JAVA UDP Echo Server/Client Example Code

說實話，我不會寫 JAVA。我個人的程式語言母語是 C++（感謝廖婉君教授），工作上比較常用的是 C，JAVA 還真的沒碰過。雖然我一直跟別人說，程式語言這東西，只要掌握住一種，要跨入其他語言並不困難，因為已經有最起碼的程式邏輯（迴圈、判斷式等），但其實我也很少去用其他的程式語言。最近因為工作的關係，不得不使用 JAVA，所以就紀錄一些 JAVA 的範例程式在這裡，以便未來使用。

在這裡要紀錄的是 UDP 的 Echo Server 和 Client 的程式。資料來源為 http://www.cs.uic.edu/~troy/spring05/cs450/sockets/socket.html。

UDP Echo Server

import java.io.*; 
import java.net.*; 
  
class UDPServer 
{ 
    public static void main(String args[]) throws Exception 
    { 
        int serverPort = 9876;
        
        try
        {
            if ( args.length > 0 )
            {
                serverPort = new Integer( args[0] ).intValue();
            }
            
            DatagramSocket serverSocket = new DatagramSocket( serverPort ); 
  
            byte[] receiveData = new byte[1024]; 
            byte[] sendData  = new byte[1024]; 
  
            while(true) 
            { 
                receiveData = new byte[1024]; 

                DatagramPacket receivePacket = new DatagramPacket( receiveData, receiveData.length ); 

                System.out.println ("Waiting for datagram packet");

                serverSocket.receive( receivePacket ); 

                String sentence = new String( receivePacket.getData() );   
                InetAddress IPAddress = receivePacket.getAddress();   
                int port = receivePacket.getPort(); 
  
                System.out.println ("From: " + IPAddress + ":" + port);
                System.out.println ("Message: " + sentence);

                String capitalizedSentence = sentence.toUpperCase(); 

                sendData = capitalizedSentence.getBytes(); 
  
                DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, port); 
  
                serverSocket.send(sendPacket); 
            } 

        }
        catch (SocketException ex) 
        {
            System.out.println("UDP Port " + serverPort + " is occupied.");
            System.exit(1);
        }

    } 
}

UDP Echo Client

import java.io.*; 
import java.net.*; 
import java.lang.Integer;
  
class UDPClient 
{ 
    public static void main(String args[]) throws Exception 
    { 
        try 
        {
            String serverHostname = new String ("127.0.0.1");
            int serverPort = 9876;

            if ( args.length > 0 )
            {
                serverHostname = args[0];
            }
            
            if ( args.length > 1 )
            {
                serverPort = new Integer( args[1] ).intValue();
            }
  
            BufferedReader inFromUser = new BufferedReader( new InputStreamReader( System.in ) ); 
  
            DatagramSocket clientSocket = new DatagramSocket(); 
  
            InetAddress IPAddress = InetAddress.getByName(serverHostname); 
            System.out.println ("Attemping to connect to " + IPAddress + ") via UDP port " + serverPort );
            
            while( true )
            {
                byte[] sendData = new byte[1024]; 
                byte[] receiveData = new byte[1024]; 
  
                System.out.print("Enter Message (q for exit): ");
                String sentence = inFromUser.readLine(); 
                
                if( sentence.equals( "q" ) )
                {
                    System.out.print("Exit! ");
                    break;
                }
                
                sendData = sentence.getBytes();         

                System.out.println ("Sending data to " + sendData.length + " bytes to server.");
                DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, serverPort ); 
                clientSocket.send(sendPacket); 

                DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length); 
  
                System.out.println ("Waiting for return packet");
                clientSocket.setSoTimeout(10000);

                try 
                {
                    clientSocket.receive(receivePacket); 
                    String modifiedSentence = new String(receivePacket.getData()); 
  
                    InetAddress returnIPAddress = receivePacket.getAddress();     
                    int port = receivePacket.getPort();

                    System.out.println ("From server at: " + returnIPAddress + ":" + port);
                    System.out.println ("Message: " + modifiedSentence); 
                }
                catch ( SocketTimeoutException ste )
                {
                    System.out.println ("Timeout Occurred: Packet assumed lost");
                }
            }  
            clientSocket.close(); 
        }
        catch (UnknownHostException ex) 
        { 
            System.err.println(ex);
        }
        catch (IOException ex) 
        {
            System.err.println(ex);
        }
    } 
}

因為我跟 JAVA 不熟，順便紀錄一下編譯的指令。

javac javaUdpServer.java
javac javaUdpClient.java
java UdpServer 9876
java UdpClient 127.0.0.1 9876