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01 import java.io.*;
02 import java.net.*;
03 public class HttpClient {
04 public static void main(String[] args) {
05 try {
06 // Demo參數:
07 if ((args.length != 1) && (args.length != 2))
08 throw new IllegalArgumentException("Wrong number of args");
09 //定義輸出流,下載的URL內容被寫入這個流
10 OutputStream to_file;
11 if (args.length == 2) to_file = new FileOutputStream(args[1]);
12 else to_file = System.out;
13 //使用URL類來把用戶指定的URL解析成幾個部分
14 URL url = new URL(args[0]);
15 String protocol = url.getProtocol();
16 if (!protocol.equals("http")) // 檢驗是否滿足支持的協議
17 throw new IllegalArgumentException("Must use 'http:' protocol");
18 String host = url.getHost();
19 int port = url.getPort();
20 if (port == -1) port = 80; //如果沒有指定端口,用默認端口
21 String filename = url.getFile();
22 // 打開壹個連接到指定主機和端口的網絡socket連接
23 Socket socket = new Socket(host, port);
24 // 通過socket來獲得輸入和輸出流
25 InputStream from_server = socket.getInputStream();
26 PrintWriter to_server = new PrintWriter(socket.getOutputStream());
27
28 // 發送HTTP GET命令給網絡服務器,指定要下載的文件
29 // 使用了壹個老版本非常簡單的HTTP協議
30 to_server.print("GET " + filename + "\n\n");
31 to_server.flush(); // 立即發送
32 // 現在讀取服務器的響應,把接收到的內容寫入文件
33 byte[] buffer = new byte[4096];
34 int bytes_read;
35 while((bytes_read = from_server.read(buffer)) != -1)
36 to_file.write(buffer, 0, bytes_read);
37 // 當服務器關閉連接時,也關閉stuff
38 socket.close();
39 to_file.close();
40 }
41 catch (Exception e) { // 發布錯誤
42 System.err.println(e);
43 System.err.println("Usage: java HttpClient <URL> [<filename>]");
44 }
45 }
46 }