Deserialisation issue - java.io.StreamCorruptedException: invalid type code: 00

Question

I'm writing a java file-transfer app, and i have some troubles with deserialisation myself-defined class Message from Datagramms.

Other topics at StackOverflow has similar issues, but i didn't found something helpful from there, so i'm sorry in advance if i missed it.

so, class Message is:

import java.io.Serializable;

public class Message implements Serializable {

    private static final long serialVersionUID = 1L;

    private int segmentID;
    private byte[] packet;
    private int bytesToWrite;

    public Message(){
        segmentID = -1;
    }

    public Message(int segmentID, byte[] packet, int bytesToWrite) {
        this.segmentID = segmentID;
        this.packet = packet;
        this.bytesToWrite = bytesToWrite;
    }

    public int getBytesToWrite() {
        return bytesToWrite;
    }

    public int getSegmentID() {
        return segmentID;
    }

    public byte[] getPacket() {
        return packet;
    }
}

nothing special. i serialise it with

public byte[] serialize(Message obj) throws IOException
{
    ByteArrayOutputStream byteStream = new ByteArrayOutputStream();
    ObjectOutputStream objectStream = new ObjectOutputStream(byteStream);
    objectStream.flush();
    objectStream.writeObject(obj);
    objectStream.flush();
    return byteStream.toByteArray();
}

and, actually, send it through datagram

msg = new byte[512];
int bytesRead;
bytesRead = fileReader.read(msg);//fileReader is FileInputStream
Message message = new Message(segmentID, msg, bytesRead);
byte[] test = serialize(message);
datagramSocket.send(new DatagramPacket(test, test.length, 
    datagramSocket.getInetAddress(), datagramSocket.getPort()));

I recieve it from other place by

byte[] buffer = new byte[8192];
DatagramPacket receivedPacket = new DatagramPacket(buffer, buffer.length);
socket.recieve(recievedPacket); //it's already connected by init packet
receiveMSG = (Message) deserialize(receivedPacket.getData(), 
    receivedPacket.getOffset(), receivedPacket.getLength());

where (try/catch blocks are deleted here)

private Object deserialize(byte[] bytes, int offset, int length)
{
    ByteArrayInputStream byteStream = new ByteArrayInputStream(bytes, offset, length);
    ObjectInputStream objectStream = new ObjectInputStream(new BufferedInputStream(byteStream));
    Object tmpMsg = new Object();
        tmpMsg = objectStream.readObject();//here catches the EOFException
    return tmpMsg;
}

why it can be? i did it according instructions at http://goo.gl/Rq1rMm
where and what i did wrong? i checked that initial messages passes (i have them, but here is only a little piece of my code) and datagram in receive is not null. Moreover, sent and received datagrams has equal length. Thanks in advance.

Answer 1

receiveMSG = (Message) deserialize(receivedPacket.getData());

You need to change this to

receiveMSG = (Message) deserialize(receivedPacket.getData(), receivedPacket.getOffset(), receivedPacket.getLength);

and adjust your 'deserialise()' method accordingly, to accept 'offset' and 'length' parameters, and to use them when constructing the ByteArrayInputStream.

EDIT The code you posted doesn't compile: the expression that constructs the outgoing datagram packet is incorrect. It should be

new DatagramPacket(test, test.length, datagramSocket.getInetAddress(), datagramSocket.getPort())

I have no idea what packetOverhead is supposed to be, but you don't need it.

NB You don't need to create a new Message() the line before you call readObject().

Answer 2

Are you sure you have received all of the bytes for the serialized objects? A DatagramSocket uses UDP under the hood. Are you sure that all of the packets have arrived?

Also, is it really a good idea to use UDP for transferring a serialized object? This is one case where TCP would seem to be more approriate. UDP does not guarantee delivery or delivery order which could easily cause a corrupted stream.

You should also really put a private static final long serialVersionUID = 1L; on your Message class.