UDP套接字处理

Question

我正在处理UDP Socket的服务器端,每次都在线上遇到NullPointerException DatagramPacket receivePacket = new DatagramPacket(receiveData, 4096);.客户端是Python文件.这是服务器的代码.这些方法setup()和draw()通过不同的加工文件调用.

import processing.net.*;
import java.io.*;
import java.net.*;
import java.util.*;

//Server myServer;
DatagramSocket serverSocket;
byte[] receiveData;
InetAddress IPAddressList;
int portList = -1;

void setup(){
  try{
      DatagramSocket serverSocket = new DatagramSocket(21567);
  }
  catch (IOException e){
    e.printStackTrace();
    System.out.println(e);
  }

   byte[] receiveData = new byte[4096];
}

void draw(){

  float P1,P2,P3;
  print ("hello");
  try{
     DatagramPacket receivePacket = new DatagramPacket(receiveData, 4096);
     serverSocket.receive(receivePacket);
     String greeting = new String(receivePacket.getData());
     System.out.println("From Client: " + greeting);
     IPAddressList = receivePacket.getAddress();
     portList= receivePacket.getPort();
     P1 = Integer.valueOf(greeting);
     print(P1);
     print (greeting);
   }
   catch (IOException e){
    System.out.println(e.getMessage());
  }

  for (int i=0;i<receiveData.length;i++){
    print(receiveData[i]);
  }
}

Answer 1

你指出的那条线NullPointerException非常有帮助.

问题是您初始化了局部变量 receiveData而不是外部范围中的字段 receiveData.

为了解决方法是只需更换线路byte[] receiveData = new byte[4096];用receiveData = new byte[4096];.

通常,这称为名称阴影:

• http://en.wikipedia.org/wiki/Variable_shadowing
• http://java.sun.com/docs/books/jls/third_edition/html/names.html#6.3.1