android - Can't show AlertDialog in DoInBackground -

i trying use alertdialog in android notify users runing in offline mode after checking internet connection.

i have used following codes:

protected void doinbackground(void... params) {             if (this.isonline()) {                     new getjson().execute();             } else {          alertdialog.builder builder1 = new alertdialog.builder(homefragment);         builder1.setmessage("internet connection not available. viewing news in offline mode.");         builder1.setcancelable(true);          alertdialog alert1 = builder1.create();         alert1.show();          try {             savefile = new saveintofile(filename);             jsonstr = savefile.read();             // log.d(tag,"offline data reading file");             if (!jsonstr.equals(null))                 new getdatas().execute();             else {              }         } catch (exception e) {             e.printstacktrace();          }     }     return null;  } 

but getting error on adding codes alertdialog. app works fine without codes alertdialog. can mistakes code , how can correct work well??

doinbackground() runs on separate thread, other main thread. can use ui elements on main thread. try using runonuithread() method inside doinbackground() show dialog.