java.io: UDP 網路程式設計 (request / response)

在 Oracle 網站上，有個簡單的 UDP 程式範例 --- Lesson: All About Datagrams，這個程式和前一個程式基本上是差不多的，差別在於，它是由 client 先送個 request 給 server 端，server 端接收到後，回應訊息給 client 端，下面是 Oracle 的範例程式。

 1 package idv.steven;
 2 
 3 import java.io.*;
 4 import java.net.*;
 5 import java.util.*;
 6  
 7 public class QuoteClient {
 8     public static void main(String[] args) throws IOException {
 9  
10         if (args.length != 1) {
11              System.out.println("Usage: java QuoteClient <hostname>");
12              return;
13         }
14  
15         //get a datagram socket
16         DatagramSocket socket = new DatagramSocket();
17  
18         //send request
19         byte[] buf = new byte[256];
20         InetAddress address = InetAddress.getByName(args[0]);
21         DatagramPacket packet = new DatagramPacket(buf, buf.length, address, 4445);
22         socket.send(packet);
23      
24         //get response
25         packet = new DatagramPacket(buf, buf.length);
26         socket.receive(packet);
27  
28         //display response
29         String received = new String(packet.getData(), 0, packet.getLength());
30         System.out.println("Quote of the Moment: " + received);
31      
32         socket.close();
33     }
34 }

client 端程式在送出 request 後，會停在第 26 行，以等待 server 端的 response。

 1 package idv.steven;
 2 
 3 import java.io.IOException;
 4 
 5 public class QuoteServer {
 6 
 7     public static void main(String[] args) throws IOException {
 8         new QuoteServerThread().start();
 9     }
10 }

 1 package idv.steven;
 2 
 3 import java.io.*;
 4 import java.net.*;
 5 import java.util.*;
 6  
 7 public class QuoteServerThread extends Thread {
 8  
 9     protected DatagramSocket socket = null;
10     protected BufferedReader in = null;
11     protected boolean moreQuotes = true;
12  
13     public QuoteServerThread() throws IOException {
14     this("QuoteServerThread");
15     }
16  
17     public QuoteServerThread(String name) throws IOException {
18         super(name);
19         socket = new DatagramSocket(4445);
20  
21         try {
22             in = new BufferedReader(new FileReader("one-liners.txt"));
23         } catch (FileNotFoundException e) {
24             System.err.println("Could not open quote file. Serving time instead.");
25         }
26     }
27  
28     public void run() {
29  
30         while (moreQuotes) {
31             try {
32                 byte[] buf = new byte[256];
33  
34                 // receive request
35                 DatagramPacket packet = new DatagramPacket(buf, buf.length);
36                 socket.receive(packet);
37  
38                 // figure out response
39                 String dString = null;
40                 if (in == null)
41                     dString = new Date().toString();
42                 else
43                     dString = getNextQuote();
44  
45                 buf = dString.getBytes();
46  
47                 // send the response to the client at "address" and "port"
48                 InetAddress address = packet.getAddress();
49                 int port = packet.getPort();
50                 packet = new DatagramPacket(buf, buf.length, address, port);
51                 socket.send(packet);
52             } catch (IOException e) {
53                 e.printStackTrace();
54                 moreQuotes = false;
55             }
56         }
57         socket.close();
58     }
59  
60     protected String getNextQuote() {
61         String returnValue = null;
62         try {
63             if ((returnValue = in.readLine()) == null) {
64                 in.close();
65         moreQuotes = false;
66                 returnValue = "No more quotes. Goodbye.";
67             }
68         } catch (IOException e) {
69             returnValue = "IOException occurred in server.";
70         }
71         return returnValue;
72     }
73 }

server 端程式啟動後，會先停在第 36 行以等待 request，當收到 request 後程式立即往下執行，server 端的回應內容只是由文字檔 one-liners.txt 中抓取一行文字將它回傳給 client 端。至於 client 端在那? 從程式第 48、49 行可以看到，由 client 端傳來的封包中，可以解析出 client 端的網址和 port 號，因此，server 端知道要將回應值傳向那裡。