netcat或nc将空字符发送到远程服务器

Question

您好我正在尝试使用以下命令将一些字符串发送到远程服务器：

    ##1.2.3.4 is IP address of remote server
    echo "hey" | nc 1.2.3.4 7728
    >> hey
    >> null

它不会出现循环，而在远程服务器中我遇到空指针异常。有人可以帮我解决这个错误吗？

NPE：

New connection with client# 2 at Socket[addr=/1.2.3.4,port=55757,localport=7728]
Exception in thread "Thread-2" java.lang.NullPointerException
    at org.o7planning.tutorial.socketthread.ServerProgram$ServiceThread.run(ServerProgram.java:85)

需要知道：

1. 如何摆脱命令？
2. 如何避免空指针异常。

PS：客户端在Linux上运行。和服务器是基于Java的。

服务器的代码：

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.net.ServerSocket;
import java.net.Socket;

public class ServerProgram {

   public static void main(String args[]) throws IOException {

       ServerSocket listener = null;

       System.out.println("Server is waiting to accept user...");
       int clientNumber = 0;

       // Try to open a server socket on port 7777
       // Note that we can't choose a port less than 1023 if we are not
       // privileged users (root)

       try {
           listener = new ServerSocket(7722);
       } catch (IOException e) {
           System.out.println(e);
           System.exit(1);
       }

       try {
           while (true) {
               // Accept client connection request
               // Get new Socket at Server.

               Socket socketOfServer = listener.accept();
               new ServiceThread(socketOfServer, clientNumber++).start();
           }
       } finally {
           listener.close();
       }

   }

   private static void log(String message) {
       System.out.println(message);
   }

   private static class ServiceThread extends Thread {

       private int clientNumber;
       private Socket socketOfServer;

       public ServiceThread(Socket socketOfServer, int clientNumber) {
           this.clientNumber = clientNumber;
           this.socketOfServer = socketOfServer;

           // Log
           log("New connection with client# " + this.clientNumber + " at " + socketOfServer);
       }

       @Override
       public void run() {

           try {

               // Open input and output streams
               BufferedReader is = new BufferedReader(new InputStreamReader(socketOfServer.getInputStream()));
               BufferedWriter os = new BufferedWriter(new OutputStreamWriter(socketOfServer.getOutputStream()));

               while (true) {
                   // Read data to the server (sent from client).
                   String line = is.readLine();

                   // Write to socket of Server
                   // (Send to client)                    
                   os.write(">> " + line);
                   // End of line.
                   os.newLine();
                   // Flush data.
                   os.flush();


                   // If users send QUIT (To end conversation).
                   if (line.equals("QUIT")) {
                       os.write(">> OK");
                       os.newLine();
                       os.flush();
                       break;
                   }
               }

           } catch (IOException e) {
               System.out.println(e);
               e.printStackTrace();
           }
       }
   }
}