Client / Server communicate smoothly only when on same machine

[Jay Crawf]

I'm creating a simple server client / server. The client has a list of names and sends them one at a time to the server, the server then tries to locate that name within an arrayList "WhatsWhat.srvrList". If the server finds the name it then returns some additional info from the arrayList to the client.

The communication works perfectly when both client and server are on the same machine, but I'm running into problems when the two are run on separate boxes. When they are run on separate machines, the client receives a seemingly random set of replies. If the client requests info on 10 names, I only end up with replies for 2 to 4. If on the other hand, I run client and server on the same box, I get all 10 replies.

Either the client or the server seems to be dropping information over the network, or somehow lagging behind. Is it unwise to be pulling data out of an arrayList as I've done? How can I ensure that the client waits to get a response for each name it sends?

Server:

class srvListen implements Runnable{
    
    public static void listen() throws IOException {

        PrintWriter out=null;
        BufferedReader in=null;
        ServerSocket sprocket=null;
        Socket clientSocket = null;
        
    while(true){
        try{
        sprocket = new ServerSocket();
        System.out.println("Server Online");
        String fromClient, outputLine="NOGO";

        try {
            sprocket = new ServerSocket(4444);
        } catch (IOException e) {
            System.err.println("Could not listen on port: 4444.");
            System.exit(1);
        }

        clientSocket = null;
        try {
            clientSocket = sprocket.accept();
        } catch (IOException e) {
            System.err.println("Accept failed.");
            System.exit(1);
        }

        out = new PrintWriter(clientSocket.getOutputStream(), true);
        in = new BufferedReader(
                new InputStreamReader(
                clientSocket.getInputStream()));

        //Client requests status of a particular server
        while ((fromClient = in.readLine()) != null) {
            //Go thru all Servers that we know of
            for (int z=0; z< WhatsWhatUI.ServerNames.size(); z++){
                //Server name recieved from Client is already being tracked, find and return its Status info
                if (WhatsWhatUI.ServerNames.contains(fromClient)){
                        int pos=0;
                        outputLine = "GO";//<-no longer needed, but client waits for this reply
                        out.println(outputLine); //Tell the client we know about this server, proceed with communication
                            for (; pos< WhatsWhatUI.srvrList.size(); pos++){
                                //send all the information we have on the requested server
                                if (fromClient.equals(WhatsWhatUI.srvrList.get(pos).Server)){                                           
                                    outputLine = WhatsWhatUI.srvrList.get(pos).Status;
                                    out.println(outputLine);
                                    outputLine = WhatsWhatUI.srvrList.get(pos).Version;
                                    out.println(outputLine);
                                    outputLine = WhatsWhatUI.srvrList.get(pos).Patch;
                                    out.println(outputLine);
                                    break;  //We've found and communicated server info, quit loop
                                }
                            }
                             break;//We've found and communicated server info, quit loop
                }
            }
        }
        }
        finally {
            out.close();
            in.close();
            clientSocket.close();
            sprocket.close();
        }
     }
    }]

Client:

public class client implements Runnable {

        public static void client() throws IOException {
              
        Socket wwSocket = null;
        PrintWriter out = null;
        BufferedReader in = null;
        
        wwSocket = new Socket("ServerName", 4444);
     
        out = new PrintWriter(wwSocket.getOutputStream(), true);
        in = new BufferedReader(new InputStreamReader(wwSocket.getInputStream()));
    
        BufferedReader stdIn = new BufferedReader(new InputStreamReader(System.in));
        String fromServer;//<-- unused, but will get out of sync without back and forth
        String toServer=null;
        
        
        for (int i =0; i< WhatsWhatUI.ServerNames.size(); i++){
            toServer = WhatsWhatUI.ServerNames.get(i);
            out.println(toServer);
            fromServer = in.readLine();//<- no longer functionally needed server response
            //if (fromServer.equals("GO"))
            processResponse(in, toServer);
        }

        out.close();
        in.close();
        stdIn.close();
        wwSocket.close();
    }
        
    //Process server reply for Known server, expect to receive Status, version, then patch 
    private static void processResponse(BufferedReader in, String Server) throws IOException {
        srvTrack myServer = new srvTrack();
        String line;
  
        myServer.setServer(Server);  //set the server name
        line = in.readLine();
        myServer.setStatus(line);    //set the server status
        line = in.readLine();
        myServer.setVersion(line);   //set the server version
        line = in.readLine();
        myServer.setPatch(line);     //set the server patch

        
        WhatsWhatUI.RetrievedList.add(myServer);
    }

[Paul Clapham]

Jay Crawf wrote:Either the client or the server seems to be dropping information over the network, or somehow lagging behind.

What that suggests to me is that when you run the two things on the same machine, they are somehow sharing data in some way other than over the network. I say that because even when you run the client and the server on the same machine, the connection is still made through the operating system's networking code.

When you run the client and server on the same machine, are you running them in separate processes?

[Jay Crawf]

Paul Clapham wrote:
When you run the client and server on the same machine, are you running them in separate processes?

I would think so, I don't even know how I could get them to run in the same process.
What I can tell you is that I built both the client and server, and moved the jar files from the development machine to another computer. Running out of separate folders on this new machine, the client and server are able to communicate without issue. But, if I move the client to yet a third machine, the communication is hit and miss... mostly miss, but some of the information does come across.

[Jay Crawf]

This is my suspicion, the server is replying to the client with data it is pulling from an ArrayList "WhatsWhatUI.srvrList" - this arraylist is not static and is being actively and continuously updated in another thread... I'm getting the feeling this isn't too safe of a thing to do. When run on the same machine everything happens fast enough so it doesn't matter, but when done over an actual network connection it's slower, and the results get wonky.

[Paul Clapham]

Jay Crawf wrote:This is my suspicion, the server is replying to the client with data it is pulling from an ArrayList "WhatsWhatUI.srvrList" - this arraylist is not static and is being actively and continuously updated in another thread... I'm getting the feeling this isn't too safe of a thing to do.

Really? I don't see any threads at all in the code you posted.

[Ulf Dittmer]

Seeing the code, and noticing that it causes problems only if run on disparate hosts, I advise to read to read Don't println to a Socket.

[Jay Crawf]

Ulf Dittmer wrote:Seeing the code, and noticing that it causes problems only if run on disparate hosts, I advise to read to read Don't println to a Socket.

Hrm, none of the hosts used was a Mac, but worth a shot though. The server's in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream())); seems totally unable to receive from the clients out.print(anything at all); though.

[Ulf Dittmer]

The problem described in the article is not Mac-specific (or OS X-specific); it would happen on any Unix/Linux-based OS.