28.12. Programming Exercises

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25.4. Serving Multiple Clients

Multiple clients are quite often connected to a single server at the same time. Typically, a server runs constantly on a server computer, and clients from all over the Internet may want to connect to it. You can use threads to handle the server's multiple clients simultaneously . Simply create a thread for each connection. Here is how the server handles a connection:

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   while   (   true   ) { Socket socket = serverSocket.accept(); Thread thread =   new   ThreadClass(socket); thread.start(); } 

The server socket can have many connections. Each iteration of the while loop creates a new connection. Whenever a connection is established, a new thread is created to handle communication between the server and the new client; and this allows multiple connections to run at the same time.

Listing 25.4 creates a server class that serves multiple clients simultaneously. For each connection, the server starts a new thread. This thread continuously receives input (the radius of a circle) from clients and sends the results (the area of the circle) back to them (see Figure 25.7). The client program is the same as in Listing 25.2. A sample run of the server with two clients is shown in Figure 25.8.

Figure 25.7. Multithreading enables a server to handle multiple independent clients.

Figure 25.8. The server spawns a thread in order to serve a client.

Listing 25.4. MultiThreadServer.java

(This item is displayed on pages 824 - 826 in the print version)

 1   import   java.io.*; 2   import   java.net.*; 3   import   java.util.*; 4   import   java.awt.*; 5   import   javax.swing.*; 6 7   public class   MultiThreadServer   extends   JFrame { 

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 8  // Text area for displaying contents  9   private   JTextArea jta =   new   JTextArea(); 10 11   public static void   main(String[] args) { 12   new   MultiThreadServer(); 13 } 14 15   public   MultiThreadServer() { 16  // Place text area on the frame  17 setLayout(   new   BorderLayout()); 18 add(   new   JScrollPane(jta), BorderLayout.CENTER); 19 20 setTitle(   "MultiThreadServer"   ); 21 setSize(   500   ,   300   ); 22 setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 23 setVisible(   true   );  // It is necessary to show the frame here!  24 25   try   { 26  // Create a server socket  27  ServerSocket serverSocket =   new   ServerSocket(   8000   );  28 jta.append(   "MultiThreadServer started at "   +   new   Date() +   '\n'   ); 29 30  // Number a client  31   int   clientNo =   1   ; 32 33   while   (   true   ) { 34  // Listen for a new connection request  35  Socket socket = serverSocket.accept();  36 37  // Display the client number  38 jta.append(   "Starting thread for client "   + clientNo + 39   " at "   +   new   Date() +   '\n'   ); 40 41  // Find the client's host name , and IP address  42  InetAddress inetAddress = socket.getInetAddress();  43 jta.append(   "Client "   + clientNo +   "'s host name is "   44 +  inetAddress.getHostName()  +   "\n"   ); 45 jta.append(   "Client "   + clientNo +   "'s IP Address is "   46 +  inetAddress.getHostAddress()  +   "\n"   ); 47 48  // Create a new task for the connection  49 HandleAClient task =   new   HandleAClient(socket); 50 51  // Start the new thread  52 task.start(); 53 54  // Increment clientNo  55 clientNo++; 56 } 57 } 58   catch   (IOException ex) { 59 System.err.println(ex); 60 } 61 } 62 63  // Inner class  64  // Define the thread class for handling new connection  65    class   HandleAClient   implements   Runnable  { 66   private   Socket socket;  // A connected socket  67 

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 68  /** Construct a thread */  69   public   HandleAClient(Socket socket) { 70   this   .socket = socket; 71 } 72 73  /** Run a thread */  74    public void   run()  { 75   try   { 76  // Create data input and output streams  77  DataInputStream inputFromClient =   new   DataInputStream(  78  socket.getInputStream());  79  DataOutputStream outputToClient =   new   DataOutputStream(  80  socket.getOutputStream());  81 82  // Continuously serve the client  83   while   (   true   ) { 84  // Receive radius from the client  85   double   radius =  inputFromClient.readDouble()  ; 86 87  // Compute area  88   double   area = radius * radius * Math.PI; 89 90  // Send area back to the client  91  outputToClient.writeDouble(area);  92 93 jta.append(   "radius received from client: "   + 94 radius +   '\n'   ); 95 jta.append(   "Area found: "   + area +   '\n'   ); 96 } 97 } 98   catch   (IOException e) { 99 System.err.println(e); 100 } 101 } 102 } 103 } 

The server creates a server socket at port 8000 (line 27) and waits for a connection (line 35). When a connection with a client is established, the server creates a new thread to handle the communication (line 49). It then waits for another connection in an infinite while loop (lines 33 “56).

The threads, which run independently of one another, communicate with designated clients. Each thread creates data input and output streams that receive and send data to a client.

This server accepts an unlimited number of clients. To limit the number of concurrent connections, you can use a thread pool with a fixed size and add tasks to the pool.