public void Combobox(){
    try{
        conexion=DriverManager.getConnection("jdbc:mysql://localhost/demo_seguridad","root","75:)82paco");
        st=conexion.createStatement();
        rs=st.executeQuery("Select nombre from clientes");
        while (rs.next()){
            this.jcb_clientes.addItem(rs.getString("nombre"));
        }
        }catch(SQLException err){
        JOptionPane.showMessageDialog(null,"Error "+err.getMessage());
    }
}