@Override
        protected String fill(String pattern, long timeMillis, int logLevel, String tag, String message) {
            if (useLongName) {
                return pattern.replace(wrappedParameter, LogLevel.getLevelName(logLevel));
            } else {
                return pattern.replace(wrappedParameter, LogLevel.getShortLevelName(logLevel));
            }
        }
Fill the original pattern string with level name.
android/android_framework/base/src/main/java/com/elvishew/xlog/flattener/PatternFlattener.java