Java NIO. SocketChannel.read method all time return 0. Why?

Question

I try understand how works java NIO. In particular, how works SocketChannel.

I wrote code below:

import java.io.*;
import java.net.*;
import java.nio.*;
import java.nio.channels.*;

public class Test {

    public static void main(String[] args) throws IOException {

        SocketChannel socketChannel = SocketChannel.open();
        socketChannel.configureBlocking(false);
        socketChannel.connect(new InetSocketAddress("google.com", 80));

        while (!socketChannel.finishConnect()) {
            // wait, or do something else...
        }

        String newData = "Some String...";

        ByteBuffer buf = ByteBuffer.allocate(48);
        buf.clear();
        buf.put(newData.getBytes());

        buf.flip();

        while (buf.hasRemaining()) {
            System.out.println(socketChannel.write(buf));
        }

        buf.clear().flip();

        int bytesRead;

        while ((bytesRead = socketChannel.read(buf)) != -1) {

            System.out.println(bytesRead);

        }

    }

}

I try connect to google server.
Send request to the server;
Read answer from the server.

but, method socketChannel.read(buf) all time return 0 and performs infinitely.

Where I made mistake??

score 6 · Accepted Answer · edited Mar 22 '18 at 20:11

Because NIO SocektChannel will not block until the data is available for read. i.e, Non Blocking Channel can return 0 on read() operation.

That is why while using NIO you should be using java.nio.channels.Selector which gives you read notification on channel if the data is available.

On the otherhand, blocking channel will wait till the data is available and return how much data is available. i.e, Blocking channel will never return 0 on read() operation.

You can read more about NIO here:

score 5 · Answer 2 · answered Jul 12 '12 at 12:21

5

You have specifically configured

socketChannel.configureBlocking(false);

If you left the default which is blocking then you would never get a length of 0 returned.

answered Jul 12 '12 at 12:21

Peter Lawrey

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Similarly you have initiated a non-blocking connect and then spun until it completes. There's a strong flavor here of trying to write blocking-mode code in non-blocking mode. – user207421 Jul 12 '12 at 12:48
there is an "endless" loop - so it doesn't matter if the channel is blocking at all. It will eat up CPU but it would read properly if there was any remaining room left in the bytebuffer. – bestsss Jul 12 '12 at 13:34

score 2 · Answer 3 · answered Jul 12 '12 at 13:32

2

While reading the title I was about the say: b/c there is no room in the buffer:

Unlike the answers so far the real culprit is buf.clear().flip(); this effectively sets the buffer like that position=limit=0 - hence no data read - change to buf.clear() and you are good to go.

Have fun w/ the NIO and use debugger before posting a question.

answered Jul 12 '12 at 13:32

bestsss

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`buffer.clear() or `buffer.compact()` are sufficient. `buffer.flip()` has a different purpose, to put the buffer into a state for getting data out of it. – user207421 Jul 12 '12 at 23:47
unless noticed, the line is part of the code - which makes the code return '0' into an endless loop. I am perfectly aware how (not) to use flip(), it is effectively limit=position, position=0) – bestsss Jul 12 '12 at 23:59
Thanks @bestsss for this. In some subtle way I run into this "no room in the buffer" issue and was puzzled why I receive 0 byte reads for awhile. – TFuto May 06 '20 at 20:16

score 0 · Answer 4 · answered Jul 12 '12 at 14:22

0

If you really want to use non-blocking I/O, then either use selector, or use nio2 (AsynchronousSocketChannel et al.) from Java 7. Using selector is tricky, better find ready working examples. Using nio2 is relatively easy.

answered Jul 12 '12 at 14:22

Alexei Kaigorodov

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Non-blocking and asynchronous I/O are two different things. Async is still blocking, it just doesn't block *you*. For example it never returns zero from read. – user207421 Nov 29 '17 at 02:41

score -1 · Answer 5 · answered Aug 08 '15 at 19:20

-1

The technical explanation is that you didn't send a complete HTTP request to the google webserver, therefore it won't send you a reply until it receives a complete request.

This explains why you are reading 0 bytes which means that no data is available for reading.

answered Aug 08 '15 at 19:20

user1187372

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9

You have no evidence whatsoever about what he sent. The read could return zero any time, not just because of an incomplete request. – user207421 Nov 29 '17 at 02:42

Java NIO. SocketChannel.read method all time return 0. Why?

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