When an exception occurs within the GUIInstaller constructor, e.g. NoClassDefFoundError, the error is displayed at the console but no message box occurs. When the installer is started by e.g. double-clicking at the JAR file in Windows, no error message can be seen and it seems like nothing happens. I suggest to move the current GUIInstaller constructor code into an init() subroutine and change the constructor to something like that: public GUIInstaller() throws Throwable { try { init(); } // call original constructor code catch (Throwable e) { // catch Throwables so that e.g. NoClassDefFoundError is included try { // try to display a message box JOptionPane.showMessageDialog (null, "Error: "+e.toString(), "Error", JOptionPane.ERROR_MESSAGE); } catch (Exception e2) {} // ignore possible exception from showMessageDialog throw e; }} // re-throw the original exception