErrorWebExceptionHandler extends AbstractErrorWebExceptionHa
 	}
 
 	/**
-	 * Log the original exception if handling it results in a Server Error.
+	 * Log the original exception if handling it results in a Server Error or a Bad Request
+	 * (Client Error with 400 status code) one.
 	 * @param request the source request
 	 * @param errorStatus the HTTP error status
 	 */
 	protected void logError(ServerRequest request, HttpStatus errorStatus) {
 		if (errorStatus.is5xxServerError()) {
 			Throwable ex = getError(request);
-			logger.error("Failed to handle request [" + request.methodName() + " "
-					+ request.uri() + "]", ex);
+			if (ex instanceof ResponseStatusException) {
+				logger.error(buildMessage(request, ex));
 			}
+			else {
+				logger.error(buildMessage(request, null), ex);
+			}
+		}
+		else if (errorStatus == HttpStatus.BAD_REQUEST) {
+			Throwable ex = getError(request);
+			if (ex instanceof ResponseStatusException) {
+				logger.warn(buildMessage(request, ex));
+			}
+			else {
+				logger.warn(buildMessage(request, null), ex);
+			}
+		}
+	}
+
+	private String buildMessage(ServerRequest request, Throwable ex) {
+		StringBuilder message = new StringBuilder("Failed to handle request [");
+		message.append(request.methodName());
+		message.append(" ");
+		message.append(request.uri());
+		message.append("]");
+		if (ex != null) {
+			message.append(": ");
+			message.append(ex.getMessage());
+		}
+		return message.toString();
 	}
 
 }