Sharing an output stream from more than 1 thread.

Question

I have a problem, an ObjectOutputStream is giving me a nullpointerexception.
I have a thread which connects to a server application and uses object input/output streams to communicate. This thread will deal with any incoming requests from the server concurrently from the main program. I presumed you could use the same Socket connection that was created in this thread to send objects from other threads (for example the main thread) but I think I presumed wrong.
This is part of the class which runs as a separate thread:
    public void connect()
        try
            mainSocket = new Socket(hostName, hostPort);
            in = new ObjectInputStream(mainSocket.getInputStream());
            out = new ObjectOutputStream(mainSocket.getOutputStream());
        catch(UnknownHostException e)
            System.out.println(e.getMessage());
        catch(IOException e)
            System.out.println(e.getMessage());
    public void run()
        connect();
        while(true)
         // (Not complete)
    public boolean verifyEmployeeName(String employeeName)
        boolean answer = false;
        try
            out.writeObject(Server.protocol.VERIFY_EMPLOYEE);
            out.writeObject(employeeName);
            answer = in.readBoolean();
        catch(IOException e)
        return answer;
    }The verifyEmployeeName() method is called externally from the main thread and generates the IOException at out.writeObject(Server.protocol.VERIFY_EMPLOYEE); (or the first point at which the socket is used). Anyway my question is; do you have to create a new socket for each thread (bear in mind I haven't yet put in synchronization features) or is this nothing to do with that?
Very grateful for any help you have to offer.

Answer 1

alex.p wrote:
Anyway my question is; do you have to create a new socket for each thread (bear in mind I haven't yet put in synchronization features) Not necessarily. But you'll have to sequentialise the communication.
Create a communication thread. Give it the control over the socket. Have it expose a BlockingQueue that takes some form of "tasks". As a thread, that communicator polls its queue, and executes the tasks. Other threads post their tasks on the queue and wait until they're completed. Look for clues in the java.util.concurrent package.