public class Test_n_Try {
	
	static Enumeration portList;
	static CommPortIdentifier portId;
	
	public static void main(String[] args) {
		String s="rxtxSerial.dll"; //на этой строчке выкидывает
		System.loadLibrary(s);
		//Runtime.getRuntime().loadLibrary(s);//на этой - тоже

		portList=CommPortIdentifier.getPortIdentifiers();
		while(portList.hasMoreElements()) {
			portId=(CommPortIdentifier)portList.nextElement();
			System.out.println(portId.getName());
		}
		//System.exit(0);
	}

}

ShtrihFiscalPrinter p=new ShtrihFiscalPrinter() {
	@Override
	public BeepCommand beep() throws Exception {
		// TODO Auto-generated method stub
		return null;
	}
...
}

import com.shtrih.jpos.*;
import com.sun.comm.*;

...
				Win32Driver driver=new com.sun.comm.Win32Driver();
				driver.initialize();
				SerialPortInterface port=(SerialPortInterface) driver.getCommPort("COM1", 0);
				FiscalPrinterProtocol protocol=new FiscalPrinterProtocol(port);
				
				com.shtrih.jpos.FiscalPrinterDevice printer=new FiscalPrinterDevice(protocol);
				try {
					printer.beep(); //здесь вываливается NullPointerException
					JOptionPane.showMessageDialog(c, "Beep!");
				} catch (Exception e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
					JOptionPane.showMessageDialog(c, e.getClass().getName()+": "+e.getMessage());
				}

				FiscalPrinterProtocol protocol=new FiscalPrinterProtocol(port);

				SerialPortInterface port=(SerialPortInterface) driver.getCommPort("COM1", 0);

private void refillPortList() {
	Enumeration<CommPortIdentifier> en=CommPortIdentifier.getPortIdentifiers();
	CommPortIdentifier portId;
        while (en.hasMoreElements()) {
            portId = en.nextElement();
            if (portId.getPortType() == CommPortIdentifier.PORT_SERIAL) jcbPorts.addItem(portId.getName()); //заполнение JComboBox
        }
}

private void refillPortList() {
	Enumeration<CommPortIdentifier> en=CommPortIdentifier.getPortIdentifiers();
	CommPortIdentifier portId;
        while (en.hasMoreElements()) {
            portId = en.nextElement();
            if (portId.getPortType() == CommPortIdentifier.PORT_SERIAL) jcbPorts.addItem(portId.getName()); //заполнение JComboBox
        }
}