how to restart a thread

Question

I tried to write a file monitor which will check the file if a new line is appended,the monitor in fact is a thread which will read the line by a randomaccessfile all the time.

Answer 1

A thread in Java cannot be re-started. Every time you need to restart the thread you must make a new one.

That said, you might want to look at:

private void setRandomFile() {
        if (!monitoredFile.exists()) {
            log.warn("File [" + monitoredFile.getAbsolutePath()
                    + "] not exist,will try again after 30 seconds");
            try {
                Thread.sleep(30 * 1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            setRandomFile();
            return;
        }
// ....
}

Here you sleep for 30 seconds if the file does not exist, then recursively call the same function. Now, I don't know what business requirements you have, but if this recursion ran long enough you will run out of stack space. Perhaps you will be better served with a while loop or even better, a little synchronisation like a Semaphore.