我有一个带有ImageView的Activity,如果它是空的并且单击,则应该从Internet加载图像并将其保存到内部存储.所以活动看起来像这样：

public class PlaceCreate extends Activity {
Context context;

@Override
public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.create);
  context = this;

  ImageView img = (ImageView) findViewById(R.id.imageView);
  img.setOnClickListener(new OnClickListener() {
    public void onClick(View v) {
        EditText edit = (EditText)findViewById(R.id.editTextName);
        Download(edit.getText().toString());
    }
  });
}

private boolean Download(String Name)
{
  try
  {
    URL u = new URL("http://example.com/img.png");
    HttpURLConnection c = (HttpURLConnection)u.openConnection();
    c.setRequestMethod("GET");
    c.setDoOutput(true);
    c.connect();
    FileOutputStream f = context.openFileOutput(Name, Context.MODE_PRIVATE);
    InputStream in = c.getInputStream();
    byte[] buffer = new byte[1024];
    int len1 = 0;
    while((len1 = in.read(buffer)) > 0 )
    {
      f.write(buffer, 0, len1);
    }
    f.close();

    FileInputStream is = openFileInput(Name);
    Bitmap bitmap = BitmapFactory.decodeStream(is);
    is.close();
    ImageView img = (ImageView) findViewById(R.id.imageViewPlan);
    img.setImageBitmap(bitmap);
  }
  catch(IOException e)
  {
    return false;
  }
  return true;
}}

问题是我在openFileOutput的行上得到了NullPointerException.我发现了几个类似的问题,但所有答案都暗示null是一个缺失的上下文.因为在这种情况下它是一个活动,它显然有一个上下文(this),它不是null.我试图通过上下文成员调用openFileOutput(带有这个暗示).两种方法都失败了.此外,我尝试将on.tatContext()从onClick传递到Download作为附加参数,并且此值不为null,但也产生了相同的异常. (如果重要,请求的文件名是有效的文件名,例如“abc”.)

有人可以对此有所了解吗？

这是一个堆栈的例子：

04-22 00:45:10.659: W/dalvikvm(330): threadid=1: thread exiting with uncaught exception (group=0x40015560)
04-22 00:45:10.909: E/AndroidRuntime(330): FATAL EXCEPTION: main
04-22 00:45:10.909: E/AndroidRuntime(330): java.lang.NullPointerException
04-22 00:45:10.909: E/AndroidRuntime(330):  at android.app.ContextImpl.openFileOutput(ContextImpl.java:420)
04-22 00:45:10.909: E/AndroidRuntime(330):  at android.content.ContextWrapper.openFileOutput(ContextWrapper.java:158)
04-22 00:45:10.909: E/AndroidRuntime(330):  at com.example.scanner.PlaceCreate.Download(PlaceCreate.java:93)
04-22 00:45:10.909: E/AndroidRuntime(330):  at com.example.scanner.PlaceCreate.access$0(PlaceCreate.java:84)
04-22 00:45:10.909: E/AndroidRuntime(330):  at com.example.scanner.PlaceCreate$3.onClick(PlaceCreate.java:67)
04-22 00:45:10.909: E/AndroidRuntime(330):  at android.view.View.performClick(View.java:2485)
04-22 00:45:10.909: E/AndroidRuntime(330):  at android.view.View$PerformClick.run(View.java:9080)
04-22 00:45:10.909: E/AndroidRuntime(330):  at android.os.Handler.handleCallback(Handler.java:587)
04-22 00:45:10.909: E/AndroidRuntime(330):  at android.os.Handler.dispatchMessage(Handler.java:92)
04-22 00:45:10.909: E/AndroidRuntime(330):  at android.os.Looper.loop(Looper.java:123)
04-22 00:45:10.909: E/AndroidRuntime(330):  at android.app.ActivityThread.main(ActivityThread.java:3683)
04-22 00:45:10.909: E/AndroidRuntime(330):  at java.lang.reflect.Method.invokeNative(Native Method)
04-22 00:45:10.909: E/AndroidRuntime(330):  at java.lang.reflect.Method.invoke(Method.java:507)
04-22 00:45:10.909: E/AndroidRuntime(330):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
04-22 00:45:10.909: E/AndroidRuntime(330):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
04-22 00:45:10.909: E/AndroidRuntime(330):  at dalvik.system.NativeStart.main(Native Method)

这里有一些更多的发现.根据堆栈,ContextImpl文件中出现错误,我发现此方法中的父局部变量为null.这是代码：

public FileOutputStream openFileOutput(String name, int mode)
  throws FileNotFoundException {
  final boolean append = (mode&MODE_APPEND) != 0;
  File f = makeFilename(getFilesDir(), name);
  try {
     FileOutputStream fos = new FileOutputStream(f, append);
     setFilePermissionsFromMode(f.getPath(), mode, 0);
     return fos;
  } catch (FileNotFoundException e) {
 }

 File parent = f.getParentFile();
 parent.mkdir();
 FileUtils.setPermissions(
     parent.getPath(),
     FileUtils.S_IRWXU|FileUtils.S_IRWXG|FileUtils.S_IXOTH,
     -1, -1);
 FileOutputStream fos = new FileOutputStream(f, append);
 setFilePermissionsFromMode(f.getPath(), mode, 0);
 return fos;
}

所以现在问题是如何确保对f.getParentFile()的调用不会返回null,为什么它会不时地工作？

解决方法:

The problem is that I get NullPointerException on the line with openFileOutput.

您在openFileOutput()的实现中获得了NullPointerException.这就是为什么您需要为SO提供堆栈跟踪,而不是仅提供异常名称.行上的异常与行触发的异常不同,尤其是NullPointerException.

基于我在ContextImpl上读取的openFileOutput(),要么：

>名称为null,或
>你有一个非常奇怪的背景,或
>您正在运行的Android版本还有一些其他可能的问题,这在当前的openFileOutput()实现中是不可见的

我将首先完全删除上下文,因为您在Activity中并且不需要它.然后,输入一些Log语句或设置断点以查看Name是什么.