Updating dynamically JTable(s)

Question

I'm working on a Java database application, where the user has one side to insert data and another to show in a JTable (in a JScrollPane) the content of the linked HSQLDB database. Everthing works so far so good, but I searched for a very long time without finding how to update my JTable dynamically when a change is made to the database (add, update or delete) for each instance of my program because several persons can work on it simultaneously. It actually works by replacing my table model for local update, but for other instances, i have set a Timer which is not very clean.

Could you help me resolving this please ?

Here is my code (I have an Interface.java for "visible" part and a SQL.java to manage my db) :

My timer

JTable table = new JTable(sql.showTable("SELECT * FROM suivi_flotte"));
Timer timer = new Timer(900000, new ActionListener() {

    @Override
    public void actionPerformed(ActionEvent e) {
        sql.updateTable();
        nbLinesOrRefresh.setText("Actualisation...");
        Timer timer2 = new Timer(1000, new ActionListener() {

            @Override
            public void actionPerformed(ActionEvent e) {
                nbLinesOrRefresh.setText("");
            }
        });
        timer2.start();
        timer2.setRepeats(false);
    }
});
timer.setDelay(900000);
timer.start();
formatTable(table);

My updateTable() method (called each time there is an insert, update or delete)

protected void updateTable() {
    Interface.f.table.setModel(showTable("SELECT * FROM suivi_flotte"));
    Interface.f.formatTable(Interface.f.table); // a method to custom JTable appearance
}

My showTable(String query) method

protected DefaultTableModel showTable(String query) {
    String[] columnsTitles = {"N°", "Propriétaire", "Service", "Grade", "Nom", "Prénom", "Mail", "N° SIM", "N° Tél", "Modèle", "N° Série", "IMEI", "N° Crypto", "Date début", "Date fin", "DS", "Commentaire", "Date de création"};
    ArrayList<String> columnNames = new ArrayList<>();
    ArrayList<Object> data = new ArrayList<>();

    connectDB(); // only to connect to db
    try (ResultSet rs = stmt.executeQuery(query)) {
        ResultSetMetaData md = rs.getMetaData();
        int columns = md.getColumnCount();

        for (int i = 1; i <= columns; i++) {
            columnNames.add(columnsTitles[i - 1]);
        }

        while (rs.next()) {
            ArrayList<Object> row = new ArrayList<>(columns);
            for (int i = 1; i <= columns; i++) {
                row.add(rs.getObject(i));
            }
            data.add(row);
        }
        shutdownDB(); // to fully disconnect from db
    } catch (SQLException e) {
        System.out.println(e.getMessage());
    }

    Vector<Vector<?>> dataVector = new Vector<>();
    Vector<String> columnNamesVector = new Vector<>();

    for (int i = 0; i < data.size(); i++) {
        ArrayList<?> subArray = (ArrayList<?>) data.get(i);
        Vector<Object> subVector = new Vector<>();
        for (int j = 0; j < subArray.size(); j++) {
            subVector.add(subArray.get(j));
        }
        dataVector.add(subVector);
    }

    for (int i = 0; i < columnNames.size(); i++) {
        columnNamesVector.add(columnNames.get(i));
    }

    DefaultTableModel tModel = new DefaultTableModel(dataVector, columnNamesVector) {
        private static final long serialVersionUID = 1L;

        public Class<?> getColumnClass(int column) {
            for (int row = 0; row < getRowCount(); row++) {
                Object o = getValueAt(row, column);
                if (o != null) {
                    return o.getClass();
                }
            }
            return Object.class;
        }

        @Override
        public boolean isCellEditable(int row, int column) {
            return false;
        }
    };

    // tModel.fireTableDataChanged(); // <-- does not work

    return tModel;
}

Thank you for your help.

UPDATE 1 : All my connection/disconnection code :

private void connectDB() {
    JFrame op = new JFrame();
    op.setAlwaysOnTop(true);
    try {
        Class.forName("org.hsqldb.jdbcDriver");
        co = DriverManager.getConnection("jdbc:hsqldb:file:db;shutdown=true");
        stmt = co.createStatement();
    } catch (SQLException | ClassNotFoundException e) {
        JOptionPane.showMessageDialog(op, "Erreur de connexion à la base de données :\n" + e.getMessage(), Interface.windowTitle, JOptionPane.ERROR_MESSAGE);
    }
}

private void openDB() throws IOException {
    try {
        BufferedReader br = new BufferedReader(new FileReader("db.sql"));
        String line;
        StringBuffer sb = new StringBuffer();
        while ((line = br.readLine()) != null) {
            sb.append(line + "\n");
        }
        br.close();
        stmt.executeUpdate(sb.toString());
    } catch (SQLException e) {}
}

private void shutdownDB() throws SQLException {
    stmt.close();
    co.close();
}

UPDATE 2 : I changed everything in SQL.java to non-static. Each SQL.[...] has been changed to sql.[...] with private SQL sql = new SQL(); on top of the class and public SQL() {} in the class. I also changed it in the above code.