BufferedReader chained to socket, how does readLine() != null work?

I am a bit confused as to what is going on here. Going through Head First Java and I am making a multithreaded server. The part I don't understand is here, where reader is a BufferedReader chained to a InputStream(socket.getInputStream()): 

class IncomingReader implements Runnable {
    public void run() {
        String message;
        try {
            while ((message = reader.readLine()) != null){
            System.out.println("client read " + message);
            incoming.append(message + "\n");
      }

in particular how does the

while ((message = reader.readLine()) != null)

activate?

What I mean is, if there is no input coming from the server, shouldn't it == null, and if it does not, does the readLine() method itself have some sort of loop that detects data from the server's stream?

I understand that readLine() "blocks" the while loop from activating until it returns a line. My question is, how does it detect that there is something in the inputstream, how does it know when it becomes null?

Jon Skeet
people
quotationmark

My question is, how does it detect that there is something in the inputstream, how does it know when it becomes null?

This is more to do with the InputStream from socket.getInputStream() than BufferedReader, really. Any call to InputStream.read() will block until there's data available, unless the stream is closed. So if the server isn't providing any data, but the socket is still open, the call will block.

If the connection is closed (by either end) then any remaining data will be read, and then future read calls will return -1 to indicate that the stream has been closed.

BufferedReader.readLine() will only return null when the underlying reader returns that there will be no more data, which in the case of InputStreamReader will only happen if the underlying stream has been closed.

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