我正在尝试通过Android中的Java套接字实现文件传输。 它工作正常，但当我关闭应用程序时，它崩溃，控制台显示nullPointerException。 我已经尝试了一切，但无法找到问题！ 我已经将ServerSocket定义为类变量，并在检查服务器套接字的状态后将其关闭在onDestroy()方法中。 这是我的代码：

package com.vinit.airdrive;

import java.io.BufferedInputStream;

import java.io.DataInputStream;

import java.io.DataOutputStream;

import java.io.File;

import java.io.FileInputStream;

import java.io.FileNotFoundException;

import java.io.IOException;

import java.io.OutputStream;

import java.net.ServerSocket;

import java.net.Socket;

import java.util.ArrayList;

import android.annotation.SuppressLint;

import android.app.Activity;

import android.os.Bundle;

import android.os.Environment;

import android.util.Log;

//Latest saved stub

@SuppressLint("SdCardPath")

public class MainActivity extends Activity {

Socket socket=null;

ServerSocket serversocket=null;

ArrayList fileArray=null;

@SuppressLint("SdCardPath")

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

File file1=new File(Environment.getExternalStorageDirectory()+"/airdrive/Sphere.pdf");

File file2=new File(Environment.getExternalStorageDirectory()+"/airdrive/Robinson.pdf");

File file3=new File(Environment.getExternalStorageDirectory()+"/airdrive/Conspiracy.pdf");

File file4=new File(Environment.getExternalStorageDirectory()+"/airdrive/Expedition.pdf");

File file5=new File(Environment.getExternalStorageDirectory()+"/airdrive/Xperia.pdf");

fileArray=new ArrayList();

fileArray.add(file1);

fileArray.add(file2);

fileArray.add(file3);

fileArray.add(file4);

fileArray.add(file5);

Thread trd = new Thread(new Runnable(){

@Override

public void run(){

initializeServer();

for (int i=0;i<=4;i++){

copyFile(fileArray.get(i), fileArray.get(i).getName());

}

}

});

trd.start();

}

private void initializeServer() {

try {

serversocket=new ServerSocket(4444);

} catch (IOException e) {

e.printStackTrace();

Log.d("Listen failed", "Couldn't listen to port 4444");

}

try {

socket=serversocket.accept();

} catch (IOException e) {

// TODO Auto-generated catch block

e.printStackTrace();

Log.d("Acceptance failed","couldn't accept the server socket connection");

}

}

private void copyFile(File file, String name) {

FileInputStream fis;

long filesize=file.length();

try {

fis = new FileInputStream(file);

BufferedInputStream bis = new BufferedInputStream(fis);

@SuppressWarnings("resource")

DataInputStream dis = new DataInputStream(bis);

byte[] mybytearray = new byte[16384];

OutputStream os;

os=socket.getOutputStream();

DataOutputStream dos = new DataOutputStream(os);

dos.writeUTF(name); //filename is also sent to client

dos.writeLong(filesize); //file size is also sent to client

long z=filesize;

int n=0;

while((z > 0) && (n = dis.read(mybytearray, 0, (int)Math.min(mybytearray.length, z))) != -1){

dos.write(mybytearray,0,n);

dos.flush();

z -= n;

}

} catch (FileNotFoundException e1) {

// TODO Auto-generated catch block

e1.printStackTrace();

} catch (IOException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

}

@Override

protected void onDestroy() {

// TODO Auto-generated method stub

super.onDestroy();

if (serversocket != null) {

try {

serversocket.close();

} catch (IOException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

}

}

}

可能是什么问题呢？ 请帮忙！

编辑：控制台输出：

03-22 01:35:38.630: E/AndroidRuntime(12302): FATAL EXCEPTION: Thread-1212

03-22 01:35:38.630: E/AndroidRuntime(12302): java.lang.NullPointerException

03-22 01:35:38.630: E/AndroidRuntime(12302): at com.vinit.airdrive.MainActivity.copyFile(MainActivity.java:83)

03-22 01:35:38.630: E/AndroidRuntime(12302): at com.vinit.airdrive.MainActivity.access$1(MainActivity.java:73)

03-22 01:35:38.630: E/AndroidRuntime(12302): at com.vinit.airdrive.MainActivity$1.run(MainActivity.java:50)

03-22 01:35:38.630: E/AndroidRuntime(12302): at java.lang.Thread.run(Thread.java:856)

EDIT2：

@Override

protected void onDestroy() {

// TODO Auto-generated method stub

if (serversocket != null && socket==null) {

try {

serversocket.close();

} catch (IOException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

}

super.onDestroy();

}