Activity 中 openFileOutput 的 NullPointerException [英] NullPointerException at openFileOutput in Activity
问题描述
我有一个带有 ImageView 的 Activity，如果它是空的并被点击，应该从 Internet 加载一个图像并将其保存到内部存储中.所以活动看起来像这样:
I have an Activity with an ImageView, which, if it is empty and clicked, should load an image from Internet and save it to internal storage. So the activity looks like this:
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 行上得到 NullPointerException.我发现了几个类似的问题，但所有的答案都暗示 null 是一个缺失的上下文.由于在这种情况下它是一个活动，它显然有一个上下文 (this)，并且它不为空.我试图通过 context 成员调用 openFileOutput(隐含 this).这两种方法都失败了.我还尝试将 v.getContext() 从 onClick 传递到 Download 作为附加参数，并且该值不为空，但产生了同样的例外.(如果很重要，请求的文件名是一个有效的文件名，例如abc".)
The problem is that I get NullPointerException on the line with openFileOutput. I found a couple of similar questions, but all answers imply that null is a missing context. As it's an activity in this case, it obviously has a context (this), and it is not null. I tried to call openFileOutput (with this implied) and via the context member. Either approach fails. Also I tried to pass v.getContext() from onClick into Download as an additional parameter, and this value was not null, yet produced the same exception as well. (If it is important, the name of the file requested is a valid filename, for example "abc".)
有人可以解释一下吗?
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)
这里还有一些发现.根据堆栈，错误发生在ContextImpl文件中，我看到该方法内部的parent局部变量为空.代码如下:
And here is some more findings. According to the stack, the error occures inside ContextImpl file, and I see that parent local variable is null inside this method. Here is the code:
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;
}
那么现在的问题是如何确保对 f.getParentFile() 的调用不会返回 null，为什么它不时起作用?
So now the question is how can I make sure that the call to f.getParentFile() will not return null, and why does it work from time to time?
推荐答案
这被证明是 Android 2.3.3 模拟器的一个特性，作为 SDK 的一部分安装在这里.如果我使用带有 Android 2.3.3 的真实设备，则不会出现此问题.如果我使用 Android 4.0.3 模拟器，代码也可以正常工作.所以，如果发生了一些奇怪的事情，在另一个环境中的测试可能会有所启发.我希望这可以帮助其他人解决类似的问题.
This is proved to be a feature of Android 2.3.3 emulator installed here as a part of SDK. The problem does not occur if I use a real device with Android 2.3.3. Also the code works ok if I use Android 4.0.3 emulator. So, if some weird things happen, tests in another environment can shed some light. I hope this can help someone else in similar problem solving.
这篇关于Activity 中 openFileOutput 的 NullPointerException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IT屋！
