How to put the content of a ByteBuffer into an OutputStream?

Look at Channels.newChannel(OutputStream). It will give you a channel given an OutputStream. With the WritableByteChannel adapter you can provide the ByteBuffer which will write it to the OutputStream.

public void writeBuffer(ByteBuffer buffer, OutputStream stream) {
   WritableByteChannel channel = Channels.newChannel(stream);

   channel.write(buffer);
}

This should do the trick!