CommPortIdentifier 中open()方法乍么没有法返回串口

mport java.io.*;
import java.util.*;
import javax.comm.*;public class SimpleWrite {
    static Enumeration portList;
    static CommPortIdentifier portId;
    static String messageString = "Hello, world!\n";
    static SerialPort serialPort;
    static OutputStream outputStream;    public static void main(String[] args) {
        portList = CommPortIdentifier.getPortIdentifiers();        while (portList.hasMoreElements()) {
            portId = (CommPortIdentifier) portList.nextElement();
            if (portId.getPortType() == CommPortIdentifier.PORT_SERIAL) {
              if (portId.getName().equals("COM1")) {
                
                    try {
                     System.out.println(portId);
                        serialPort = (SerialPort)
                            portId.open("SimpleWriteApp", 2000);
                        
                    } catch (PortInUseException e) {
                     e.printStackTrace();
                    }
                    try {
                        outputStream = serialPort.getOutputStream();
                    } catch (IOException e) {
                     e.printStackTrace();
                    }
                    try {
                        serialPort.setSerialPortParams(9600,
                            SerialPort.DATABITS_8,
                            SerialPort.STOPBITS_1,
                            SerialPort.PARITY_NONE);
                    } catch (UnsupportedCommOperationException e) {}
                    try {
                        outputStream.write(messageString.getBytes());
                    } catch (IOException e) {}
                }
            }
        }
    }
}javax.comm.CommPortIdentifier@276af2
javax.comm.PortInUseException: Port currently owned by SimpleWriteApp
at javax.comm.CommPortIdentifier.open(CommPortIdentifier.java:337)
at Simple.SimpleWrite.main(SimpleWrite.java:55)
Exception in thread "main" java.lang.NullPointerException
at Simple.SimpleWrite.main(SimpleWrite.java:61)