ObjectOutputStream and Sockets.

[Nina Foley]

Hi, I'm new to Java and I'm trying to put together a client side program which will send a message (string) to a server side application, and depending on the content of the message, the application will perform an appropriate action. Using some example code from the net, I managed to put together a client and server program in java that worked.

However when I change the IP Address and socket to point to the real server side application, I get an invalid stream header error. I know the application receives the connection at least as it displays a brief message something like "Winsock1 Connection from <IP Address>" but then instantly goes to "Winsock1 <IP Address> disconnected". I tried using the 'HelloDevice' from UniversalComm (really useful program) to see if I could determine what was being passed, and it does show the message, but the message is preceded with ....t.. (byte code AC ED 00 05 74 00 0F).

I've read some articles that indicate that this may be to do with the encoding and the way the message is passed across, but I'm not really understanding it. Can someone please point me in the right direction with this? This is the code I'm using:-

public class MYClient {
   public static void main(String[] args) throws
       UnknownHostException, IOException, ClassNotFoundException, InterruptedException {
         //get the localhost IP address.
         InetAddress host = InetAddress.getLocalHost();

         //pass in the message
         String msgText = args[0];

         //initialise sockets & IO
         Socket socket = null;
         ObjectOutputStream oos = null;
         ObjectInputStream ois = null;

         //establish socket connection to server
         socket = new Socket(host.getHostName(), 1197);

         //write to socket using ObjectOutputStream
         oos = new ObjectOutputStream(socket.getOutputStream());

         //inform user of request send
         System.out.println("Sending request to Server");

         //send message
         oos.writeObject(msgText);
         oos.flush();

         //read the server response message
         ois = new ObjectInputStream(socket.getInputStream());
         String message = (String) ois.readObject();

         System.out.println(message);

        //close resources
        ois.close();
        oos.close();

        Thread.sleep(100);
   }
}

[Campbell Ritchie]

Welcome to the Ranch

I have added code tags and indentation to your code and new line characters so the lines are not too long. Always do that: doesn't it look better.
Don't know much about sockets myself. Are you using Java7/8? If so you should probably use try with resources

try (
   Socket socket = ...
   ObjectOutputStream oos = new ObjectOutputStream(socket.getOutputStream());
   ObjectInputStream ois = new ObjectInputStream(socket.getInputStream())
{
   // reading or writing code.
     System.out.println("Sending request to Server");
     oos.writeObject(msgText);
     oos.flush();
     System.out.println(ois.readObject());
}
catch (IOException exc)
{
   // handle Exception
}
// no need for finally nor to close the streams explicitly

You can use try with resources with all sorts of readers writers etc. That way they are all automatically closed without your having to write explicit close calls. I went through the Java® Tutorials and can't see what you are doing wrong. They use PrintWriters and BufferedReaders there rather than object streams. Readers and writers are more appropriate for Strings and Streams for binary data, but since Strings are always Serializable, I can't see that being a real problem.
Why are you using Thread.sleep()? I cannot see that either helping or hindering.
I presume you have the server side working at the same time.
Try using only English letters and numbers in the text (e rather than é) which should obviate any encoding problems. Can you specify an encoding on the sockets? Probably not. I don't think the hex you showed will cause problems because you don't have 04 or 1a (=ctrl‑D/ctrl‑Z) which might be interpreted as end of transmission there.

That is about as much as I know, I am afraid.

[Campbell Ritchie]

Have you verified that port 1197 is available?
There is an error in the code I posted. Line 4 needs another ) at its end.

[Nina Foley]

Thanks for your prompt reply! Unfortunately I was drowning in work and this is the first chance I've had to get back to the issue.
Re Thread.sleep, there was additional code which has been removed since and it should have been removed also.
Re the inputs, all characters entered were ASCII - an example is BMSG0001001001X where the X is a terminator when read by the server app.
1197 was / is available as a port.

Since then... I looked at the code you mentioned in the tutorials and changed to PrintWriter to try it - Voila! no more leading characters and the message is processed accordingly.... but,
and there's always the but; I'm not getting the reply. The normal reply is OK, only 2 bytes, - and when I used the 'HelloDevice' to test, it works. It would appear my code is failing to execute to the end.

I'll go trawling for examples / forum posts on this issue and see if I can work out what's going on. If I can't, I may be posting again!

Thanks again

[Campbell Ritchie]

So far so good … well done