code: static String getSimpleFormat(boolean useProxy) {
          String format =
              AccessController.doPrivileged(
                  new PrivilegedAction<String>() {
                      public String run() {
                          return 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.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