TCP socket: Why BufferedReader.readLine() get NULL?

Question

I have a classic Java client/server app, where client and server exchange messages by a TCP connection.

When one elemement, let's call it Receiver (no matter if client or server), has to receive a message, it listens and waits for a message coming from the other element, let's call it Sender. The receiver implements it by the following code:

BufferedReader myBufferedReader = new BufferedReader(new InputStreamReader(mySocket.getInputStream()));
receivedMessageCompleto = myBufferedReader.readLine();

The sender, after some time, will send a text message implementing the following code:

String messageCompleto = "whatever text";
String packetSend = messageCompleto + '\n' ;  
DataOutputStream myDataOutputStream = new DataOutputStream(mySocket.getOutputStream());
myDataOutputStream.writeBytes(packetSend ); 

The probelm is: in some cases "myBufferedReader.readLine()" get null value, but I'm quite sure that the sender didn't send a message NULL. The question is: why?

According to my understanding the Receiver should remain blocked until it receives something (let's consider that timeout is set to infinite), then "myBufferedReader.readLine()" should return a value only when something arrives. The point is that I'm sure that the sender didn't send a null message.

Any idea???

Thank you very much in advance Fausto

Answer 1

public String readLine()
                throws IOException

Reads a line of text. A line is considered to be terminated by any one of a line feed ('\\n'), a carriage return ('\\r'), or a carriage return followed immediately by a linefeed.

Returns:

A String containing the contents of the line, not including any line-termination characters, or null if the end of the stream has been reached

Basically you get null if the underlying stream hits EOF.

Answer 2

As it says in the documentation for the readLine method :

Returns: A String containing the contents of the line, not including any line-termination characters, or null if the end of the stream has been reached

So, in your case, end-of-stream has been reached. Probably, the remote end of the socket was closed.