三次握手，四次挥手

Socket

群聊

客户端

package com.atguigu.net;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.PrintStream;
import java.net.Socket;
import java.net.UnknownHostException;
import java.util.Scanner;

/*
 * 群聊
 */
public class Client4 {
	public static void main(String[] args) throws UnknownHostException, IOException {
		// 1、连接服务器
		Socket socket = new Socket("127.0.0.1", 9999);

		// 2、开启两个线程，一个收消息，一个发消息
		SendThread st = new SendThread(socket);
		ReceiveThread rt = new ReceiveThread(socket);

		st.start();
		rt.start();

		// 等发送线程停下来再往下走
		try {
			st.join();
		} catch (InterruptedException e) {
			e.printStackTrace();
		}

		// 等接收线程停下来，再往下走，断开连接
		try {
			rt.join();
		} catch (InterruptedException e) {
			e.printStackTrace();
		}

		socket.close();
	}

	static class SendThread extends Thread {
		private Socket socket;

		public SendThread(Socket socket) {
			super();
			this.socket = socket;
		}

		public void run() {
			try {
				// 键盘输入
				Scanner input = new Scanner(System.in);
				OutputStream out = socket.getOutputStream();
				PrintStream ps = new PrintStream(out);
				while (true) {
					// 从键盘输入
					System.out.print("请输入要发送的消息：");
					String content = input.nextLine();

					// 给服务器发送
					ps.println(content);

					// 如果bye，就结束发送
					if ("bye".equals(content)) {
						break;
					}
				}
				input.close();
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
	}

	static class ReceiveThread extends Thread {
		private Socket socket;

		public ReceiveThread(Socket socket) {
			super();
			this.socket = socket;
		}

		public void run() {
			try {
				InputStream in = socket.getInputStream();
				InputStreamReader isr = new InputStreamReader(in);
				BufferedReader br = new BufferedReader(isr);

				while (true) {
					String line = br.readLine();
					if("bye".equals(line)){
						break;
					}
					System.out.println(line);
				}
			} catch (IOException e) {
				e.printStackTrace();
			}
		}
	}
}

服务端

package com.atguigu.net;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.PrintStream;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.ArrayList;
import java.util.Iterator;

public class Server4 {
	private static ArrayList<Socket> online = new ArrayList<Socket>();
	
	public static void main(String[] args) throws IOException {
		//1、开启服务器
		ServerSocket server = new ServerSocket(9999);
		
		while(true){
			//2、接收客户端的连接
			Socket socket = server.accept();
			
			//把这个客户端加入到online中
			online.add(socket);
			
			//每一个客户端独立的线程
			MessageHandler mh = new MessageHandler(socket);
			mh.start();
		}
	}

	private static class MessageHandler extends Thread{
		private Socket socket;
		private String ip;
		
		public MessageHandler(Socket socket) {
			super();
			this.socket = socket;
			this.ip = socket.getInetAddress().getHostAddress();
		}

		public void run(){
			//这个客户端的一连接成功，线程一启动，就可以告诉其他人我上线了
			sendToOthers(ip+"上线了");
			
			//（1）接收当前的客户端发送的消息
			try {
				InputStream in = socket.getInputStream();
				InputStreamReader isr = new InputStreamReader(in);
				BufferedReader br = new BufferedReader(isr);
				
				String content;
				while((content = br.readLine()) !=null){
					//收到一句，转发一句
					sendToOthers(ip+"说:" + content);
					
					if("bye".equals(content)){
						//给自己发一句bye
						OutputStream out = socket.getOutputStream();
						PrintStream ps = new PrintStream(out);
						ps.println("bye");
						
						break;
					}
				}
				sendToOthers(ip+"下线了");
			} catch (IOException e) {
				sendToOthers(ip+"掉线了");
			}
		}
		
		//因为转发的代码也很长，独立为一个方法
		public void sendToOthers(String str){
			//遍历所有online的客户端
			Iterator<Socket> iterator = online.iterator();
			while(iterator.hasNext()){
				Socket on = iterator.next();
				if(!on.equals(socket)){//只给其他客户端转发
					try {
						OutputStream out = on.getOutputStream();
						PrintStream ps = new PrintStream(out);
						
						ps.println(str);
					} catch (IOException e) {
						//说明on这个客户端要么下线了，要么掉线了
						iterator.remove();
					}
				}
			}
		}
	}	
}

UDP广播

package com.atguigu.net;

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.InetAddress;
import java.net.MulticastSocket;
import java.util.Scanner;

public class GuangBo {
	private volatile static boolean exit = false;
	private static Scanner input = new Scanner(System.in);

	public static void main(String[] args) throws IOException {
		MulticastSocket socket = new MulticastSocket(9999);
		InetAddress ip = InetAddress.getByName("230.0.0.1");
		socket.joinGroup(ip);
		socket.setLoopbackMode(false);

		SendThread s = new SendThread(socket, ip);
		ReceiveThread r = new ReceiveThread(socket);

		s.start();
		r.start();

		try {
			s.join();
		} catch (InterruptedException e) {
			e.printStackTrace();
		}

		socket.close();
		input.close();
	}

	static class SendThread extends Thread {
		private MulticastSocket socket;
		private InetAddress ip;

		public SendThread(MulticastSocket socket, InetAddress ip) {
			super();
			this.socket = socket;
			this.ip = ip;
		}

		public void run() {
			try {
				while (!exit) {
					System.out.print("输入广播消息：");
					String message = input.nextLine();
					if ("bye".equals(message)) {
						exit = true;
						break;
					}

					byte[] data = (socket.getInetAddress().getHostAddress() + ":" + message).getBytes();
					DatagramPacket dp = new DatagramPacket(data, data.length, ip, 9999);

					socket.send(dp);
				}
			} catch (IOException e) {
				e.printStackTrace();
			}
		}

	}

	static class ReceiveThread extends Thread {
		private MulticastSocket socket;

		public ReceiveThread(MulticastSocket socket) {
			super();
			this.socket = socket;
		}

		public void run() {
			try {
				while (!exit) {
					byte[] data = new byte[1024];
					DatagramPacket dp = new DatagramPacket(data, data.length);
					socket.receive(dp);
					String str = new String(data, 0, dp.getLength());
					System.out.println(str);
				}
			} catch (IOException e) {
				exit = false;
			}
		}
	}
}