当端口存在时,为什么我会得到一个JavaNoSuchPortException?

    final String portName = getSerialPort();
    try {
        final CommPortIdentifier id = CommPortIdentifier.getPortIdentifier(portName);
        port = (SerialPort) id.open(getName(), 1000);
    } catch (NoSuchPortException nspe) {
        report(SeverityCode.LEVEL2, getName(), "PIN Pad is not connected to " + portName + " port, or the port does not exist.");
        return;
    } catch (PortInUseException piue) {
        report(SeverityCode.LEVEL2, getName(), portName + " port is already in-use by some other device. Reason: " + piue.getMessage());
        return;
    }