Java NIO: transferFrom until end of stream

Question

I'm playing around with the NIO library. I'm attempting to listen for a connection on port 8888 and once a connection is accepted, dump everything from that channel to somefile.

I know how to do it with ByteBuffers, but I'd like to get it working with the allegedly super efficient FileChannel.transferFrom.

This is what I got:

ServerSocketChannel ssChannel = ServerSocketChannel.open();
ssChannel.socket().bind(new InetSocketAddress(8888));

SocketChannel sChannel = ssChannel.accept();
FileChannel out = new FileOutputStream("somefile").getChannel();

while (... sChannel has not reached the end of the stream ...)     <-- what to put here?
    out.transferFrom(sChannel, out.position(), BUF_SIZE);

out.close();

So, my question is: How do I express "transferFrom some channel until end-of-stream is reached"?

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^{Edit: Changed 1024 to BUF_SIZE, since the size of the buffer used, is irrelevant for the question.}

Answer 1

There are few ways to handle the case. Some background info how trasnferTo/From is implemented internally and when it can be superior.

• 1st and foremost you should know how many bytes you have to xfer, i.e. use FileChannel.size() to determine the max available and sum the result. The case refers to FileChannel.trasnferTo(socketChanel)
• The method does not return -1
• The method is emulated on Windows. Windows doesn't have an API function to xfer from filedescriptor to socket, it does have one (two) to xfer from the file designated by name - but that's incompatible with java API.
• On Linux the standard sendfile (or sendfile64) is used, on Solaris it's called sendfilev64.

in short for (long xferBytes=0; startPos + xferBytes<fchannel.size();) doXfer() will work for transfer from file -> socket. There is no OS function that transfers from socket to file (which the OP is interested in). Since the socket data is not int he OS cache it can't be done so effectively, it's emulated. The best way to implement the copy is via standard loop using a polled direct ByteBuffer sized with the socket read buffer. Since I use only non-blocking IO that involves a selector as well.

That being said: I'd like to get it working with the allegedly super efficient "? - it is not efficient and it's emulated on all OSes, hence it will end up the transfer when the socket is closed gracefully or not. The function will not even throw the inherited IOException, provided there was ANY transfer (If the socket was readable and open).

I hope the answer is clear: the only interesting use of File.transferFrom happens when the source is a file. The most efficient (and interesting case) is file->socket and file->file is implemented via filechanel.map/unmap(!!).