Question

We have a java socket program where the server gets data from many devices and works fine. At times the server needs to send some command to the devices. When it sends individual commands it works fine. The problem comes when it sends multiple commands, only the first one is successful. We cant figure out why the rest fails. Below is the snippet showing how the message is sent. Should I set a delay after a message is sent?

public static void main(String[] args) {   

      new sServer7888();

   }
sServer7888() {


    try{
    final ServerSocket serverSocketConn = new ServerSocket(7888);               
    while (true){
    try{
       Socket socketConn1 = serverSocketConn.accept();
          new Thread(new ConnectionHandler(socketConn1)).start();                       
    }
    catch(Exception e){
        e.printStackTrace(System.out);
    }
       }
    } 
    catch (Exception e)     {
         e.printStackTrace(System.out);
    }

}


class ConnectionHandler implements Runnable {

  private Socket receivedSocketConn1;
    ConnectionHandler(Socket receivedSocketConn1) {
      this.receivedSocketConn1=receivedSocketConn1;
    }

    public void run() {

      while ((nextChar=readIn1.read()) != -1) {

         completeMessage += (char) nextChar;     
         if (nextChar == '*')
    {
         String[] splitResult = completeMessage .split(",");    
         String header=splitResult[0].trim().substring(0,4);

         if((header.equals("$ACK")){

          //update the message sent from the server as already acknowledge.
         }     
         else{
          //run query to find if there are any message to be sent out to the devices
          while(rsOC1.next()){
            commandText = rsOC1.getString("commandText");
            writeOut1.write(commandText);
            writeOut1.write("\r\n");
            writeOut1.flush(); 
          }

          //now process the normal message receive from the devices.
         } 
        completeMessage="";
       }   
      }
   }
}

Answer 1

If your device is sending ACK on Every message and Server is able to receive it then you can proceed in following way with your server side program.
EDIT
I have updated the code as per the requirement analysis. Let me know if any discrepancy is found after implementing it.

Thread.sleep(1000) is not the reliable solution for above case because
we are not knowing how long the device might take to execute previous
command sent by Server .

    public void run() 
    {

        int i = -1;
        ArrayList<String> list = new ArrayList<String>();
        while ((nextChar=readIn1.read()) != -1) 
        {
            boolean isCompleteMessage = readMessage(nextChar);
            if (isCompleteMessage)
            {
                String[] splitResult = completeMessage .split(",");    
                String header=splitResult[0].trim().substring(0,4);
                if((header.equals("$ACK"))
                {
                    String id = null;
                    if (i != -1)
                    {
                        id = list.get(i);
                        id = id.substring(0,id.indexOf("^"));
                    }
                    //update the message sent from the server as already acknowledge using id extracted above.
                    if ( i == 0)
                    {
                        list.remove(i);
                        if (list.size() == 0)
                        {
                            i = -1;
                        }
                        else
                        {
                            commandText = list.get(i);
                            writeOut1.write(commandText.substring((commandText.indexOf("^")) + 1));
                            writeOut1.write("\r\n");
                            writeOut1.flush(); 
                        }
                    }
                }     
                else
                {
                    //process here the normal message receive from the devices.
                    if (i == -1)
                    {
                        list = getRecords();
                        if (list.size() > 0)
                        {
                            i = 0;
                            commandText = list.get(i);
                            writeOut1.write(commandText.substring((commandText.indexOf("^")) + 1));
                            writeOut1.write("\r\n");
                            writeOut1.flush(); 
                        }
                    }
                    else 
                    {
                        commandText = list.get(i);
                        writeOut1.write(commandText.substring((commandText.indexOf("^")) + 1));
                        writeOut1.write("\r\n");
                        writeOut1.flush(); 
                    }
                } 
                completeMessage = "";
            }   
        }
   }
   public boolean readMessage(int nextChar)
   {
        completeMessage += (char)nextChar;
        if (((char)nextChar) == '*')
        {
            return true;
        }
        else
        {
            return false;
        }
   }
   //Retreive all commands from database and returns the ArrayList containing those commands.
   public ArrayList<String> getRecords()
   {
       ArrayList<String> list = new ArrayList<String>();
       Statement stat = null;
       ResultSet rsOC1 = null;
       try
       {
            stat = con.createStatement();
            rsOC1 = stat.executeQuery("Query for message retrieval from database");
            while (rsOC1.next())
            {
                String sElement = rs0C1.getString("commandID") + "^" + rs0C1.getString("commandText");
                list.add(sElement);
            }
       }
       catch (Exception ex){}
       finally
       {
           if (rs0C1 != null)
           {
               try
               {
                    rs0C1.close();   
               } catch () {}
           }
           if (stat != null)
           {
               try
               {
                    stat.close();   
               } catch () {}
           }
           return list;
       }
   }