安卓：openFileOutput抛出NullPointerException异常

问题描述

我想字符串写入文件和我使用openFileOutput：

I'm trying to write Strings into a file and for that i use openFileOutput:

 FileOutputStream fOut =  openFileOutput("samplefile.txt",Context.MODE_PRIVATE);

由于某些原因，应用程序崩溃是由于一个NullPointerException异常
（我看，它发生在模拟器上，而不是实际的设备，所以我迷上我的电话，你猜怎么上 - CRASHED TOO :-(）

For some reason the app crashes due to a NullPointerException(I read that it happens on the emulator and not on the actual device so I hooked my phone and guess what - CRASHED TOO :-( )

这里是LogCat中输出：

here is the LogCat output:

04-18 22:48:25.520: E/AndroidRuntime(12045): FATAL EXCEPTION: main
04-18 22:48:25.520: E/AndroidRuntime(12045): java.lang.NullPointerException
04-18 22:48:25.520: E/AndroidRuntime(12045):    at    android.content.ContextWrapper.openFileOutput(ContextWrapper.java:165)
04-18 22:48:25.520: E/AndroidRuntime(12045):    at     com.example.tester.GenerateXml.listToTextFile(GenerateXml.java:48)
04-18 22:48:25.520: E/AndroidRuntime(12045):    at com.example.tester.MainActivity$1.parseAppListToXML(MainActivity.java:81)
04-18 22:48:25.520: E/AndroidRuntime(12045):    at com.example.tester.MainActivity$1.onClick(MainActivity.java:62)
04-18 22:48:25.520: E/AndroidRuntime(12045):    at android.view.View.performClick(View.java:3517)
04-18 22:48:25.520: E/AndroidRuntime(12045):    at android.view.View$PerformClick.run(View.java:14155)
04-18 22:48:25.520: E/AndroidRuntime(12045):    at android.os.Handler.handleCallback(Handler.java:605)
04-18 22:48:25.520: E/AndroidRuntime(12045):    at android.os.Handler.dispatchMessage(Handler.java:92)
04-18 22:48:25.520: E/AndroidRuntime(12045):    at android.os.Looper.loop(Looper.java:154)
04-18 22:48:25.520: E/AndroidRuntime(12045):    at android.app.ActivityThread.main(ActivityThread.java:4624)
04-18 22:48:25.520: E/AndroidRuntime(12045):    at java.lang.reflect.Method.invokeNative(Native Method)
04-18 22:48:25.520: E/AndroidRuntime(12045):    at java.lang.reflect.Method.invoke(Method.java:511)
04-18 22:48:25.520: E/AndroidRuntime(12045):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:809)
04-18 22:48:25.520: E/AndroidRuntime(12045):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:576)
04-18 22:48:25.520: E/AndroidRuntime(12045):    at dalvik.system.NativeStart.main(Native Method)

这是我的code：

here is my code:

package com.example.tester;

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.util.List;

import android.app.Activity;
import android.content.Context;
import android.content.pm.ApplicationInfo;
import android.util.Log;
import android.util.Xml;

public class GenerateXml extends Activity{

private List<ApplicationInfo> packages;
private static final String TAG = MainActivity.class.getName();
private static final String FILENAME = "myFile.txt";

public void listToTextFile(List<ApplicationInfo> _packages) {


    try { // catches IOException below
         final String TESTSTRING = _packages.toString();

         // ##### Write a file to the disk #####
         /* We have to use the openFileOutput()-method
          * the ActivityContext provides, to
          * protect your file from others and
          * This is done for security-reasons.
          * We chose MODE_WORLD_READABLE, because
          *  we have nothing to hide in our file */
      //

         FileOutputStream fOut =       openFileOutput("samplefile.txt",Context.MODE_PRIVATE);

         OutputStreamWriter osw = new OutputStreamWriter(fOut);

         // Write the string to the file
         osw.write(TESTSTRING);
         /* ensure that everything is
          * really written out and close */
         osw.flush();
         osw.close();
     }catch (IOException e){
         //Log.e(TAG,"could not open file out stream", e);
     }
}

}

任何想法？

推荐答案

好像你已经与实例化你的 GenerateXml 活动新GenerateXml（） 。无法实例活动的方式 - 他们将无法正确设置为上下文和等

Seems like you've instantiated your GenerateXml activity with new GenerateXml(). You cannot instantiate activities that way - they won't be properly set up as Contexts and so on.

从code你张贴，它看起来像 GenerateXml 在所有的不应该是一个活动。删除延伸活动，并通过在上下文如在需要的地方的参数，例如：

From the code you posted, it seems like GenerateXml should not be an Activity at all. Remove the extends Activity and pass in a Context as an argument where it's needed, for example:

public void listToTextFile(Context context, List<ApplicationInfo> _packages) {
    //...
    ... context.openFileOutput(...);

这篇关于安卓：openFileOutput抛出NullPointerException异常的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！