how to read from standard input non-blocking?

Using BufferedReader.available() as suggested by Sibbo isn’t reliable. Documentation of available() states:

Returns an estimate of the number of bytes that can be read… It is never correct to use the return value of this method to allocate a buffer.

In other words, you cannot rely on this value, e.g., it can return 0 even if some characters are actually available.

I did some research and unless you are able to close the process input stream from outside, you need to resort to an asynchronous read from a different thread. You can find an example how to read without blocking line by line here.

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Update: Here is a simplified version of the code from the link above:

public class NonblockingBufferedReader {
    private final BlockingQueue<String> lines = new LinkedBlockingQueue<String>();
    private volatile boolean closed = false;
    private Thread backgroundReaderThread = null;

    public NonblockingBufferedReader(final BufferedReader bufferedReader) {
        backgroundReaderThread = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    while (!Thread.interrupted()) {
                        String line = bufferedReader.readLine();
                        if (line == null) {
                            break;
                        }
                        lines.add(line);
                    }
                } catch (IOException e) {
                    throw new RuntimeException(e);
                } finally {
                    closed = true;
                }
            }
        });
        backgroundReaderThread.setDaemon(true);
        backgroundReaderThread.start();
    }

    public String readLine() throws IOException {
        try {
            return closed && lines.isEmpty() ? null : lines.poll(500L, TimeUnit.MILLISECONDS);
        } catch (InterruptedException e) {
            throw new IOException("The BackgroundReaderThread was interrupted!", e);
        }
    }

    public void close() {
        if (backgroundReaderThread != null) {
            backgroundReaderThread.interrupt();
            backgroundReaderThread = null;
        }
    }
}