Java Networking: Explain InputStream and OutputStream in Socket

Question

I have created a server by using ServerSocket. After that, I have created Client using Socket, and connect to this server.

After that, I do

Answer 1

A socket is an abstraction that you use to talk to something across the network. See diagram below...

In Java, to send data via the socket, you get an OutputStream (1) from it, and write to the OutputStream (you output some data).

To read data from the socket, you get its InputStream, and read input from this second stream.

You can think of the streams as a pair of one-way pipes connected to a socket on the wall. What happens on the other side of the wall is not your problem!

In your case, the server has another socket (the other end of the connection) and another pair of streams. It uses its InputStream (2) to read from the network, and its OutputStream (3) to write the same data back across the network to your client, which reads it again via its InputStream (4) completing the round trip.

      Client                                                     Server

1. OutputStream -->\                                     /--> 2. InputStream -->
                    Socket <--> network <--> ServerSocket                       |
4. InputStream  <--/                                     \<--3. OutputStream <--

Updated: in reply to comment:

Note that the streams and sockets just send raw bytes; they have no notion of a "message" at this level of abstraction. So if you send X bytes and another X bytes, then read X bytes and read another X bytes, then your system behaves as if there are two messages, because that's how you've divided up the bytes.

If you send X bytes, and another X bytes, then read a reply of length 2X, then you might be able to read a single combined "message", but as you've noticed, the underlying implementation of the streams can choose when to deliver chunks of bytes, so it might return X bytes, then X bytes, later, or 2X at once, or 0.5X four times...

Answer 2

InputStream and OutputStream are two completely separate streams. What you write into one has no a priori relation to what you read from the other. The InputStream gives you whatever data the server decides to send to you. I would also like to comment on this piece of your code:

sockOutput.write(msg.getBytes(), 0, test.length());
sockOutput.write("Hello StackOverFlow".getBytes(), 0, test.length());

You use the length of a string test (not shown in your code), which has nothing to do with the byte array you are passing as the first argument. This can cause an ArrayIndexOutOfBoundsException or truncation of your intended message.

Additional comments to your updated question

Reviewing your server-side code, it is not quite correctly written. You need to have try { handleConnection(...); } finally { socket.close(); } to ensure proper cleanup after an error, as well as when completing normally. Your code never closes anything on the server side.

Finally, and most critically, your entire code is written in a way that can result in a deadlock. Normally you need a separate thread to read and to write; otherwise the following may happen:

1. You attempt to write some data to the output;
2. The server reads it and tries to respond with data in your input;
3. But, since the buffers are too small, you don't manage to send everything because the server wants to first send something to you, then receive the rest; but you don't get to the receiving part before you have sent everything you've got.