Socket program to send and receive user defined objects not working

Question

I have a user defined class Message, whose object I would like to pass between the client and the server.

The Message class is as follows:

import java.io.Serializable;

public class Message implements Serializable
{
    String CorS;
    int data_id;
    int status_id;
    Integer value;
    boolean withdraw;

    public Message()
    {
        CorS = null;
        data_id = 0;
        status_id = 0;
        value = 0;
        withdraw = false;
    }

    public Message(String CorS, int data_id, int status_id, Integer value)
    {
        this.CorS = CorS;
        this.data_id = data_id;
        this.status_id = status_id;
        this.value = value;
    }

    public Message(boolean withdraw)
    {
        this.withdraw = withdraw;
    }
}

The code in the client side which sends the object to the server is as follows:

Socket s = null;
    ObjectInputStream in = null;
    ObjectOutputStream out = null;

    String hostname = null;
    int port_no = 0;

    HashMap<String, Integer> map = null;

    Message m = null;

    map = servers.get("Server" + server);

    for(String key : map.keySet())
    {
        hostname = key;
        port_no = map.get(key);
    }
    //System.out.println(hostname + " " + port_no);

    s = new Socket(hostname, port_no);
    in = new ObjectInputStream(new BufferedInputStream(s.getInputStream()));
    out = new ObjectOutputStream(new BufferedOutputStream(s.getOutputStream()));

    s_now = s;

    m = new Message(client, data, 0, 0);
    out.writeObject(m);
    out.flush();
    System.out.println("Sent obj");

Similarly, the code on the Server side is as follows:

while (true)
    {
        try 
        {
            System.out.println("Waiting for client on port " + serverSocket.getLocalPort() + "...");
            Socket server = serverSocket.accept();
            System.out.println("Just connected to " + server.getRemoteSocketAddress());

            ObjectInputStream in = new ObjectInputStream(new BufferedInputStream(server.getInputStream()));
            //ObjectOutputStream out = new ObjectOutputStream(server.getOutputStream());

            Message m = (Message) in.readObject();

            System.out.println(m.value);
        } 

        catch (IOException e) 
        {
            e.printStackTrace();
            break;
        }
        catch (ClassNotFoundException e) 
        {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

The problem is that the object is not getting printed. The output I get is as follows:

Waiting for client on port 1051...
Just connected to /127.0.0.1:59216

Any help in this regard will be greatly appreciated. Thanks :)

user207421 · Accepted Answer · 2013-11-29 00:31:46Z

8

You need to create the ObjectOutputStream before the ObjectInputStream at both ends.

The reason is that, as described in the Javadoc, the respective constructors write and read a stream header. So the input stream constructor can't return until the output stream constructor at the peer has executed. So if you construct both input streams first there is a deadlock.

edited Nov 29, 2013 at 0:31

answered Nov 28, 2013 at 2:38

user207421

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1 Comment

Maddy Over a year ago

Thanks. That worked. But could you tell me why this is the case? Any reasonable explanation?

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