客户端通过键盘录入用户名。
服务端对这个用户名进行校验

如果该用户存在，在服务端显示 xxx，已登录。
并在客户端显示 xxx，欢迎光临。

如果该用户存在，在服务端显示xxx，尝试登录。
并在客户端显示 xxx，该用户不存在。

最多登录三次。

import java.io.*;
import java.net.*;
class LoginClient
{
    public static void main(String[] args) throws Exception
    {
        Socket s = new Socket("192.168.1.108",10007);

        BufferedReader bufr = 
            new BufferedReader(new InputStreamReader(System.in));

        PrintWriter out = new PrintWriter(s.getOutputStream(),true);

        BufferedReader bufIn = 
            new BufferedReader(new InputStreamReader(s.getInputStream()));

        for(int x=0; x<3; x++)
        {
            String line = bufr.readLine();
            if(line==null)
                break;

            out.println(line);

            String info = bufIn.readLine();
            System.out.println("info:"+info);
            if(info.contains("欢迎"))
                break;
        }

        bufr.close();
        s.close();
    }
}

class UserThread implements Runnable
{
    private Socket s;
    UserThread(Socket s)
    {
        this.s = s;
    }
    public void run()
    {
        String ip = s.getInetAddress().getHostAddress();
        System.out.println(ip+"....connected");
        try
        {
            for(int x=0; x<3; x++)
            {
                BufferedReader bufIn = new BufferedReader(new InputStreamReader(s.getInputStream()));

                String name = bufIn.readLine();
                if(name==null)
                    break;

                BufferedReader bufr = new BufferedReader(new FileReader("user.txt"));
                PrintWriter out = new PrintWriter(s.getOutputStream(),true);
                String line = null;

                boolean flag = false;
                while((line=bufr.readLine())!=null)
                {
                    if(line.equals(name))
                    {
                        flag = true;
                        break;
                    }
                }
                if(flag)
                {
                    System.out.println(name+",已登录");
                    out.println(name+"欢迎光临");
                    break;
                }
                else
                {
                    System.out.println(name+",尝试登录");
                    out.println(name+"用户不存在");
                }
            }
            s.close();
        }
        catch (Exception e)
        {
            throw new RuntimeException(ip+"校验失败");
        }
    }
}

class LoginServer
{
    public static void main(String[] args) throws Exception
    {
        ServerSocket ss = new ServerSocket(10007);
        while(true)
        {
            Socket s = ss.accept();
            new Thread(new UserThread(s)).start();
        }
    }
}