客户端 java.net.Socket
服务端 java.net.ServerSocket

客户端和服务端直接是逻辑连接，其中包含一个IO对象(字节流对象), 服务端用客户端的流跟客户端交互。

serversocket.accept()获取客户端的Socket请求对象

Server端

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package file.upload;

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

public class FileUploadServer {
    public static void main(String[] args) throws IOException {
        ServerSocket server = new ServerSocket(8000);
        //获取客户端socket对象
       while(true){
           //获取客户端Socket对象
           Socket accept = server.accept();
           //网络字节输入流获取客户端数据
           new Thread(new Runnable() {
               //一个客户端连接，建立一个线程
               @Override
               public void run() {
                   try{
                       InputStream inputStream = accept.getInputStream();
                       //判断upload文件夹是否存在
                       File file = new File("D:\\upload");
                       if(!file.exists()){
                           file.mkdirs();
                       }
                       FileOutputStream fileOutputStream = new FileOutputStream(file +"\\"+System.currentTimeMillis()+".gif");

                       int len = 0;
                       byte[] bytes = new byte[1024 * 10];
                       while ((len = inputStream.read(bytes)) != -1) {
                           //写入本地
                           fileOutputStream.write(bytes, 0, len);
                       }

                       accept.getOutputStream().write("接收完毕".getBytes());
                       fileOutputStream.close();

                   }catch (IOException e){
                       e.printStackTrace();
                   }finally {
                       try {
                           accept.close();
                       }catch (IOException e){
                           e.printStackTrace();
                       }
                   }

               }
           });

       }
//        server.close();
    }
}

Client

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package file.upload;

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

public class FileUploadClient {
    public static void main(String[] args) throws IOException {
        FileInputStream file = new FileInputStream("D:\\vim.gif");
        byte[] bytes = new byte[1024 * 10];
        int readLen = 0;

        Socket socket = new Socket("127.0.0.1", 8000);

        OutputStream outputStream = socket.getOutputStream();
        while ((readLen = file.read(bytes)) != -1) {//=-1读取不到结束标记
            //网络字节输出流 上传到服务器
            outputStream.write(bytes,0,readLen);
        }
        //禁用此套接字的输出流。对于TCP任何以前写入的数据都会被发送并且后面跟TCP的正常终止序列
        socket.shutdownOutput();

        //获取服务器输入流 接收服务器反馈
        InputStream inputStream = socket.getInputStream();
        while ((readLen = inputStream.read(bytes)) != -1) {
            System.out.println(new String(bytes, 0, readLen));
        }

        file.close();
        socket.close();
    }
}