如何在Java中使用jssc从serialport读取数据？

我使用jssc库从串行端口读取和写入数据。

package serial;

import jssc.*;

public class Serial {

public static void main(String[] args) {
    String[] portNames = null;
    portNames = SerialPortList.getPortNames();
    for (String string : portNames) {
        System.out.println(string);
    }

    if (portNames.length == 0) {
        System.out.println("There are no serial-ports");
    } else {

        SerialPort serialPort = new SerialPort("com2");
        try {
            serialPort.openPort();

            serialPort.setParams(SerialPort.BAUDRATE_9600,    SerialPort.DATABITS_8, SerialPort.STOPBITS_1,
                    SerialPort.PARITY_NONE);
            serialPort.setFlowControlMode(SerialPort.FLOWCONTROL_RTSCTS_IN | SerialPort.FLOWCONTROL_RTSCTS_OUT);

            PortReader portReader = new PortReader(serialPort);

            serialPort.addEventListener(portReader, SerialPort.MASK_RXCHAR);

            serialPort.writeString("S");

            serialPort.closePort();
        } catch (Exception e) {
            System.out.println("There are an error on writing string to port т: " + e);
        }
    }
}
}

package serial;

import jssc.SerialPort;
import jssc.SerialPortEvent;
import jssc.SerialPortEventListener;
import jssc.SerialPortException;

public class PortReader implements SerialPortEventListener {

SerialPort serialPort;
public PortReader(SerialPort serialPort) {
    this.serialPort = serialPort;
}

@Override
public void serialEvent(SerialPortEvent event) {
    System.out.println("started");
    if (event.isRXCHAR() && event.getEventValue() > 0) {
        try {
            String receivedData = serialPort.readString(event.getEventValue());
            System.out.println("Received response: " + receivedData);
        } catch (SerialPortException ex) {
            System.out.println("Error in receiving string from COM-port: " + ex);
        }
    }
}
}

我使用虚拟串口和端口，此代码将数据写入串行端口。我确定数据正确写入了串行端口。

但我无法从串行端口读取数据。