How do you get successive slices out of a ByteBuffer?

Question

I have a ByteBuffer that contains a large file (100 MB):

    java.nio.ByteBuffer byteBuffer = ByteBuffer.wrap(multipartFile.getBytes());
    writeChannel.write(byteBuffer);
    writeChannel.closeFinally();

I can only legally write 1 MB to the writeChannel at a time.

How do I slice up the contents of the ByteBuffer and write only a 1 MB slice at a time into the writeChannel?

Why? Just write the buffer and let the system decide how to packetize it or whatever you are trying to do. — user207421, Jul 05 '11 at 00:41

score 11 · Accepted Answer · answered Jul 04 '11 at 23:03

You can use ByteBuffer#slice() to get a duplicate view of your base ByteBuffer instance, then bump the position along to expose a sliding window of content. Alternately, you can just do the same to your base buffer if you don't need to expose it to any other consumers.

You can change the starting position of your view of the content via the single-argument Buffer#position(int) method, and change the end position of your view via Buffer#limit(int). So long as you're mindful not to push the view beyond the limit of the underlying buffer, you can do the following:

final ByteBuffer view = base.slice();
for (int start = base.position(), end = base.limit(), stride = 1000000;
     start != end;
     start = view.limit())
  consume(view.position(start)
              .limit(start + Math.min(end - start, stride)));

I didn't test it, but it looks correct. It's possible to rewrite to avoid the initial setting of the position, which isn't strictly necessary here, but it incurs either some repetition or more awkward special case treatment of the first time through.

I left it this way to preserve the basic for loop structure.

james_bond · Answer 2 · 2011-07-04T23:10:37.283

As far as I know, write() on the writeChannel (which for the name I supposed is of type SocketChannel) will "attempt to write up to r bytes to the channel, where r is the number of bytes remaining in the buffer, that is, dst.remaining(), at the moment this method is invoked". (according to this)

The description of ByteBuffer.remaining() says that this method will "Return the number of elements between the current position and the limit."

So my guessing is not that you can't write the entire ByteBuffer, but that your code should call flip() on the ByteBuffer object so it become:

java.nio.ByteBuffer byteBuffer = ByteBuffer.wrap(multipartFile.getBytes());
byteBuffer.flip();
writeChannel.write(byteBuffer);
writeChannel.closeFinally();

As its said in Buffer.flip(): "After a sequence of channel-read or put operations, invoke this method to prepare for a sequence of channel-write or relative get operations"

How do you get successive slices out of a ByteBuffer?

2 Answers2