效果图：

一，学习UDP的简单使用步骤

接收端：

 DatagramSocket socket = null;
        try {
            //1.开启自己的端口（客户端连接的端口）
            socket = new DatagramSocket(9999);
            //2.接收数据
            byte[] buffer = new byte[1024];
            DatagramPacket packet = new DatagramPacket(buffer,0,buffer.length);
            socket.receive(packet);//堵塞接收
            //3.处理数据
            byte[] target = packet.getData();
            int len=0;
            if (target.length!=0){
                for (int i=0;i<target.length;i++){
                    if (target[i]=='\0'){
                        len=i;
                        break;
                    }
                }

            }
            //4.输出数据
            System.out.println("from:"+packet.getAddress());
            System.out.println(new String(target,0,len));

        } catch (Exception e) {
            e.printStackTrace();
        }finally {
            if (socket!=null){
                socket.close();
            }
        }

发送端：

		//1.建立连接
        DatagramSocket socket = null;
        //2.获得连接对象
        //获取对方的IP和端口号进行连接
        InetAddress Ip = null;
        int port = 9999;
        //4.创建数据包
        String msg = "你好";
        DatagramPacket packet = null;
        try {
            socket = new DatagramSocket();
            Ip = InetAddress.getByName("localhost");
            //数据,数据的长度起始，发送的地址
            packet = new DatagramPacket(msg.getBytes(),0,msg.getBytes().length,Ip,port);
            //5.发送包
            socket.send(packet);

        } catch (Exception e) {
            e.printStackTrace();
        }finally {
            //6.关闭资源
            if (socket != null){
                socket.close();
            }
        }

二，多线程实现UDP在线聊天

1.创键服务器端

package chat;

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

public class ServerThread implements Runnable{
    private final String theOther;//对方的姓名
    private final int selfPort;//自己开放的的端口号
    private DatagramSocket socket;
    public ServerThread(int selfPort,String theOther){//构造器
        this.selfPort = selfPort;
        this.theOther = theOther;
        try {
            //开启服务器
            socket = new DatagramSocket(selfPort);
        } catch (SocketException e) {
            e.printStackTrace();
        }
    }
    public void run() {
        //接收数据
        byte[] container;
        DatagramPacket packet;//包
        byte[] date;
        String receiveMsg = "";//信号灯
        int len = 0;
        try {
            while (!receiveMsg.equals("bye")){//如果对方说：bye，则退出聊天
                container = new byte[1024];
                packet = new DatagramPacket(container,0,container.length);
                socket.receive(packet);//接收数据包
                date =packet.getData();//获取数据
                //处理数据
                for (int i = 0; i < date.length; i++) {
                    if (date[i]=='\0'){
                        len = i;
                        break;
                    }
                }
                //将收到的数据转换为字符串
                receiveMsg = new String(date,0, len, StandardCharsets.UTF_8);
                System.out.println(theOther+":"+receiveMsg);
            }
//              保存聊天记录（扩展内容）
//              FileOutputStream out = new FileOutputStream(new File(theOther+"的聊天记录.txt"));
//              out.write(receiveMsg.getBytes(StandardCharsets.UTF_8));

        } catch (IOException e) {
            e.printStackTrace();
        }finally {
            //关闭服务器
            if (socket != null){
                socket.close();
            }

        }

    }
}

2.建立客户端

package chat;

import java.io.BufferedInputStream;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.*;
import java.nio.charset.StandardCharsets;

public class ClientThread implements Runnable{
     private DatagramSocket socket = null;
     private BufferedReader bufferedReader= null;//缓冲流
     private final String toIp;//对方的IP
     private final int toPort;//对方的端口号

    public ClientThread(String toIp, int toPort) {
        this.toIp = toIp;
        this.toPort = toPort;
        try {
            socket = new DatagramSocket();
            bufferedReader = new BufferedReader(new InputStreamReader(System.in));//获取键盘输入
        } catch (SocketException e) {
            e.printStackTrace();
        }
    }

    public void run() {
        String SendMsg = "";//发视频美好的消息
        byte[] date;
        DatagramPacket packet;
        try {
        while(!SendMsg.equals("bye")) {//自己发送‘bye’时关闭发送端

            SendMsg = bufferedReader.readLine();//读取键盘输入内容
            date = SendMsg.getBytes(StandardCharsets.UTF_8);//字符串转换为字节进行传输
            packet = new DatagramPacket(date,0,date.length,new InetSocketAddress(toIp,toPort));
            socket.send(packet);//发送数据
        }
        } catch (IOException e) {
                e.printStackTrace();
        }finally {
            //关闭数据流
            if (bufferedReader!=null){
                try {
                    bufferedReader.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if(socket!=null){
                socket.close();
            }

        }

    }
}

3.用户使用：

用户1：

package chat;

public class User01 {
    public static void main(String[] args) {
        System.out.println("小明的聊天室！！！");
        new Thread(new ServerThread(9000,"小红")).start();//开启接收
        new Thread(new ClientThread("localhost",9001)).start();//开启发送
    }
}

用户2：

package chat;

public class User02 {
    public static void main(String[] args) {
        System.out.println("小红的聊天室！！！");
        new Thread(new ServerThread(9001,"小明")).start();
        new Thread(new ClientThread("localhost",9000)).start();
    }
}

多人聊天思路：
用户—>公共服务器接收端打包数据–>服务器发送端发送给各个用户—>用户