NIO异常java.nio.BufferUnderflowException

Question

package client;

import java.io.DataInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SocketChannel;
import java.util.concurrent.TimeUnit;

public class NioClient {
    public static void main(String[]args){
         ByteBuffer buffer = ByteBuffer.allocate(1024);
            SocketChannel socketChannel = null;
            ByteBuffer res=ByteBuffer.allocate(1024);
            try
            {
                socketChannel = SocketChannel.open();
                socketChannel.configureBlocking(false);
                socketChannel.connect(new InetSocketAddress("localhost",8080));
                if(socketChannel.finishConnect())
                {
                    int i=0;
                    int size=0;
                    while(true)
                    {
                        TimeUnit.SECONDS.sleep(1);
                        String info = "I'm "+i+++"-th information from client";
                        buffer.clear();
                        buffer.put(info.getBytes());
                        buffer.flip();
                        while(buffer.hasRemaining()){
//                            System.out.println(buffer);
                            socketChannel.write(buffer);
                        }
                        size=socketChannel.read(res);
                        while((size)!=0){
                            res.flip();
                             byte result[]=new byte[size];
                            res.get(result);
                             System.out.println(new String(result));
                             res.clear();
                        }
                    }
                }
            }
            catch (IOException | InterruptedException e)
            {
                e.printStackTrace();
            }
            finally{
                try{
                    if(socketChannel!=null){
                        socketChannel.close();
                    }
                }catch(IOException e){
                    e.printStackTrace();
                }
            }
    }
}

解决方法：
将while循环上的赋值放到while循环里

while((size=socketChannel.read(res))!=0){

为什么会这样，求解释？？？

另附赠服务端代码

package client;

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

import message.IMessage;
import message.LoginMessage;

public class Server2 {
    public static void main(String[] args) throws IOException {
        Server2 socketService = new Server2();
        socketService.oneServer();

    }

    public void oneServer() {
         ServerSocket serverSocket = null;
         InputStream in=null;
         OutputStream out =null;
           try
           {
               System.out.println(" server start");
               serverSocket = new ServerSocket(8080);
               int Size = 0;
               byte[] res = new byte[1024];
               while(true){
                   Socket clntSocket = serverSocket.accept();
                   SocketAddress clientAddress = clntSocket.getRemoteSocketAddress();
                   System.out.println("Handling client at "+clientAddress);
                   in =  clntSocket.getInputStream();
                   out=clntSocket.getOutputStream();
                   int i=0;
                   while((Size=in.read(res))!=-1){
                       byte[] temp = new byte[Size];
                       System.arraycopy(res, 0, temp, 0, Size);
                       System.out.println(new String(temp));
                       String status="success"+i;
                       i++;
                       out.write(status.getBytes());
                       out.flush();
                   }
                  
               }
           }
           catch (IOException e)
           {
               e.printStackTrace();
           }
           finally{
               try{
                   if(serverSocket!=null){
                       serverSocket.close();
                   }
                   if(in!=null){
                       in.close();
                   }
               }catch(IOException e){
                   e.printStackTrace();
               }
           }
    }
}

Answer 1

while((size=socketChannel.read(res))!=0){

上面写的是读完了在读一遍，重新给size赋值，发现size=0,结束；
而while((size)!=0) size 是一个固定的值，你循环里面又没有重新赋值