Java/File Input Output/WritableByteChannel

Read all available bytes from one channel and copy them to the other

/*
 * Copyright (c) 2004 David Flanagan.  All rights reserved.
 * This code is from the book Java Examples in a Nutshell, 3nd Edition.
 * It is provided AS-IS, WITHOUT ANY WARRANTY either expressed or implied.
 * You may study, use, and modify it for any non-commercial purpose,
 * including teaching and use in open-source projects.
 * You may distribute it non-commercially as long as you retain this notice.
 * For a commercial use license, or to purchase the book, 
 * please visit http://www.davidflanagan.ru/javaexamples3.
 */
//package je3.nio;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.Channels;
import java.nio.channels.ReadableByteChannel;
import java.nio.channels.WritableByteChannel;
public class FileCopy3 {
  public static void main(String[] args) throws IOException {
    // Open file streams and get channels for them.
    ReadableByteChannel in = new FileInputStream(args[0]).getChannel();
    WritableByteChannel out;
    if (args.length > 1)
      out = new FileOutputStream(args[1]).getChannel();
    else
      out = Channels.newChannel(System.out);
    // Do the copy
    copy(in, out);
    // Exception handling and stream-closing code has been omitted.
  }
  // Read all available bytes from one channel and copy them to the other.
  public static void copy(ReadableByteChannel in, WritableByteChannel out) throws IOException {
    // First, we need a buffer to hold blocks of copied bytes.
    ByteBuffer buffer = ByteBuffer.allocateDirect(32 * 1024);
    // Now loop until no more bytes to read and the buffer is empty
    while (in.read(buffer) != -1 || buffer.position() > 0) {
      // The read() call leaves the buffer in "fill mode". To prepare
      // to write bytes from the bufferwe have to put it in "drain mode"
      // by flipping it: setting limit to position and position to zero
      buffer.flip();
      // Now write some or all of the bytes out to the output channel
      out.write(buffer);
      // Compact the buffer by discarding bytes that were written,
      // and shifting any remaining bytes. This method also
      // prepares the buffer for the next call to read() by setting the
      // position to the limit and the limit to the buffer capacity.
      buffer.rupact();
    }
  }
}