Java: client-sever app.- WHILE on ObjectInputStream read thread

问题

This must be very easy but I am obviously missing some basic understanding. I have simple client server application

server:

public class GameServer{
   private Socket clientSocket;
   private ServerSocket serverSocket;
   private ArrayList<ObjectOutputStream> oosPlayers=new ArrayList<ObjectOutputStream>();   

   public static void main(String args[]){
       GameServer server = new GameServer();
       server.startGame();
   }   

   public void startGame(){  
      try{
         serverSocket = new ServerSocket(10008);  
         System.out.println("Server started at:" + serverSocket);
         while(true){
             System.out.println("Server: Waiting for client requests...");
             clientSocket=serverSocket.accept();
             System.out.println("Server: Client connection accepted at:"+clientSocket);
             //remember player's output stream
             oosPlayers.add(new ObjectOutputStream(clientSocket.getOutputStream()));
             //read data from clients 
             Thread receiverThread = new Thread(new DataReceiver());
             receiverThread.start();
         }
      }
      catch(IOException ex){
          System.out.println(ex.getMessage());
      }
   }
   public class DataReceiver implements Runnable{
       public void run(){
           try{
               String mess;
               ObjectInputStream ois = new ObjectInputStream(clientSocket.getInputStream());
               while((mess=(String)ois.readObject()) != null){                         
                   System.out.println("Server: "+mess);
                   send();
               }
           }
           catch(Exception ex){ System.out.println(ex.getMessage()); }
       }
       public void send(){
           for (ObjectOutputStream oos : oosPlayers){
               try{
                   oos.writeObject(new String("There is "+oosPlayers.size()+" player(s) at the board"));                   
                   oos.flush();
               }
               catch(Exception ex){ System.out.println(ex.getMessage()); }  
           }
       }
   }        
}

client:

public class GameClient implements ActionListener{
   private Socket socket;
   private ObjectInputStream  ois;
   private ObjectOutputStream oos;
   private JButton button;

   public static void main(String args[]){
       GameClient client = new GameClient();  
       client.drawBoard();
   }
   public void drawBoard(){
       JFrame frame = new JFrame("A game");
       frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
       button = new JButton("Send");
       button.addActionListener(this);
       configureConnection();
       Thread receiverThread = new Thread(new DataReceiver());
       receiverThread.start();

       frame.getContentPane().add(new JPanel().add(button));
       frame.setSize(200,200);
       frame.setVisible(true);
   }   
   public void configureConnection(){
       try{
           socket = new Socket("127.0.0.1", 10008);
           ois = new ObjectInputStream(socket.getInputStream());
           oos = new ObjectOutputStream(socket.getOutputStream());
       }
       catch(Exception ex) { System.out.println(ex.getMessage()); }
   }   

   public void actionPerformed(ActionEvent ev){
       if (ev.getSource()==button){
          try{
              oos.writeObject(new String("Some data from client"));
              oos.flush();
          }
          catch(Exception ex){ System.out.println(ex.getMessage()); }         
       }
   }   

   public class DataReceiver implements Runnable{
       public void run(){
           try{
               String mess;
               while((mess=(String)ois.readObject())!= null){                  
                   System.out.println("Client:"+mess);
               }
           }
           catch(Exception ex){ System.out.println(ex.getMessage()); }
       }
   }   
}

My question relates to receiverThread on both sides. When I use WHILE during read from ObjectInputStream (in RUN method), sever listens to clients requests and properly dispatches them to all ObjectOutputStream s. If I change this WHILE to IF data are read (and send) from server only once per each client. The same applies to client's WHILE/IF.

My understanding is that when I run receiverThread it somehow stops the thread at WHILE and keeps reading from input stream, whereas using IF let's the thread to finish (and thereby stops read process). How is that possible that although WHILE condition is not satisfied i.e. initially (no client got connected yet) it keeps receiverThread alive. The same happens after receiverThread already read data and there is nothing left in the stream.

Would appreciate some explanation on this basic issue.

Regards Marcin

回答1:

If you change while to if in your run() method you read only one object and exit. If your client-server protocol is very simple, e.g. they send only one object per session both versions are equal. Otherwise the system will not function properly: one all other objects will not arrive.

来源：https://stackoverflow.com/questions/7997106/java-client-sever-app-while-on-objectinputstream-read-thread