实现的功能

客户端发送消息到服务端，服务端收到后打印该消息并给客户端反馈信息

基本思路

使用类 ServerSocket，创建一个绑定到特定窗口的服务器套接字。
然后用它的accept方法侦听客户端对其的连接，并接受其发送的Socket。
Socket有一个获取输入输出流的方法。
如果要发送消息，使用输出流。
如果要接受消息，使用输入流。

服务端

import java.io.*;
import java.net.ServerSocket;
import java.net.Socket;

public class UpperServer {
    public static void main(String[] args) {
        try {
            ServerSocket ss = new ServerSocket(23333);
            Socket s = ss.accept();
            BufferedReader bufIn = new BufferedReader(new InputStreamReader(s.getInputStream()));
            BufferedWriter bufOut = new BufferedWriter(new OutputStreamWriter(s.getOutputStream()));
            PrintWriter out = new PrintWriter(bufOut, true);
            System.out.println("From: " + s.getInetAddress().getHostAddress());
            String str;
            //readLine方法需要读到换行符
            //缓冲区需要使用flush刷新才能发送
            while ((str = bufIn.readLine()) != null) {
                System.out.println(str);
                out.println(str.toUpperCase());
                /*
                bufOut.write(str.toUpperCase());
                bufOut.newLine();
                bufOut.flush();
                */
            }
            s.close();
            ss.close();

        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

客户端

import java.io.*;
import java.net.Socket;
import java.net.UnknownHostException;

public class UpperClient {
    public static void main(String[] args) {
        try {
            Socket s = new Socket("192.168.149.1", 23333);
            BufferedReader bufr = new BufferedReader(new InputStreamReader(System.in));
            BufferedWriter bufOut = new BufferedWriter(new OutputStreamWriter(s.getOutputStream()));
            PrintWriter out = new PrintWriter(bufOut, true);
            BufferedReader bufIn = new BufferedReader(new InputStreamReader(s.getInputStream()));
            String line;
            String str;
            while ((line = bufr.readLine()) != null) {
                //bufOut.write(line);
                //bufOut.newLine();
                // bufOut.flush();
                if (line.equals("over")) {
                    break;
                }
                out.println(line);
                str = bufIn.readLine();
                System.out.println("Server: " + str);
            }
            s.close();
        } catch (UnknownHostException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

END