public class DatabaseHandler {

    public static Connection getConnection() throws SQLException, IOException {

        Properties props = new Properties();
        try(InputStream in = Files.newInputStream(Paths.get("files/database.properties.txt"))){
            props.load(in);
        }
        String url = props.getProperty("url");
        String username = props.getProperty("username");
        String password = props.getProperty("password");

        return DriverManager.getConnection(url, username, password);
    }

    static public void singUpUser (User user) {
        String sqlCommand = "INSERT INTO "+ Const.TABLE + "(" + Const.USERS_FIO + "," + Const.USERS_PASS + ","
                + Const.USERS_MAIL + ")" + " VALUES (?,?,?);";

        try {
            Class.forName("com.mysql.jdbc.Driver").getDeclaredConstructor().newInstance();

            try (Connection conn = getConnection()) {
                System.out.println("Подключение успешно");

                PreparedStatement preparedStmt = conn.prepareStatement(sqlCommand);
                preparedStmt.setString (1, user.getFioField());
                preparedStmt.setString (2, user.getPassword());
                preparedStmt.setString (3, user.getMail());

                preparedStmt.execute();

                System.out.println("Данные внесены");
            }
        } catch (Exception ex) {
            System.out.println("Ошибка подключения");

            System.out.println(ex);
        }
    }

    public ResultSet getUser (User user){
        ResultSet resultSet = null;
        String sqlCommand2 = "SELECT * FROM  "+ Const.TABLE + " WHERE " + Const.USERS_MAIL + "=? AND " +
                Const.USERS_PASS + "=?";

        try {
            Class.forName("com.mysql.jdbc.Driver").getDeclaredConstructor().newInstance();

            try (Connection conn = getConnection()) {
                System.out.println("Подключение успешно");

                PreparedStatement preparedStmt = conn.prepareStatement(sqlCommand2);
                preparedStmt.setString (1, user.getPassword());
                preparedStmt.setString (2, user.getMail());

                resultSet = preparedStmt.executeQuery();

                System.out.println("OK!");
            }
        } catch (Exception ex) {
            System.out.println("Ошибка подключения");
            System.out.println(ex);
        }
        return resultSet;
    }
}

public class Controller {

    @FXML
    private TextField passField;

    @FXML
    private TextField passwordField;

    @FXML
    private TextField fioField;

    @FXML
    private Button registrButton;

    @FXML
    private TextField mailField;

    @FXML
    private Button enter;

    @FXML
    private TextField loginField;

    @FXML
    void initialize() {
        enter.setOnAction(event -> {
            String loginText = loginField.getText().trim();
            String passwordText = passField.getText().trim();
            if (!loginText.equals("") && !passwordText.equals("")) {
                loginUser(loginText, passwordText);
            } else System.out.println("введите данные");
        });

        registrButton.setOnAction(event -> {
            signUpNewUser();
        });
    }

    private void signUpNewUser() {
        DatabaseHandler databaseHandler = new DatabaseHandler();

        String fio = fioField.getText();
        String pass = passwordField.getText();
        String mail = mailField.getText();

        User user = new User(fio, pass, mail);

        databaseHandler.singUpUser(user);
    }

    private void loginUser(String loginText, String passwordText) {
        DatabaseHandler databaseHandler = new DatabaseHandler();
        User user = new User();
        user.setMail(loginText);
        user.setPassword(passwordText);
        ResultSet resultSet = databaseHandler.getUser(user);

        int counter = 0;

        try {
            while (resultSet.next()){
                counter++;
             }
        } catch (SQLException ex){
            ex.printStackTrace();
        }

        if (counter >= 1 ) {
            System.out.println("OK");
        }
    }
}

package server;

public class User {
    private String fioField;
    private String password;
    private String mail;

    public User(String fioField, String password, String mail) {
        this.fioField = fioField;
        this.password = password;
        this.mail = mail;
    }

    public User() {

    }

    public String getFioField() {
        return fioField;
    }

    public void setFioField(String fioField) {
        this.fioField = fioField;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public String getMail() {
        return mail;
    }

    public void setMail(String mail) {
        this.mail = mail;
    }
}