Scanner.nextLine() blocks when using InputStream from Socket

Question

When I receive data using Socket.getInputStream() directly (without some kind of interface like Scanner), it doesn't block. But, when I try to use a Scanner (similar to how we receive Strings from System.in), it does. I was wondering the reason for this, and how the InputStream that a connected Socket supplies to you is different from the InputStream in in System.

The Client used for testing (used for both servers)

The code that hangs:

public class Server {

    public static void main(String[] args) {
        try {
            ServerSocket ss = new ServerSocket(15180);
            Socket socket = ss.accept();

            Scanner scanner = new Scanner(socket.getInputStream());
            //read data from client
            while(true) {
                String data = scanner.nextLine();
                System.out.println("Received data!");
            }

        }catch(IOException e) {
            e.printStackTrace();
        }
    }
}

The code that doesn't block:

public class Server {
    public static void main(String[] args) {
        try {
            ServerSocket ss = new ServerSocket(15180);
            Socket socket = ss.accept();

            //read data from client
            while(true) {
                int data = socket.getInputStream().read();
                System.out.println("Received data!");
            }

        }catch(IOException e) {
            e.printStackTrace();
        }
    }
}

Answer 1

(I think you've already figured this out but ...)

The readLine() method returns the rest of the current line. That is, all unconsumed characters up to the next "end of line" sequence, or the "end of stream", which ever comes first. It will block, waiting until the current line (according to the above) is available.

So if your socket readLine() call blocks, it is waiting for the remote to either send an end-of-line marker (e.g. '\n'), or close its socket output stream (which will result in an "end-of-stream" at this end).

Q: Why does it "work" when you read from the console?

A: The console adds an "end-of-line" sequence to the stream whenever you hit ENTER. (Precisely what sequence is added is OS dependent, but the Scanner class will cope with all common varieties, and some unusual ones too.)

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The lesson here is that you should only use Scanner.readLine() if the input stream is line oriented; i.e. if whatever wrote / generated the stream is including "end-of-line" markers.