服务端

import java.io.*;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class MutilServerDemo {
    public static void main(String[] args){
        ExecutorService es = Executors.newFixedThreadPool(3);//创建一个固定线程数的线程池,每接收一个客户端的请求就生成一个线程，因此先创建一个线程池

        try {
            ServerSocket server = new ServerSocket(6666);
            System.out.println("服务器已启动，等待客户端连接...");

            while(true){ //用一个循环，去循环的接收客户端发送过来的请求，每一个客户端的请求就new一个线程
                Socket s = server.accept();  //等待客户端的连接，有一个连接就执行下面的new一个线程任务，所以当有多个客户端连接时就有多个线程
                System.out.println(s.getInetAddress().getHostAddress());
                es.execute(new UserRunnable(s));
            }
        } catch (IOException e) {
            e.printStackTrace();
        }

    }
}

/**
 * 创建自定义线程，用来处理客户端请求的线程任务
 */
class UserRunnable implements Runnable{
    private Socket s;
    public UserRunnable(Socket s){
        this.s=s;
    }
    @Override
    public void run() {
        try {
            //读取客户端发送过来的消息
            BufferedReader br = new BufferedReader(new InputStreamReader(s.getInputStream()));
            String info= br.readLine();
            System.out.println(info);

            //消息重新发送(写入)到客户端
            PrintWriter pw = new PrintWriter(new BufferedOutputStream(s.getOutputStream()));
            pw.println("echo："+info);
            pw.flush();
            pw.close();
            br.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

客户端

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

public class MutilClientDemo {
    public static void main(String[] args){
        Scanner input = new Scanner(System.in);
        try {
            Socket socket = new Socket("localhost",6666);
            PrintWriter pw = new PrintWriter(new BufferedWriter(new OutputStreamWriter(socket.getOutputStream())));
            System.out.println("请输入...");
            String in=input.nextLine();
            pw.println(in);
            pw.flush();

            BufferedReader br = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            String info=br.readLine();
            System.out.println(info);
            pw.close();
            br.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}