Just to wrap this up, here's what I found:
You really have to know how big the header message is, because if you
simply wrap an input stream around the entire concatenated buffer, the
deserialization doesn't know where the end of the header is. This strikes
me as a shortcoming of protobuf, but it is what it is. I used code
something like(in java):
protected static byte[] makePacket(Message header, Message message) throws
IOException {
// We cannot simply concatenate the two messages; protobuf will fail to
deserialize them.
// So we add the header length at the front. But we don't know it yet...
ByteArrayOutputStream os = new ByteArrayOutputStream();
os.write(new byte[4]); // we will update this later
CodedOutputStream cos = CodedOutputStream.newInstance(os);
header.writeTo(cos);
cos.flush();
// NOW we know the header length
int headerLength = os.size() - 4;
message.writeTo(cos);
cos.flush();
byte[] result = os.toByteArray();
// Set header size
ByteBuffer bb = ByteBuffer.wrap(result);
bb.order(ByteOrder.LITTLE_ENDIAN);
bb.putInt(0, headerLength);
return result;
}
Then for deserialization:
protected static Message getResponseMessage(MessageFactory messageFactory,
byte[] packet) throws Exception {
ByteBuffer bb = ByteBuffer.wrap(packet);
bb.order(ByteOrder.LITTLE_ENDIAN);
int headerLength = bb.getInt(0);
CommonWrapper.ResponseHeader header;
{
InputStream is = new ByteArrayInputStream(packet, 4, headerLength);
CodedInputStream cis = CodedInputStream.newInstance(is);
header = CommonWrapper.ResponseHeader.parseFrom(cis);
}
InputStream is = new ByteArrayInputStream(packet, 4 + headerLength,
packet.length - (4 + headerLength));
return messageFactory.createMessage(header.getResponseMessageType(), is);
}
The MessageFactory is a hand-rolled class for creating messages from full
name; but that's for a different thread.
john
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