ByteBuffer returns null characters (character 0)

Question

I'm trying to implement a simple client-server application, using NIO.

As an exercise, communication should be text-based and line-oriented. But when the server reads the bytes sent by the client, it gets nothing, or rather, the buffer is filled with a bunch of zeroes.

I'm using a selector, and this is the code triggered, when the channel is readable.

private void handleRead() throws IOException {
    System.out.println("Handler Read");
    while (lineIndex < 0) {
        buffer.clear();
        switch (channel.read(buffer)) {
            case -1:
            // Close the connection.
            return;

            case 0:
            System.out.println("Nothing to read.");
            return;

            default:
            System.out.println("Converting to String...");
            buffer.flip();
            bufferToString();
            break;
        }
    }
    // Do something with the line read.
}

In this snippet, lineIndex is an int holding the index at which the first \n occurred, when reading. It is initialized with -1, meaning there's no \n present.

The variable buffer references a ByteBuffer, and channel represents a SocketChannel.

To keep it simple, without Charsets and whatnot, this is how bufferToString is coded:

private void bufferToString() {
    char c;
    System.out.println("-- Buffer to String --");
    for (int i = builder.length(); buffer.remaining() > 1; ++i) {
        c = buffer.getChar();
        builder.append(c);
        System.out.println("Appending: " + c + "(" + (int) c + ")");
        if (c == '\n' && lineIndex < 0) {
            System.out.println("Found a new-line character!");
            lineIndex = i;
        }
    }
}

The variable builder holds a reference to a StringBuilder.

I expected getChar to do a reasonable convertion, but all I get in my output is a bunch (corresponding to half of the buffer capacity) of

Appending: (0)

Terminated by a

Nothing to read.

Any clues of what may be the cause? I have similar code in the client which is also unable to properly read anything from the server.

---

If it is of any help, here is a sample of what the writing code looks like:

private void handleWrite() throws IOException {
    buffer.clear();
    String msg = "Some message\n";
    for (int i = 0; i < msg.length(); ++i) {
        buffer.putChar(msg.charAt(i));
    }
    channel.write(buffer);
}

I've also confirmed that the result from channel.write is greater than zero, reassuring that the bytes are indeed written and sent.

Answer 1

Turns out, this was a buffer indexing problem. In the server, a flip() was missing before writing to the socket. In the client code, a few flip() were missing too, after reading and before writing. Now everything works as expected.

Current writing code (server side):

private void handleWrite() throws IOException {
    String s = extractLine();
    for (int i = 0, len = s.length(); i < len;) {
        buffer.clear();
        while (buffer.remaining() > 1 && i < len) {
            buffer.putChar(s.charAt(i));
            ++i;
        }
        buffer.flip();
        channel.write(buffer);
    }
    // some other operations...
}