while (in.read(buf) >= 0 || buf.position != 0) {
     buf.flip();
     out.write(buf);
     buf.compact();    // In case of partial write
}

In my application, the throughput increased by 6% by replacing compact() with this method.

public static void compact(ByteBuffer bb) {
    if (bb.remaining() == 0)
        bb.clear();
    else
        bb.compact();
}

Note: this is not the increase in a micro-benchmark, but across the entire application!

Your Mileage May Vary of course