code: public void activateOptions() {
          if (follow) {
              if (target.equals(SYSTEM_ERR)) {
                 setWriter(createWriter(new SystemErrStream()));
              } else {
                 setWriter(createWriter(new SystemOutStream()));
              }
          } else {
              if (target.equals(SYSTEM_ERR)) {
                 setWriter(createWriter(unwrap(System.err)));
              } else {
                 setWriter(createWriter(unwrap(System.out)));
              }
          }
  
          super.activateOptions();
    }
docs: Prepares the appender for use.