Re: Cancel Reading Over A Socket

Knute Johnson <>
Mon, 21 May 2007 20:50:19 -0700
Jason Cavett wrote:

I am attempting to cancel a running process that is reading data
coming in via a socket from an external (non-Java) application. The
cancel *does* work, except I always get an exception. Here is the
code in question...

    private void process() throws ProcessException {
        socketWriter.println("START PROCESSING");

        try {
            File outFile = new File(workingFolder, "output.txt");
            FileWriter fw = new FileWriter(outFile, false);
            BufferedWriter bw = new BufferedWriter(fw);

            String line = socketReader.readLine();
(line.indexOf(NativeProcessDistributed.COMPLETED_RUN) == -1) {
                line = socketReader.readLine();

        } catch (IOException e) {
            throw new

Basically, I start the processing (via START PROCESSING written to the
application) and then watch the input for a string that means the run
has completed. Until then, I write out all output to an output file
(so someone can examine the output if anything goes wrong with the

I want the user to cancel the run at any time in the GUI via a cancel
button. This is the method that's fired when the cancel button is

    public void cancel() {
        try {
            // write a suicide file/flag to force the processing to
            File suicide = new File(runFolder + SUICIDE_FILE);


            status = CANCELLED;
        } catch (IOException e) {

The cleanup() method is as follows:

    private void cleanup() {
        try {
        } catch (IOException e) {

What's happening (as far as I can tell) is that I'm closing the
clientSocket while the "while" loop in the process() method is still
looping, thus causing the exception to be thrown. I am not sure how
to sync the two methods up so that I can cancel the process AND the
while loop in the distribute method stops (the socketReader.readLine()
is what is throwing the actual exception, BTW). It may be a threading
issue, but I'm having difficulty figuring out what the issue actually

Thanks for any help with this frustrating problem.

If the stream or socket is closed an IOException is thrown. For a
normal stop (when you've read the stop flag) I would close the stream
after the loop. To force a termination, close the socket, that will
also close all streams and throw a SocketException.

try {
     while (true) {
         String str = reader.readLine();
         if ("end message".equals(str))
             break; // you are done
         else if (str == null)
             break; // end of stream
         // process str here
     reader.close(); // normal exit, close stream
     // close your file i/o too
} catch (IOException ioe) {
     // print error message if needed

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