code: static String getSimpleFormat(boolean useProxy) {
          String format = System.getProperty(FORMAT_PROP_KEY);
          if (useProxy && proxy != null && format == null) {
              format = proxy.getProperty(FORMAT_PROP_KEY);
          }
  
          if (format != null) {
              try {
                  // validate the user-defined format string
                  String unused = String.format(format, new Date(), "", "", "", "", "");
              } catch (IllegalArgumentException e) {
                  // illegal syntax; fall back to the default format
                  format = DEFAULT_FORMAT;
              }
          } else {
              format = DEFAULT_FORMAT;
          }
          return format;
      }
docs: java.util.logging and read its configuration