Facing issue in java MultiThreading

Question

Im facing one problem in streaming data capture for reading the broadcast data during multithreading, pls help or suggest,

Actually there is one class which is reading data from one of the udp socket. Another class accepts the tcp connection from every client request, creates a thread for every client and request the same udp class for data. The thing is working with 1st thread which gets created. But when i request with another client from another pc/ip the packets get losted to the 2nd client/thread

I have made a workaround by creating a list where im storing the Threads outputstream object and looping it to send the data to all the client. But this is just temporary as it ll delay the packets if clients/connections gets increased.

code for reading UDP Data

   public class EventNotifier    
    {
      private InterestingEvent ie;
      public DatagramSocket clientSocket;
      public String[] split_str;

      byte[] receiveData;

      HashMap<String, String> secMap = new HashMap<String, String>();


      public EventNotifier(InterestingEvent event)
      {
        ie = event; 
     clientSocket = new DatagramSocket(9050);
        receiveData = new byte[500];
      }



      public String getDataFeed(String client_id)
      {
        try 
        {
             DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
             clientSocket.receive(receivePacket);
             String s = new String(receivePacket.getData());
             String split_str = s.split(","); 
             if(secMap.containsValue(split_str[0]))
                  return s;
             else
                  return "";
             } catch(Exception e3) {}
      }
     }// end of eventNotifier class

code for multithreading handling client requests

public class multiServer 
{

    static protected List<PrintWriter> writers = new ArrayList<PrintWriter>();
    static String client_id = "";
    public static void main(String[] args)
    { 
       try
       {
           ServerSocket servsock = new ServerSocket(8858);
           Socket incoming;
           while(true)
           {
             incoming = servsock.accept();
             multiServerThread  connection = new multiServerThread(incoming);

             Thread t1 = new Thread(connection);
             t1.start();
           }
       }
       catch(IOException e)
       {
            System.out.println("couldnt make socket");
       }
    }
}

class multiServerThread extends Thread implements InterestingEvent
{

    Socket incoming;
    PrintWriter out=null;
    PrintWriter broad=null;
    BufferedReader in = null;
    String cliString=null;
    private EventNotifier en; 
    int id;

    public static String udp_data;

    public void interestingEvent(String str1)
    {
          this.udp_data = str1;
    }
    public String getUdpData()
    {
      String _udp_data = this.udp_data;
      return _udp_data;
    }
    multiServerThread(Socket incoming)
    {  
        this.incoming=incoming;
        en = new EventNotifier(this);
    } 
    public void run()
    {
        try
        {
            out = new PrintWriter(incoming.getOutputStream(), true);
            in = new BufferedReader(new InputStreamReader(incoming.getInputStream()));
        cliString = in.readLine();
        multiServer.writers.add(out);
        while(true)
                {
               try
                   {
                    udp_data = en.getDataFeed(cliString);
                    if(udp_data!=null && udp_data.length()>0)
                    {
              //workaround for serving the data to all cleints who are connected    
                        for (int i=0; i<multiServer.writers.size();i++)
                        {
                            broad=multiServer.writers.get(i);
                            broad.println(udp_data.trim());
                        }
             //else will directly write to the outputstream object for every thread which is connected
             // out.println(udp_data.trim());
                    }

               }
               catch (Exception e)
               {
                 System.out.println("exception "+e);
               }
               Thread.sleep(1);

            }
        } catch(IOException e)
        {
            System.out.print("IO Exception :: "+ e);
        }
        catch(InterruptedException e)
        {
            System.out.print("exception "+ e);
        }        
    }
}

Answer 1

You need mutual exclusion (or a different design).

For example, what will happen if two threads call multiServer.writers.add(out); concurrently?

From the ArrayList Javadocs

Note that this implementation is not synchronized. If multiple threads access an ArrayList instance concurrently, and at least one of the threads modifies the list structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more elements, or [...])

Another problem is two calling udp_data = en.getDataFeed(cliString); concurrently. The second thread might overwrite the result of the first. You'll loose data!

What happens if one thread calls for (int i=0; i<multiServer.writers.size();i++) while another thread is busy doing multiServer.writers.add(out); ? The size may have increased, before out has actually been added to the list!

public class multiServer 
{

    private List<PrintWriter> writers = new ArrayList<PrintWriter>();

    public synchronized void addWriter(PrintWrite out) {
         writers.add(out);
    }
    public synchronized void serveAllWriters(String data) {
         for (int i=0; i<multiServer.writers.size();i++)
         {
             broad=multiServer.writers.get(i);
             broad.println(data);
         }
    }
}

Now when a thread tries to add a writer, the synchronized s will make sure no other thread is add ing or printing. So multiServerThread should be fixed to use the new methods:

class multiServerThread extends Thread implements InterestingEvent
{
      //...
      private String udp_data;
      //...
      myMultiServer.addWriter(out);

      //...
      udp_data = en.getDataFeed(cliString);
      if(udp_data!=null && udp_data.length()>0)
            myMultiServer.serveAllWriters(udp_data.trim());
      //...
}

There might be more problems, not sure I don't fully understand your code. The question you must ask yourself is, can another thread read and/or write the same data or object? Yes? Then you'll need proper synchronization.