Serialization problem: IOException in writeObject()

[John Cebedo]

Hi, I am in the middle of a networking project. The project is a pair of programs: a Server and a Client. The Client will ask the Server for some file and the Server will try to send the requested file to the Client.

My problem is that I keep getting an IOException when trying to use the ObjectOutputStream's writeObject() method. This happens when I am trying to send packets to the Client. In my project, a packet is represented by an instance of a class I made called Frame which implements Serializable.

This simple block of code generates the error:

try {
    out.writeObject(frame);
} catch (IOException ex) {
    System.err.println("Could not send the frame");
    System.exit(1);
}

Any hints on what I am doing wrong?

[Jelle Klap]

Could you please provide the stack trace and any and all code snippets involving the ObjectOuputStream (initialization and usage).

[Rob Spoor]

Can you also tell us some more about class Frame:
1) what are its non-static non-transient fields?
2) what is its super class?
3) is that class also Serializable?
4a) if so, goto step 1 for the super class
4b) if not, does that class have a constructor without parameters?

For a class to be serializable, all non-static non-transient fields have to be primitives or Serializable themselves. Any object being stored in such a field, like in arrays, Collections or models, have to be primitive or Serializable too.
The same holds for the super class, unless there is a constructor without parameters.

[Jelle Klap]

Shoudn't all those oversights result in either an InvalidClassException or a NotSerializableException though?
[ April 08, 2008: Message edited by: Jelle Klap ]

[John Cebedo]

Here is the stack trace:

Exception in thread "main" java.net.SocketException: Connection reset
        at java.net.SocketInputStream.read(Unknown Source)
        at sun.nio.cs.StreamDecoder.readBytes(Unknown Source)
        at sun.nio.cs.StreamDecoder.implRead(Unknown Source)
        at sun.nio.cs.StreamDecoder.read(Unknown Source)
        at java.io.InputStreamReader.read(Unknown Source)
        at java.io.BufferedReader.fill(Unknown Source)
        at java.io.BufferedReader.readLine(Unknown Source)
        at java.io.BufferedReader.readLine(Unknown Source)
        at Server.main(Server.java:92)

Here is my Frame class:

import java.io.Serializable;
 
public class Frame implements Serializable {
    public Frame(int seqNum, byte[] payload) {
        this.seqNum = seqNum;
        this.payload = payload;
    }
    
    public int getSeqNum() {
        return seqNum;
    }
    
    public byte[] getPayload() {
        return payload;
    }
    
    int seqNum;
    byte[] payload;
}

[Jelle Klap]

Looks like a call to the Socket's InputStream.read() method is being attempted after the connection has been broken by the remote party.
Not so much a serialization problem as far as I can tell.
In any case the Frame class is perfectly serializable.

[John Cebedo]

Oops, I didn't copy the stack trace correctly. It should look like this:

Exception in thread "main" java.net.SocketException: Connection reset
        at java.net.SocketInputStream.read(Unknown Source)
        at sun.nio.cs.StreamDecoder.readBytes(Unknown Source)
        at sun.nio.cs.StreamDecoder.implRead(Unknown Source)
        at sun.nio.cs.StreamDecoder.read(Unknown Source)
        at java.io.InputStreamReader.read(Unknown Source)
        at java.io.BufferedReader.fill(Unknown Source)
        at java.io.BufferedReader.readLine(Unknown Source)
        at java.io.BufferedReader.readLine(Unknown Source)
        at Server.main(Server.java:92)
Could not send the frame

After adding the line

ex.printStackTrace();

inside the catch block, I get this additional stack trace:

java.net.SocketException: Connection reset by peer: socket write error
        at java.net.SocketOutputStream.socketWrite0(Native Method)
        at java.net.SocketOutputStream.socketWrite(Unknown Source)
        at java.net.SocketOutputStream.write(Unknown Source)
        at java.io.ObjectOutputStream$BlockDataOutputStream.drain(Unknown Source)
        at java.io.ObjectOutputStream$BlockDataOutputStream.setBlockDataMode(Unknown Source)
        at java.io.ObjectOutputStream.writeNonProxyDesc(Unknown Source)
        at java.io.ObjectOutputStream.writeClassDesc(Unknown Source)
        at java.io.ObjectOutputStream.writeOrdinaryObject(Unknown Source)
        at java.io.ObjectOutputStream.writeObject0(Unknown Source)
        at java.io.ObjectOutputStream.writeFatalException(Unknown Source)
        at java.io.ObjectOutputStream.writeObject(Unknown Source)
        at SendFrame.run(SendFrame.java:15)

[John Cebedo]

Since it is now a socket problem, here are the lines of code involving ObjectOutputStream:

ObjectOutputStream out = new ObjectOutputStream(clientSocket.getOutputStream());

if (!file.exists())
    out.writeObject("NO SUCH FILE EXISTS!");
else {
    out.writeObject("SUCCESS!");

SendFrame sf = new SendFrame(frame, out, timeout);
sf.start();

And here is the SendFrame class:

import java.io.IOException;
import java.io.ObjectOutputStream;
        
        
public class SendFrame extends Thread {
    public SendFrame(Frame frame, ObjectOutputStream out, double timeout) {
        this.frame = frame;
        this.out = out;
        this.timeout = timeout;
    }
    
    public void run() {
        while (true) {
            try {
                out.writeObject(frame);
            } catch (IOException ex) {
                System.err.println("Could not send the frame");
                ex.printStackTrace();
                System.exit(1);
            }
            
            try {
                Thread.sleep((int) timeout * 1000);
            } catch (InterruptedException ex) {
                break;
            }
        }
    }
    
    Frame frame;
    ObjectOutputStream out;
    double timeout;
}

[Rob Spoor]

Is this the only writing code? Could you also show us the reading code?

[John Cebedo]

Code involving the BufferedReader:

BufferedReader in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
        
String fullPath = dirName + in.readLine();

My entire algorithm attempting to simulate the Stop-and-Wait scheme:

if (!file.exists())
    out.writeObject("NO SUCH FILE EXISTS!");
else {
    out.writeObject("SUCCESS!");
 
    // start sending data
    FileInputStream fstream = new FileInputStream(file);
            
    int i = 0;
 
    // start reading in bytes
    while ((c = fstream.read()) != -1) {
        payload[i] = (byte) c;
 
        // if max payload size is reached, send it out
        if (i == (frameSize - 1)) {
            Frame frame = new Frame(seqNum, payload);
                    
            // start the thread to send the frame, will keep trying to send the frame...
            SendFrame sf = new SendFrame(frame, out, timeout);
            sf.start();
 
            // when ACK is received, interrupt thread
            String ack = in.readLine();
            if (ack.compareTo("RECEIVED!") == 0)
                sf.interrupt();
                    
            payload = new byte[frameSize];
            i = 0;
            seqNum++;
            continue;
        }
                
        i++;
    }
            
    fstream.close();
}

EDIT: added comments in the code explaining how I'm sending the frames
[ April 08, 2008: Message edited by: John Cebedo ]