swing - Java Program is not Calling Method -

public void startscanning() {      // check if there webcam available     if (cameraview.getwebcampanel() == null) {         return;     }      // make sure webcam not null     if (cameraview.getwebcam() == null) {         return;     }      if (!cameraview.getwebcam().isopen()) {         cameraview.getwebcam().open();     }      // make sure webcam open     if (cameraview.getwebcam().isopen()) {          // create swingworker thread run in background         worker = new swingworker<void, void>() {                       try {                         qrresult = new multiformatreader().decode(bitmap);                         //system.out.println("qrresults: " + qrresult);                            try {                             // new error handling                             int length = qrresult.gettext().length();                             if(length != 23){                                 joptionpane.showmessagedialog(null, "username , password correct");                                 startscanning();                             }                             // end of error handling 

i omitted of syntax, reason startscanning() method not called @ end. dialog box shown, method isn't called. can please explain why?

i suppose, it's related joptionpane.showmessagedialog. mesagedialog has closed before program proceeds. , can throw headlessexception if don't have graphics device show dialog at.

check open windows , implement catch try block.

a small example program show what's happening:

public static void main(string[] args) {     joptionpane.showmessagedialog(null, "message");     system.out.println("done"); } 

the console output done appear, if have graphics device, , after closed dialog window.