Basically I am doing some networking with a client and server sending "packets" back and forth to each other. I have it working with basic variable data such as ints or strings passing back and forth, however now I want to pass an object.
So I know I have to serialize the data of the object to pass it through the socket. That is working as well (as I can get the correct information if I serialize then de-serialize right away) but the problem comes in when my server receives a packet.
My server interprets packet data based on the first 2 characters of the packet. So 01foobar is a type of packet correlating to whatever "01" is assigned to and 02foobar is a different packet as well. So I don't know the best way to do this with an object attached. What is mean is this...
The way I have tried to do it right now is, serialize my object and get it's string. Then append on 03 to the front. So basically I have a string that looks like 03[B#3e9513b7 (or whatever) then do getBytes() on that string which gives me another byte[] (so I can send it through the socket). Then when the server receives that information, I can append the 03 off and I'm left with just [B#3e9513b7. The problem is, [B#3e9513b7 is now a string, and not a byte[] and in order to deserialize I need to send it the same byte[] as it gave me when it serialized that data. So that got me looking into a way to make [B#3e9513b7 BE the byte[] (aka, so when I do toString() on that new byte[] it returns [B#3e9513b7) but was having issues assigning it like that because it would give me a new byte[] for [B#3e9513b7 as a string. So obviously then, when I send it to be deserialized it has a byte[] that it doesn't know what to do with and throws an error.
So I have to imagine there's a better way to do this, and I'm just making things more complicated than they should be. Any recommendations? I can provide code snippets if needed.
Thanks guys!
Edit: I guess I should mention that I am using Java with using UDP sockets.
If you are looking for a reliable and efficient solution for client-server communication, I would suggest to look at Netty.
Regarding how to serialize/deserialize your objects, you have many choices as Java serialization, XML, JSON ...
You would have to pass your serialized objects in UDP datagrams. However, be aware that UDP datagram size is limited. If you're exchanging big objects, you may want to switch to TCP transport which is more reliable.
You may also want to look at SOAP/REST web services.
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I have a server which receives some object from a client and sends back that object to client later.
the problem is that the server doesn't necessarily have the class of that object so serializing that object in client and sending it to server and reading it with ObjectInputStream will throw ClassNotFoundException.
I thought about reading the serialized object as string with DataInputStream and store the resulting string in server but it did't seem like the right approach.
here's the code which serialize and send object via socket in client(session.dos is the socket DataOutputStream)
ObjectOutputStream oos = new ObjectOutputStream(Connector.session.dos);
oos.writeObject(Connector.session.game);
what can I do to read the unknown object in server?
or is serializing the wrong approach altogether? then what should i do?
If you're up for a different and exciting approach, you could do worse than use Apache Avro (there's bound to be a Java variant of it). The reason why is that the wire format in Avro has the original IDL schema (which is actually JSON in Avro's case) in it. This means that the recipient doesn't necessarily need to have ever had sight of the schema to receive serialised data and interpret data. So you could receive Avro serialised classes as a byte array, store them and ignore the content, but just in case you ever changed your mind the data can be interpretted.
If that is of interest, there's a getting started page here.
I am new to Socket programming, and I am trying to write a simple chat application.My server utilizes a HashSet of Strings to store user name information and I want to pass the HashSet from the server to the client. I was able to do this before with ObjectOutputStream and ObjectInputStream over regular Sockets. However, now I am using SocketChannel and it does not allow this I get the following exception:
java.nio.channels.IllegalBlockingModeException
So i searched the web on how to do this and everyone says something different, and most go way over my head. Is there a simple way to do this without completely overriding the writeObject and readObject methods myself?
Thank You in advance.
I recommend you either don't use non-blocking mode or don't use serialization.
If you must, you need to serialize to a ByteArrayOutputStream, get the length, send the length, send the bytes, receive the length, keep reading, accumulating data until you have the correct length of data, put it into a ByteArrayInputStream, and deserialize from that. Not easy, especially the hand-waving around 'keep reading'. And it wastes both time and space.
i'm trying to learn more about networking so i decided to do a small project using sockets. I have already been successful is creating an echo server which i know is pretty basic.
The next step would be to send all kinds of files (text files, images maybe even videos?) from the server to the client and vice versa. I have been trying to do so with FIleOutputStream, ObjectOutputStream (and input obviously) and i even tried Byte Buffer but by reading the Java docs i'm starting to understand that i really dont know much about Data....
so my questions are:
Can I convert all type of data (text, images, videos) into bytes, send it and then convert it all into the data that it used to be, or do i need a specific function for each type of data?
Can I convert a text file or an image into an Object and then send it with ObjectOutputFile? If so then how can i decompress it in the other side? I've tried ObjectOutputFile.readObject but i dont really know what to do after that.
Here's what I've been trying to do (I won't post all the code because, only this particular part is important i believe:
Server: (out is a DataOutputStream)
ObjectOutputStream oos = new ObjectOutputStream(out);
Path path = Paths.get("stuff/Folder.jpg");
byte[] data = Files.readAllBytes(path);
oos.write(data);
client (in is a DataInputStream):
ObjectInputStream ois = new ObjectInputStream(in);
ois.readObject();
Thanks for all the help in advance, if you think that you know an article or a tutorial that would help me learn more about this it will also help!
Can I convert all type of data (text, images, videos) into bytes, send it and then convert it all into the data that it used to be?
Sure.
Or do I need a specific function for each type of data?
That's not an "or" - if you want to convert several types of data into bytes yourself, then you probably need a separate function for each type. Note that if you are sending files, you don't need any conversion because the content of a file is already just bytes.
Can I convert a text file or an image into an Object and then send it with ObjectOutputFile?
You could send it with ObjectOutputStream.
If so then how can i decompress it in the other side?
With ObjectInputStream.
I've tried ObjectOutputFile.readObject but i dont really know what to do after that.`
There is no such method in Java as ObjectOutputFile.readObject.
it is pretty certain that you can convert any data into bytes. However, the problem is how you could convert it back? Typically, we use Base64 encoder to encode image and when clients get that we can convert it back by Base64 decoder. There is a lot of way to do this. Please Google it.
Yes, you can use Object data to send. You may need to define a new class for the object and then call data back by getter. In client side, you need to declare the Object value as ois.readObject(). Like:
Object object = ois.readObject();
Just add a little bit advice, I feel you are not so familiar with socket right now. So, a text book like Computer Networking(Kurose Ross) will be helpful. Also, if you plan to transmit data between C and Java, then you cannot use Object because C does not support it natively.
I am using TCP Sockets and I am a beginner in Java and Sockets too. The scenario is that the client,depending on server 's answer, may send either int or a string. I need to save the output stream of the client, in a variable on the server 's side, so i can use it in if statements and so on. But how could I do this when i don t know if the stream sent is an integer or a string?
I have made a very simple example, because my code is huge and messy and i don t want to make it more complex
Client Side:
serverSentence=inFromServer.readLine();
if (serverSentence.equals("Hello"))
{
anInt=readUserInput.nextInt();
outToServer.write(anInt);
}
else
{
outToServer.writeBytes("Hello word!"+'\n');
}
So how I could do this, I mean saving in a variable the Client 's output stream, in the Server 's side (or the opossite), so I could use it in loops and ifs?
Use Object. A reference typed as an Object may refer to Integer or String. Another story is of course deserializing the incoming data as either. If the problem has to do with not knowing how to interpret serialized data, include some sort of flag to indicate what the data is.
You should design your protocol of communication between server and client in such a way that server would be able to read some information (you could call it standard header), and based on that would know how to interpret the remaining bytes sent by the client.
And then you'll know how to read the data from client, and where to store it.
I need to write an UDP server which will wait for packets from uncorrelated devices (max 10000 of them) sending small packets periodically; do some processing with the payload and write the results on SQL. Now I'm done with the SQL part through jdbc, but the payload bytes keep bugging me, how should I access them? Until now I've worked with the payload mapped to a string and then converting the string to hex (two hex chars representing one byte). I'm aware that there's a better way to do this but I don't know it...
Do you not just want to create a DatagramSocket and receive DatagramPackets on it?
You need to specify a maximum length of packet by virtue of the buffer you use to create it, but then you'll be able to find out how much data was actually sent in the packet using getLength().
See the Java Tutorial for more details and an example.