java网络编程，简单的客户端和服务器端

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.ServerSocket;
import java.net.Socket;

public class HelloWorldServer {
    
    public static void main(String[] args) {
        ServerSocket server = null;
        BufferedReader reader = null;
        Socket client = null;
        try {
            server = new ServerSocket(9000);//创建ServerSocket对象，并绑定端口
            client = server.accept();//调用accept方法，等待客户端的连接
            InputStream in = client.getInputStream();
            reader = new BufferedReader(new InputStreamReader(in));
            String str = reader.readLine();
            System.out.println(str);
        } catch (IOException e) {
            e.printStackTrace();
        }finally{
            try{
                if(client!=null){
                    client.close();
                }
            }catch(IOException e){
                e.printStackTrace();
            }
        }
    }

}

import java.io.IOException;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.net.Socket;
import java.net.UnknownHostException;
import java.util.Scanner;

public class HelloWorldClient {
    
    public static void main(String[] args) {
        Socket client = null;
        PrintWriter writer = null;
        try {
            client = new Socket("www.feifeishi.cn", 9000);//www.feifeishi.cn是我的云服务器
            OutputStream out = client.getOutputStream();
            writer = new PrintWriter(out);
            Scanner sc=new Scanner(System.in);
            String aa=sc.nextLine();
            writer.print(aa);
            writer.flush();    
        } catch (UnknownHostException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }finally{
            try{
                if(writer!=null){
                    writer.close();
                }
                if(client!=null){
                    client.close();
                }
            }catch(IOException e){
                e.printStackTrace();
            }
        }
    }

}