import 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")) {
                //if (portId.getName().equals("/dev/term/a")) {
  try {
                        serialPort = (SerialPort)portId.open("SimpleWriteApp", 2000);
                    } catch (PortInUseException e) { System.out.println( "" + e ); }//运行到此产生异常,为什么？
                    try {
                        outputStream = serialPort.getOutputStream();
                    } catch (IOException e) {}
                    try {
                        serialPort.setSerialPortParams(9600,
                            SerialPort.DATABITS_8,
                            SerialPort.STOPBITS_1,
                            SerialPort.PARITY_NONE);
                    } catch (UnsupportedCommOperationException e) {}
                    try {
                        outputStream.write(messageString.getBytes());
                    } catch (IOException e) {}
                }
            }
        }
    }
}

产生异常的位置到底在哪里？你把System.out.println(""+e)改成e.printTrace()再观察一下出错信息，可以知道准确的错误定位。
估计是2000端口已经被别的应用程序占用了。你换个端口号看看