Behavior of Java sockets when closing output stream

Question

Can someone explain the following behavior in Java sockets:

The general idea is this:

Open socket, Obtain I/O streams.
Write request, Close out stream
Read Response, Close in stream
Close socket.

Here's my question / issue.

If I use a PrintWriter for output, and then close it, It closes the whole socket, and the subsequent read operation fails miserably.

Instead if I directly use the socket's shutdownOutput() method, it correctly closes the output stream channel, while keeping the socket alive.

Why would closing the PrintWriter object take the whole socket down with it?

have you tried flushing with .flush() on your connections before closing your printwriter to see if the problem doesnt happen? — vcetinick, Jan 17 '12 at 05:59
@vcetinick That makes no difference. `close()` calls `flush()`. — user207421, Aug 23 '23 at 08:14

score 34 · Accepted Answer · edited Oct 24 '16 at 13:02

This may be what your code looks like:

Socket socket;
PrintWriter pw = new PrintWriter(socket.getOutputStream());
pw.close();

Now, let's have a look at the description of getOutputStream() method of Socket.

getOutputStream

public OutputStream getOutputStream() throws IOException

Returns an output stream for this socket. If this socket has an associated channel then the resulting output stream delegates all of its operations to the channel. If the channel is in non-blocking mode then the output stream's write operations will throw an IllegalBlockingModeException.

Closing the returned OutputStream will close the associated socket.

Returns:

an output stream for writing bytes to this socket.

Throws:

IOException - if an I/O error occurs when creating the output stream or if the socket is not connected.

from the description above, we know closing the returned OutputStream will close the associated socket.

Now, when you close the PrintWriter, it'll close the associated OutputStream which will close the associated socket.

RTFM eh? :) I guess that explains it thoroughly.Thanks Bloodwolf! — Lenny Markus, Jan 17 '12 at 14:49
@Alanmars how to avoid such situation. What if I want to keep the socket open even after closing the OutputStream. Is there any way to do this? — vishal munde, Jun 13 '16 at 10:08
I'm still missing the answer to `why`. "Why would closing the PrintWriter object take the whole socket down with it?" — AlikElzin-kilaka, Jul 17 '18 at 08:01

score 3 · Answer 2 · edited Oct 24 '16 at 12:40

It is probably because calling the close() method of PrintWriter is tracing back through the hierarchy and calling the close() method of the SocketOutputStream as well. As part of the close() method for the SocketOutputStream it also calls the close() method for the Socket as well, which would in term close the SocketInputStream as well. Calling the shutdownOutput() function instead sends any previously written data followed by TCP's normal connection termination sequence. It then disables the output stream.

Behavior of Java sockets when closing output stream

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