protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    response.setContentType("text/html");

    int periodNum = Integer.valueOf(request.getAttribute("periodNum2").toString());
    int firstNum = Integer.valueOf(request.getAttribute("firstNum2").toString());
    Map<String, Card> map = new HashMap();
    // fill map

    new SelectCard().execute(map);
    request.getRequestDispatcher(PAGE_RESULT).forward(request, response);
}

public class SelectCard {
   protected static DBConnectionPool pool = DBConnectionPool.getInstance();

public void execute(Map<String, Card> map){
    Connection connection = pool.getConnection();
    if (connection == null) {
        System.out.println("Can't receive connection!");
        LogSingleton.appendLog("Can't receive connection!");
    }
    String SELECT_QUERY = "SELECT * from Majong.dbo.Cards where id = %d";

    Statement stmt = null;
    ResultSet resultSet= null;
    String query = String.format(Locale.ENGLISH, SELECT_QUERY,
            map.get("west").getId()
    );
    try {
        stmt = connection.createStatement(); // tomcat утверждает, что в этом месте NullPointerException
        resultSet = stmt.executeQuery(query);
        while(resultSet.next()){
            // work with result
                }
            }
        }

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

        try {
            if (stmt != null) {
                stmt.close();
            }
            if(resultSet != null){
                resultSet.close();
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
        pool.freeConnections(connection);
    }
}
}

public class DBConnectionPool {

static private String URL ;
private static String user = "";
private static String password = "";
private static int maxConn;
private final String DRIVER_NAME = "net.sourceforge.jtds.jdbc.Driver";
private final String JDBC_URL = "jdbc:jtds:sqlserver://localhost:1433/Majong;";
private final int MAX_COUNT = 3;

PrintWriter log;
static private List<Connection> freeConnections = new ArrayList<>();
static private DBConnectionPool instance = new DBConnectionPool();

private DBConnectionPool() {
    this.URL = JDBC_URL ;
    this.maxConn = MAX_COUNT;
    loadDrivers();
}

private void loadDrivers() {
    try {
        DriverManager.registerDriver(new net.sourceforge.jtds.jdbc.Driver());
        System.out.println("Registered JDBC driver ");
    } catch (SQLException e) {
        e.printStackTrace();
        System.out.println("Can't register JDBC driver");
    }
}

public static synchronized DBConnectionPool getInstance() {

    if (instance == null) {
        instance = new DBConnectionPool();
    }

    return instance;
}


public synchronized Connection getConnection() {

    Connection con = null;
    if (!freeConnections.isEmpty()) {
        con = (Connection)freeConnections.get(freeConnections.size()-1);
        freeConnections.remove(con);

        try {
            if (con.isClosed()) {
                System.out.println("Removed bad connection ");
                con = getConnection(); // Try again recursively
            }
        } catch (SQLException e) {
            System.out.println("Removed bad connection ");
            con = getConnection(); // Try again recursively
        } catch (Exception e) {
            System.out.println("Removed bad connection ");
            con = getConnection();// Try again recursively
        }
    } else {
        con = newConnection();
    }

    return con;
}
private Connection newConnection() {

    Connection con = null;
    try {
        if (user == null) {
            con = DriverManager.getConnection(URL);
        } else {
            con = DriverManager.getConnection(URL, user, password);
        }
    }  catch (SQLException e) {
        e.printStackTrace();
        return null;
    }catch (ClassNotFoundException e) {
        e.printStackTrace();
    }

    return con;
}
public synchronized void freeConnections(Connection con) {

    if ( (con != null) && (freeConnections.size() <= maxConn) ) {
        freeConnections.add(con);
    }
}
public synchronized void released() {
    Iterator allConnections = freeConnections.iterator();

    while (allConnections. hasNext()) {
        Connection con = (Connection) allConnections.next();

        try {
            con.close();
            System.out.println("Closed connection for pool");
        }

        catch (SQLException e) {
            System.out.println("Can't close connection for pool");
        }

    }

    freeConnections.clear(); }