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Pure Java implementation of libzmq
There is a newer version: 0.6.0
/*
    Copyright (c) 2007-2014 Contributors as noted in the AUTHORS file

    This file is part of 0MQ.

    0MQ is free software; you can redistribute it and/or modify it under
    the terms of the GNU Lesser General Public License as published by
    the Free Software Foundation; either version 3 of the License, or
    (at your option) any later version.

    0MQ is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU Lesser General Public License for more details.

    You should have received a copy of the GNU Lesser General Public License
    along with this program.  If not, see .
*/

package zmq;

import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.channels.WritableByteChannel;

public interface Transfer
{
    public int transferTo(WritableByteChannel s) throws IOException;
    public int remaining();

    public static class ByteBufferTransfer implements Transfer
    {
        private ByteBuffer buf;

        public ByteBufferTransfer(ByteBuffer buf)
        {
            this.buf = buf;
        }

        @Override
        public final int transferTo(WritableByteChannel s) throws IOException
        {
            return s.write(buf);
        }

        @Override
        public final int remaining()
        {
            return buf.remaining();
        }
    }

    public static class FileChannelTransfer implements Transfer
    {
        private Transfer parent;
        private FileChannel channel;
        private long position;
        private long count;
        private int remaining;

        public FileChannelTransfer(ByteBuffer buf, FileChannel channel, long position, long count)
        {
            parent = new ByteBufferTransfer(buf);
            this.channel = channel;
            this.position = position;
            this.count = count;
            remaining = parent.remaining() + (int) this.count;
        }

        @Override
        public final int transferTo(WritableByteChannel s) throws IOException
        {
            int sent = 0;
            if (parent.remaining() > 0) {
                sent = parent.transferTo(s);
            }

            if (parent.remaining() == 0) {
                long fileSent = channel.transferTo(position, count, s);
                position += fileSent;
                count -= fileSent;
                sent += fileSent;
            }

            remaining -= sent;

            if (remaining == 0) {
                channel.close();
            }

            return sent;
        }

        @Override
        public final int remaining()
        {
            return remaining;
        }
    }
}