Correctly casting serialised objects

Wulfmann · 2013-09-17T19:33:50

Hi guys, I am trying to implement a network architecture in java for a small project etc., however I am unsure of what the design pattern/best-practice is for correctly identifying objects, which are coming through as bytes via DatagramPackets, so that I can then make the appropriate cast where necessary and reconstruct them. The objects that are being communicated are various messages which will make up the client/server communication protocol. I managed to dig up an old thread discussing this same issue and the agreed solution seems to be a dispatcher. Is this the best way of going about this? I decided to give it a try as it seems like a decent solution. My message classes implement the Externalizable interface and I have roughly followed the design discussed in said old thread below: http://www.gamedev.net/topic/634511-concept-of-a-basic-custom-packet-in-use-for-sending-data-to-server-and-client-back-and-forth/ In particular the last 2 posts (really 1 post but with the second-last post quoted): I was worried that I have to write a few packets entirely in bytes. What a relief. I don't think that's a bad thing. You can model the contents of packets as structs/classes, and then actually generate the reading/writing to/from byte arrays as functions, perhaps on a general interface that each of your messages implement. reading/writing fields of a message isn't that hard to do manually, and gives you great control over how your protocol grows. class WhisperMessage extends Message { public int toPlayer; public string text; private static const int code = 10; // should not conflict with any other message class private static const registration = new MessageRegistration<WhisperMessage>(code); // for finding class on receipt public WhisperMessage(int p, string t) { super(code); toPlyaer = p; text = t; } protected void writeData(ByteStream strm) { strm.writeInt(toPlayer); strm.writeString(text); } protected void readData(ByteStream strm) { toPlayer = strm.readInt(); text = strm.readString(); } }; Class Message would have public read/write functions that take care of also adding the type code and length fields to the output stream, as well as static functions to read message instances from a byte stream (you need to register the code with some dipatcher). Also, please apologize if my Java is not quite syntactically correct; it's been several years since I last wrote any. Better yet, make the Message class realize the Externalizable interface, which provide those public read and write methods and is seamless pluggable to Java's default serialization framework.http://docs.oracle.com/javase/7/docs/api/java/io/Externalizable.html I am however confused as to how I would implement the dispatcher (and to a degree the Message superclass - what does the constructor do with the code?) described in the second last post (I consider myself somewhat a beginner, especially with serialisation) and was hoping someone could elaborate? tl;dr: What is the best (or at least an elegant) way of correctly identifying and reconstructing serialised objects? How would one link up the Message implementation, described in the post above, to a dispatcher? Cheers, Wulf

Networking and Multiplayer Programming

Started by Wulfmann September 14, 2013 04:03 AM

11 comments, last by hplus0603 10 years, 7 months ago

TheChubu

9,484

September 17, 2013 09:51 AM

You could do, say:

obj.getClass().cast(obj);

Dunno what would accomplish but it can be done (and I find it rather funny :D )

In any case, here is an IBM article about Java serialization, might help http://www.ibm.com/developerworks/library/j-5things1/

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Wulfmann

115

Author

September 17, 2013 11:28 AM

Thanks for all the replies and a great discussion! I ended up implementing a Factory class as smr and Bacterius have suggested and believe it is a suitable solution for now. I am trying to hack up the network layer reasonably quickly so I can begin to implement the more exciting details of the game, but at the same time I want it to be a reasonable solution that follows some common conventions.

This is why I have opted to use the Externalizable interface rather than retain full control over the data.

Also, thank you for your explanation of the Dispatcher Sprangle, I will most likely end up implementing something of this sort when I move onto message handling.

hplus0603

11,917

September 17, 2013 07:33 PM

You should never go through pain Object IMO. If you manufacture objects from a factory, make them derive from IFactoryOutput. Make the interface have whatever polymorphic functionality you need, such as "handleIncomingMessage" or whatever.

switch() is a signal that you're doing something wrong.

enum Bool { True, False, FileNotFound };

Correctly casting serialised objects

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Correctly casting serialised objects

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