[NIO] custom messages from the netwerk

Question

Hi you all,
Yes this is another NIO question :)
Ok I have the following situation, I've got a NIO server which reads custom messages from the netwerk. These messages consists of an interger and then a variable byte array.
int -> length of the message
byte array -> data of message (could be anything)
On my local network it works fine because I don't have to worry about my read() not reading all the bytes the client send. So I can just use the ByteBuffer.getInt() to read the int and create a buffer for the byte array etc.
But... on the intenet there is no garentee all the bytes are read. So if only the first 3 bytes of the whole message are in my read buffer and I call the getInt() I'll get an exception.
How would you fix this? The only garentee is that I will always have 1 byte (except when a connection is closed or other error)
I could create of small bytebuffer of 4 bytes and wait till it's full and then read the int, but what if I also have a length that can consist out of like 5 or 6 bytes... and I don't know by forehand if it's 4 or 5 or 6... or just 2...
Any idear would be welcome... or even code if you have the time...
Grtz
David

Answer 1

Use ByteBuffer[] { lengthField, dataField } as a structure for messages. Set the size of the length field statically as part of the protocol. Set dataField's limit to 0. Do readsinto this structure (there is an appropriate read method for an array of ByteBuffers) until there lengthField.remaining() == 0. At this point, read the int/long/whatever from lengthField and set dataField's limit to that. Then do the reads necessary until dataField.remaining() == 0. At this point, you'll have your whole message.
The reason that I say "do reads" is that on any particular read you may only get one byte's worth of data. Therefore you have to hold this partially completed message in some variable, or as an attachment to the SelectionKey which was selected. As the Channel continually gets selected for OP_READ, you can do the above steps, and if there's not enough left in the channel, you just leave the message partially completed for the next OP_READ. Once it's complete, you do what you need to with it, and replace the attachment or variable with an empty ByteBuffer[] of the above format.
--Dan