private Connection conn = null;
private Statement st=null;
private ResultSet rs=null;

    public void setConnection(){
             host="localhost";
             database="test";
             username="testtest";
             password="testtest";
             String url = "jdbc:mysql://" + host + "/" +database+ "";
             try {
                 Class.forName ("com.mysql.jdbc.Driver");
                 conn = DriverManager.getConnection (url, username, password);
                 System.out.println("Succesfuly connected to the database!");

             } catch(Exception e){
                 System.out.println("Failed to connect to the database.");
                 e.printStackTrace();
             }
        }

    private boolean checkConnection(){
            try {
                ResultSet cc = st.executeQuery("SELECT 1");
                return true;
            } catch (SQLException e) {
                int n = JOptionPane.showConfirmDialog(
                        frame,
                        "The connection was closed, reconnect?",
                        "Database security measure",
                        JOptionPane.YES_NO_OPTION);
                if(n == JOptionPane.YES_OPTION){
                    try {
                        conn.close();
                    } catch (SQLException e1) {
                        e1.printStackTrace();
                    }
                      setConnection();
                    } else if ( n == JOptionPane.NO_OPTION){
                        frame.setVisible(false);
                        new LoginWindowB();
                    }
                return false;
            }
        }
    btn1.addActionListener(new ActionListener(){
                @Override
                public void actionPerformed(ActionEvent arg0) {
                    if(checkConnection()){
                        try {
                            rs = st.executeQuery("SELECT * FROM " + selectedTable);
                            loadData(rs);

                        } catch (SQLException e) {
                            e.printStackTrace();
                        }
                    } else {

                    }
                }
            });