Java AT command writing errors.

Dear All,
I am running a java application on centos. For now I have a gsm modem connected via the the usb cable. Below is the message I get when I type the command dmesg | grep tty

serial8250: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
00:0c: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
usb 2-2: pl2303 converter now attached to ttyUSB0
pl2303 ttyUSB0: pl2303 converter now disconnected from ttyUSB0
usb 2-1: pl2303 converter now attached to ttyUSB0
pl2303 ttyUSB0: pl2303 converter now disconnected from ttyUSB0
usb 2-1: pl2303 converter now attached to ttyUSB0

The based on that I am running a java application that trying to send sms via the gsm modem. Why I am using /dev/ttyS0 because /dev/ttyUSB0 is not part of the port lists. Below is my codes. When I run then I get an error as below. Can some one help me please? Thank you.

Exception in thread "main" java.lang.RuntimeException:
Error opening "/dev/ttyS0"
lockf(): Resource temporarily unavailable
at com.sun.comm.LinuxDriver.getCommPort(LinuxDriver.java:66)
at javax.comm.CommPortIdentifier.open(CommPortIdentifier.java:369)
at PortWriter.<init>(PortWriter.java:15)
at PortWriter.main(PortWriter.java:34)

Code:

import java.io.*;
import javax.comm.*;
import java.util.*;

public class PortWriter
{
    static Enumeration ports;
    static CommPortIdentifier pID;
    static OutputStream outStream;
    SerialPort serPort;
    static String messageToSend = "AT+CMGS='+60162210456'testing\0x1A";
    
    public PortWriter() throws Exception{
        serPort = (SerialPort)pID.open("PortWriter",2000);
        outStream = serPort.getOutputStream();
        serPort.setSerialPortParams(9600, SerialPort.DATABITS_8,
        SerialPort.STOPBITS_1,
        SerialPort.PARITY_NONE);
    }    
    
    public static void main(String[] args) throws Exception{
        ports = CommPortIdentifier.getPortIdentifiers();
        
        while(ports.hasMoreElements())
        {
            pID = (CommPortIdentifier)ports.nextElement();
            System.out.println("Port " + pID.getName());
            
            if (pID.getPortType() == CommPortIdentifier.PORT_SERIAL)
            {
                if (pID.getName().equals("/dev/ttyS0"))
                {
                    PortWriter pWriter = new PortWriter();
                    System.out.println("COM1 found");
                }
            }
        }
        outStream.write(messageToSend.getBytes());
    }
}