这一篇文章我们将讲述网络编程中UDP服务器客户端的编程代码

1、前置知识

UDP协议全称是用户数据报协议，在网络中它与TCP协议一样用于处理数据包，是一种无连接的协议。

UDP的特点有：无连接、尽最大努力交付、面向报文、没有拥塞控制

本文讲的回响服务器是指客户端向服务器发送一个报文，从服务器那里得到一条一模一样的回响报文

该代码需要在同一个包下创建两个类，服务器UdpEchoServer类，客户端UdpEchoClient类

2、服务器端代码

（1）完整代码

先放上完整代码

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.SocketException;
import java.nio.charset.StandardCharsets;

public class UdpEchoServer {
    public static DatagramSocket socket = null;

    public UdpEchoServer(int port) throws SocketException {
        //手动指定服务器端口号
        socket = new DatagramSocket(port);
    }

    public static void start() throws IOException {
        while (true){
            System.out.println("服务器启动");

            DatagramPacket requestPacket = new DatagramPacket(new byte[1024],1024);
            socket.receive(requestPacket);

            String request = new String(requestPacket.getData(),0,requestPacket.getLength());
            String response = process(request);

            DatagramPacket responsePacket = new DatagramPacket(response.getBytes(),response.getBytes().length,requestPacket.getSocketAddress());
            socket.send(responsePacket);

            System.out.printf("[%s:%d] req:%s,resp:%s",requestPacket.getAddress(),requestPacket.getPort(),request,response);
            System.out.println();
        }
    }

    public static String process(String request){
        return request;
    }

    public static void main(String[] args) throws IOException {
        UdpServer server = new UdpServer(9090);
        server.start();
    }

}

（2）代码讲解

构造方法中手动指定服务器端口号

public UdpEchoServer(int port) throws SocketException {
    //手动指定服务器端口号
    socket = new DatagramSocket(port);
}

start()方法中进行信息的接收、处理、发送 

public static void start() throws IOException {
        while (true){
            System.out.println("服务器启动");

            //创建接收报文，用socket进行接收请求报文
            DatagramPacket requestPacket = new DatagramPacket(new byte[1024],1024);
            socket.receive(requestPacket);

            //将请求报文转化为请求字符串，用process()方法进行处理，得到响应字符串
            String request = new String(requestPacket.getData(),0,requestPacket.getLength());
            String response = process(request);

            //将得到的响应字符串转化为响应报文，并用socket进行发送
            DatagramPacket responsePacket = new DatagramPacket(response.getBytes(),response.getBytes().length,requestPacket.getSocketAddress());
            socket.send(responsePacket);

            //打印客户端IP地址和端口号信息
            System.out.printf("[%s:%d] req:%s,resp:%s",requestPacket.getAddress(),requestPacket.getPort(),request,response);
            System.out.println();
        }
}

process()方法

直接返回接收到的字符串

public static String process(String request){
        return request;
}

3、 客户端代码

 （1）完整代码

import java.io.IOException;
import java.net.*;
import java.util.Scanner;

public class UdpEchoClient {
    public static DatagramSocket socket = null;
    private static String ip = null;
    private static int port = 0;

    public UdpEchoClient(String ip,int port) throws SocketException {
        //系统自动为客户端分配端口
        socket = new DatagramSocket();
        this.ip = ip;
        this.port = port;
    }

    public static void start() throws IOException {
        System.out.println("客户端启动！");
        Scanner scanner = new Scanner(System.in);
        while (true){
            System.out.print("->");
            String request = scanner.next();
            DatagramPacket requestPacket = new DatagramPacket(request.getBytes(),request.getBytes().length, InetAddress.getByName(ip),port);
            socket.send(requestPacket);

            DatagramPacket responsePacket = new DatagramPacket(new byte[1024],1024);
            socket.receive(responsePacket);

            String response = new String(responsePacket.getData(),0,responsePacket.getLength());
            System.out.println(response);
        }
    }

    public static void main(String[] args) throws IOException {
        UdpEchoClient client = new UdpEchoClient("127.0.0.1",9090);
        client.start();
    }
}

（2）代码讲解

客户端由于可能有多个，所以需要系统自动为客户端分配端口

当初始化socket时，客户端还需要记下服务器端的ip和端口 

public static DatagramSocket socket = null;
private static String ip = null;
private static int port = 0;

public UdpEchoClient(String ip,int port) throws SocketException {
        //系统自动为客户端分配端口
        socket = new DatagramSocket();
        //记录ip和端口号
        this.ip = ip;
        this.port = port;
}

start()方法 

public static void start() throws IOException {
        System.out.println("客户端启动！");
        Scanner scanner = new Scanner(System.in);
        while (true){
            System.out.print("->");
            String request = scanner.next();

            //将读取到的字符串转化为请求报文，并用socket进行发送
            DatagramPacket requestPacket = new DatagramPacket(request.getBytes(),request.getBytes().length, InetAddress.getByName(ip),port);
            socket.send(requestPacket);

            //创建回应报文来接收响应
            DatagramPacket responsePacket = new DatagramPacket(new byte[1024],1024);
            socket.receive(responsePacket);

            //将响应报文转换成字符串，并打印出来
            String response = new String(responsePacket.getData(),0,responsePacket.getLength());
            System.out.println(response);
        }
}