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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);}
}