Question

From Java, I run a C++ app which logs its progress as it proceeds, and want to monitor that there is some progress continually, so (in Java) I monitor process.getInputStream() and process.getErrorStream(). When I run the C++ app from the command line, the log appears one line at a time as it progresses (about 1 line very 1-2 seconds). From the Java monitor, I’m not getting the data in realtime, and am not sure who is buffering it.

I can look at the C++ code (though it’s rather complex) and try to put flush() calls as necessary, but the data appears in realtime when I run it from cmd.exe, so before I do that, I’d like to check out if there is something else along the way that I have to do. Is there something in Java I have to do to get the data in realtime, to make sure the input stream is not buffered?

I’m running in Windows XP and above.

I tried

            for (;;) {
                int c = is.read();
                if (c < 0) {
                    break;
                }
                System.out.print(c);
                System.out.flush();
            }
            if (true) {
                return;
            }

and also tried

            InputStreamReader isr = new InputStreamReader(is);
            BufferedReader br = new BufferedReader(isr);
            String line = null;
            while ((line = br.readLine()) != null) {
                synchronized (lastActivityTime) {
                    lastActivityTime = java.util.Calendar.getInstance().getTimeInMillis();
                }
                if (pw != null) {
                    pw.println(type + line);
                    pw.flush();
                }
            }

Answer 1

It is the source process that is doing the buffering, in the stdio library. There is nothing you can do about that at the Java end. You have to put fflush() calls into the C++ code, or put up with it and wait. The data appears in real time when you run from cmd.exe because stdio behaves differently when attached to what it thinks is a real terminal.