Programming Tutorials

Read from a COM port using Java program

By: Steven Lim in Java Tutorials on 2006-12-11  

Most projects that deal with hardware and devices, needs to communicate with them using the COM port of the PC or Server. For example if there is a modem that is connected to a server via its COM port and the Java program has to read the output of the modem then the Java program has to read the COM port for any incoming data.

This sample Java program can be used to Read from a COM port for incoming data and process it. Note that you will need to change the Port number to COM1 or COM2 or any other ports as required.

Also if you are using unix based machines then you will have to uncomment the /dev/term/a instead of COM.

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

public class SimpleRead implements Runnable, SerialPortEventListener {
    static CommPortIdentifier portId;
    static Enumeration portList;

    InputStream inputStream;
    SerialPort serialPort;
    Thread readThread;

    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")) {
                    SimpleRead reader = new SimpleRead();

    public SimpleRead() {
        try {
            serialPort = (SerialPort)"SimpleReadApp", 2000);
        } catch (PortInUseException e) {System.out.println(e);}
        try {
            inputStream = serialPort.getInputStream();
        } catch (IOException e) {System.out.println(e);}
	try {
	} catch (TooManyListenersException e) {System.out.println(e);}
        try {
        } catch (UnsupportedCommOperationException e) {System.out.println(e);}
        readThread = new Thread(this);

    public void run() {
        try {
        } catch (InterruptedException e) {System.out.println(e);}

    public void serialEvent(SerialPortEvent event) {
        switch(event.getEventType()) {
        case SerialPortEvent.BI:
        case SerialPortEvent.OE:
        case SerialPortEvent.FE:
        case SerialPortEvent.PE:
        case SerialPortEvent.CD:
        case SerialPortEvent.CTS:
        case SerialPortEvent.DSR:
        case SerialPortEvent.RI:
        case SerialPortEvent.OUTPUT_BUFFER_EMPTY:
        case SerialPortEvent.DATA_AVAILABLE:
            byte[] readBuffer = new byte[20];

            try {
                while (inputStream.available() > 0) {
                    int numBytes =;
                System.out.print(new String(readBuffer));
            } catch (IOException e) {System.out.println(e);}

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