Continuously read objects from an ObjectInputStream in Java

Question

I have a problem using an ObjectInputStream and I have been struggling with it for 2 days now. I tried to search for a solution but unfortunately found no fitting answer.

I am trying to write a client/server application in which the client sends objects (in this case a configuration class) to the server. The idea is that connection keeps alive after sending the object so it is possible to send a new object if necessary.

Here are the important parts of my client code:

mSocket = new Socket("192.168.43.56", 1234);

mObjectIn = new ObjectInputStream(mSocket.getInputStream());
mObjectOut = new ObjectOutputStream(mSocket.getOutputStream());

mObjectOut.writeObject(stubConfig);
mObjectOut.flush();

In the above code, I left out some try/catch blocks to keep the code readable for you.

The server side looks as follows:

mHostServer = new ServerSocket(port);
mSocket = mHostServer.accept();

// create streams in reverse oreder
mObjectOut = new ObjectOutputStream(mConnection.getOutputStream());
mObjectOut.flush();
mObjectIn = new ObjectInputStream(mConnection.getInputStream());

while (mIsSocketConnected)
{
    StubConfig = (StubConfiguration)mObjectIn.readObject(); 
}

What I want to achieve is that as long at the socketconnection is alive, the server is listening for incoming config objects.

When I run my program however, I got an EOFException in the while loop at server side. I receive the first config object without any problems in the first iteration of the while loop but after that I get an EOFException every time readObject() is called.

I am looking for a way to solve this. Can anyone put me in the good direction?

EDIT: What I read about the EOFException is that it is thrown when you want to read from a stream when the end of it is reached. That means that for some reason the stream ended after the object has been send. Is there a way to reinitialize the streams or so??

Answer 1

try using this

Server side
1.Server running on a separate thread

public class ServeurPresence implements Runnable {

public final static int PORT = 20000 ;
public final static String HOSTNAME = "localhost" ;
public static enum Action {CONNEXION, MSG, DECONNEXION,USER, FINCLASSEMENT};

ServerSocket serveur ;
static List<String> names ;

 */
public ServeurPresence() 
{

    System.out.println("Start Server...");
    try 
    {
        serveur = new ServerSocket(PORT) ;
        new Thread(this).start();
        //javax.swing.SwingUtilities.invokeLater(new Runnable() { public void run() {   createAndShowGUI();}    }   );
    } 
    catch (IOException e)
    {
        e.printStackTrace();
    }
}
/**
 * @param args
 */
public static void main(String[] args)
{
    new ServeurPresence();

}
@Override
public void run() 
{
    System.out.println("server runs");

        while(true)
        {
            try {

                Socket sock = serveur.accept();
                ServiceClientsThread thread= new ServiceClientsThread(sock);
                thread.start();

            }
            catch (IOException e) 
            {
                System.out.println("Error with  socket");
                e.printStackTrace();        
            }
        }

}

}

2. A Thread to handle each Client:ServiceClientThread

    import java.io.DataInputStream;
    import java.io.DataOutputStream;
    import java.io.IOException;
    import java.net.Socket;
    import java.util.ArrayList;
    import java.util.List;


 public class ServiceClientsThread extends Thread{
    private Socket sock ;
ServiceClientsThread(Socket sock)
{
        //super();
        this.sock=sock;
}

@Override
public void run() 
{
    DataInputStream is ;
    DataOutputStream os ;
    String name =null ;

    try {

        is = new DataInputStream(sock.getInputStream()) ;
        os = new DataOutputStream(sock.getOutputStream()) ;
        ServeurPresence.Action act ;

        do {
            // read Action              
            act = ServeurPresence.Action.valueOf(is.readUTF()) ; // read string -> enum
            System.out.println("action :"+act);
            switch (act) {

            case CONNEXION :
                name = is.readUTF(); //read client name
                System.out.println("Name :"+name);
                os.writeUTF("Hi");//send welcome msg
                break ;
            case MSG :
                String msg = is.readUTF();
                 os.writeUTF("OK");//response
                break ;
            case DECONNEXION :
                System.out.println(name+" is logged out");
                break ;
            }

        } while (act!=ServeurPresence.Action.DECONNEXION) ;

        // the end
        is.close();
        os.close();
        sock.close();

    } catch (IOException e) 
    {
        System.out.println("Error with "+name+" socket");
        e.printStackTrace();        
    }
}

}

3. Client side

    import java.io.DataInputStream;
    import java.io.DataOutputStream;
    import java.io.IOException;
    import java.net.InetAddress;
    import java.net.Socket;
    import java.net.UnknownHostException;


public class Client {
/**
 * 
 */
Client(String name)
{
    System.out.println("Start Client...");

    try {

        Socket sock = new Socket(ServeurPresence.HOSTNAME,ServeurPresence.PORT) ;
        DataOutputStream os = new DataOutputStream(sock.getOutputStream()) ;
        DataInputStream is = new DataInputStream(sock.getInputStream()) ;

                    System.out.println("Send "+name+" to server");

        // CONNECTION : Action then value
        os.writeUTF(ServeurPresence.Action.CONNEXION.name()) ; // send action : write enum -> String
        os.writeUTF(name) ; // send the name

                    //read server welcome msg
        String msg = is.readUTF();
                    System.out.println("Welcome msg: "+msg);

        /*  Your actions here : see example below */
        try 
        {
            Thread.currentThread().sleep(4000);
            os.writeUTF(ServeurPresence.Action.MSG.name()) ; // send action : write enum -> String
            os.writeUTF("My message here") ; // send msg
            Thread.currentThread().sleep(4000);
            msg = is.readUTF();//server response message
        } 
        catch (InterruptedException e) 
        {
            e.printStackTrace();
        }
        /************************************************/

        //CLOSE
        os.writeUTF(ServeurPresence.Action.DECONNEXION.name()) ; // send action
        System.out.println("Log out");
        os.close();
        sock.close();

    }
    catch (UnknownHostException e) 
    {
        System.out.println(ServeurPresence.HOSTNAME+ " unknown");
        e.printStackTrace();
    } 
    catch (IOException e)
    {
        System.out.println("Impossible to connect to "+ServeurPresence.HOSTNAME+ ":"+ServeurPresence.PORT);
        e.printStackTrace();
    } 
}

}

4. In your case use readObject()/writeObject() instead of readUTF()/writeUTF() to write your config objects

Answer 2

EOFException is thrown by readObject() when the peer has closed the connection. There can never be more data afterwards. Ergo you can't have written multiple objects at all: you closed the connection instead.

Answer 3

Try this and let me know how it goes:

while (1==1)
{
    StubConfig = (StubConfiguration)mObjectIn.readObject();
    Thread.sleep(100); //Saves CPU usage
}

Answer 4

Very late answer, but just for future reference. I have been having problems sending Objects via sockets because the method flush() is not working properly.

I solved this problem just by switching flush() to reset().