《Java编程艺术》章节选登。作者：高永强 清华大学出版社 （即将出版）

23.2.5  Datagram编程（2）

如下为利用Datagram编写的用户端程序：

 

//这个程序存在本书配套资源目录Ch23名为DatagramClientTest.java
import java.io. *;
import java.net.*;
import java.util.*;
public class DatagramClientTest {
    public static void main(String[] args) {
        try {
            DatagramSocket socket = new DatagramSocket();
                                                //创建Datagram插座
            byte[] buf = new byte[256];         //创建缓冲
            InetAddress address = InetAddress.getByName("127.0.0.1");                                                   //利用本地计算机
            sending(socket, buf, buf.length, address, 1688);
                                                //通过端口1688发送空邮包
             String received = receiving(socket, buf, buf.length);
                                                //接收服务器邮包
            System.out.println(received);       //打印内容，即慰问和提示
            Scanner sc = new Scanner(System.in);//创建键盘输入扫描
            while (sc.hasNextLine()) {          //如果有键盘输入，则继续
                String line = sc.nextLine();    //得到键入内容
                if (!line.trim().equals("quit")) {//如果不是停止
                    buf = new byte[256];        //清除缓冲
                    buf = line.getBytes();      //将键入内容装入缓冲
                    sending(socket, buf, buf.length, address, 1688);
                                                //调用发送方法
                    received = receiving(socket, buf, buf.length);
                                                //接收服务器发来的邮包
                    System.out.println(received);//打印
                    buf = new byte[256];        //清除缓冲
                    sending(socket, buf, buf.length, address, 1688);
                                                //发送空邮包
                    received = receiving(socket, buf, buf.length);
                                                    //接收邮件长度信息
                    System.out.println(received);   //打印这个信息
                }
                else break;                         //中断循环
            }
            socket.close();                         //关闭
        }
        catch (IOException e) {
            e.printStackTrace();
        }
    }
    //自定义静态方法发送邮包至服务器
    public static void sending(DatagramSocket socket, byte[] buf, int length,   InetAddress address, int port) {
        DatagramPacket sendPacket = new DatagramPacket(buf, length, address,        port);
        try {
            socket.send(sendPacket);                //调用发送
        }catch (IOException e) {
            e.printStackTrace();
        }
    }
    //自定义静态方法接收从服务器发来的邮包
    public static String receiving(DatagramSocket socket, byte[] buf, int   length) {
        DatagramPacket receivePacket = new DatagramPacket(buf, length);
        String received = null;
        try {
            socket.receive(receivePacket);          //调用接收
            received = new String(receivePacket.getData(), 0,                       receivePacket.getLength());             //得到信息
        } catch (IOException e) {
            e.printStackTrace();
        }
        return received;
    }
 }

 

可以看到，用户和服务器通过邮包进行通讯和数据传递。当用户需要得到服务器发送过来的信息时，首先发送一个空邮包给服务器，然后服务器利用这个邮包，将数据发还给用户。如果用户需要将发给服务器的信息转换为大写字母时，也首先将这个信息通过邮包发给服务器，经过处理后，服务器利用这个邮包将新内容发还给用户。
    如同上一小节讨论过的远程用户-服务器运行模拟，将这个例子的用户端代码中本地计算机IP地址127.0.0.1修改为作为服务器的远程计算机IP地址，如192.168.15.101，则可进行联网运行。具体步骤可参考上一小节的例子。（续完）