问为什么出现此错误-无法获取连接，池错误超时等待空闲对象

EN

public CartItem getCustomerCartItem(int customerId, int productId, int productOfCityId) {
    Connection con = ConnectionPool.getInstance().getConnection();
    PreparedStatement st = null;
    ResultSet rs = null;
    CartItem ci = null;

    CityProduct cityProduct = productService.getCityProduct(productId, productOfCityId);
    if (cityProduct == null) {
        return null;
    }

    String query = "SELECT ci.* FROM customer_cart_item_mapping cim INNER JOIN cart_item ci ON ci.Id = cim.CartItemId WHERE "
            + "cim.CustomerId = ? AND ci.ProductOfCityId = ?";

    try {
        st = con.prepareCall(query);

        st.setInt(1, customerId);
        st.setInt(2, productOfCityId);

        rs = st.executeQuery();

        if (rs.first()) {
            ci = new CartItem();
            ci.setId(rs.getInt("Id"));
            ci.setCityProduct(cityProduct);
            ci.setQuantity(rs.getInt("Quantity"));
            ci.setCreatedDate(rs.getDate("CreatedDate"));
            ci.setUpdatedDate(rs.getDate("UpdatedDate"));
        }
    } catch (SQLException ex) {
        Logger lgr = Logger.getLogger(CartItemDaoImpl.class.getName());
        lgr.log(Level.SEVERE, ex.getMessage(), ex);

    } finally {
        DBUtil.close(con, st, rs);
    }

    return ci;
}

public static void close(Connection c, Statement s, ResultSet r) {
    try {
        if (r != null) {
            r.close();
        }
        if (s != null) {
            s.close();
        }
        if (c != null) {
            ConnectionPool.getInstance().freeConnection(c);
        }

    } catch (SQLException ex) {
        Logger.getLogger(DBUtil.class.getName()).log(Level.SEVERE, null, ex);
    }
}

public class ConnectionPool {
private static ConnectionPool pool=null;
private static DataSource dataSource = null;

public synchronized static ConnectionPool getInstance(){
    if (pool == null){
        pool = new ConnectionPool();
    }
    return pool;
}

private ConnectionPool(){
    try{
        InitialContext ic = new InitialContext();
        dataSource = (DataSource) ic.lookup("java:/comp/env/jdbc/prod_db");
    }
    catch(NamingException e){
        System.out.println(e);
    }
}

public Connection getConnection(){
    try{
        return dataSource.getConnection();
    }
    catch (SQLException sqle){
        System.err.println(sqle);
        return null;
    }
}

public void freeConnection(Connection c){
    try{
        c.close();
    }
    catch (SQLException sqle){
        System.err.println(sqle);
    }
}

<Context antiJARLocking="true" path="/">
  <Resource auth="Container" connectionProperties="useEncoding=true;" driverClassName="com.mysql.jdbc.Driver" initialSize="2"
        logAbandoned="true" maxIdle="40" maxTotal="70" maxWaitMillis="1000" minEvictableIdleTimeMillis="600000" minIdle="2" name="jdbc/winni_prime_db"
        password="password" removeAbandoned="true" removeAbandonedTimeout="90" testWhileIdle="true" timeBetweenEvictionRunsMillis="90000"
        type="javax.sql.DataSource" url="jdbc:mysql://localhost:3306/prod_db"
        username="sqladmin" validationQuery="SELECT 1"/>