How to handle different stdout behaviour in extern

Question

Hi I am trying to execute external program from Java program and read the stdout message in real time, without waiting for the program to exit. However, i found that there are different stdout behaviour in different .exe program, and I don't know how to handle it.

Example 1:
server1.exe is a console program. When i run it, it will continuously listening on a port. When a client is connected to it, it will generate 1 line of stdout output every 1 second. It will not exit unless i press "ctrl-C".

In a command prompt, I run this:

server1.exe > stdout.out 2> stderr.err

When client is connected to it, I found that stdout.out file will be updated in real time. Even though server1.exe is still running, I can open stdout.out file and read the stdout output in real time.

Example 2:
Similar to server1.exe, server2.exe is also a console program. When i run it, it will also continuously listening on a port. When client is connected to it, it will generate 1 line of stdout output every 1 second. It will not exit unless i press "ctrl-C".

In a command prompt, I run this:

server2.exe > stdout.out 2> stderr.err

Even though client has connected to server2.exe, I found that stdout.out file is empty. As long as server2.exe is still running, no stdout is written to stdout.out file. That file is not updated in real time. When i press ctrl-C, it suddenly write many lines of output to stdout.out file.

Assuming that i press ctrl-C at t=11, it will write all stdout output from t=1 until t=11 into the stdout.out file. Before this, at t=10, the stdout.out file is empty.

The program in example 2 is giving me problem because I am unable to read the stdout in real time in my Java program. My java program is as below:

process = Runtime.getRuntime().exec(command);

input = new BufferedReader(new InputStreamReader(process.getInputStream()));

String inputtext = null;

while ((inputtext = input.readLine()) != null)
{
    //print out the text in Real Time, when the .exe program is still running
}

May i know why the program in example 2 will not generate stdout output unless I press ctrl-C?

The strange thing is, when i run that program in console window manually, I can see the stdout output printed on the console window every 1 second. But when I try to read it from Java using inputtext = input.readLine(), inputtext will be null as long as the program is still running (I have tested it by printing out inputtext). When I press ctrl-C, the BufferedReader will suddenly be filled with all the pending stdout output.

How can I read stdout of server2.exe in real time?

Answer 1

For short: You need to flush the buffers.

System.out.flush()

You need to do this after every chunk of relevant data written on these streams, try doing it after every line print.

Answer 2

The way you describe things, there is some buffering happening in your second server. The server might decide to buffer output internally, unless it is connected to a live interactive console window.

While there may be ways to work around this, I would address this in the server2 source code. Whenever that application writes its once-per-second output, it should flush its output streams afterwards. Perhaps there is some option to enable that behaviour. If there isn't, and if the sources of that program are outside your control, kindly ask the developers to add flushing, in order to allow for better integration.