关闭窗口的一个小问题！


this.addWindowListener(new WindowAdapter(){
            @Override
            public void windowClosing(WindowEvent e)
            {
                if(!panel.workFlag)
                {
                    JOptionPane.showMessageDialog(null,"请先注销后退出!", "",JOptionPane.WARNING_MESSAGE);
                    e.getOldState();
                    return;
                }
                else
                {
                    System.exit(0);
                }
            }
        });
怎么让关闭窗口弹出个警告对话框后，不关闭窗口？有懂的朋友指点一下