使用Android FileProvider删除文件时出现NullPointerException

Question

我在使用Android File API时无法正常工作。

这是我要实现的目标，我正在从亚马逊下载视频，这些视频在完全下载后应保存在本地。 我正在以相当简单的方式执行此操作（代码已缩短，我只显示了基本行）：

inputStream = connection.getInputStream();
fileOutputStream = context.openFileOutput("my_video", Context.MODE_PRIVATE);
// Read bytes (and store them) until there is nothing more to read(-1)
do {
    int numread = inputStream.read(buffer);
    if (numread <= 0){
        break;                  
    }
    fileOutputStream.write(buffer, 0, numread);
} while (true);

其中inputStream和fileOutputStream是实例变量。

如果视频完全下载，这实际上效果很好。 在这种情况下，一切都很好，之后我可以在本地访问视频。

但是，在应用程序中可能会发生视频下载中断并因此需要取消的情况。 如果是这种情况，我想删除那里但显然不完整的文件。

删除文件的代码如下：

FileProvider fileProvider = new FileProvider();
File newFile = new File(context.getFilesDir(), "my_video");     
Uri contentUri = FileProvider.getUriForFile(context, FILE_PROVIDER, newFile);
fileProvider.delete(fileToDelete, null, null);

最后一行fileProvider.delete(fileToDelete, null, null); 抛出NullPointerException ，我对其进行了调试，并看到fileProvider已初始化，因此我强烈认为我用来调用delete的URI存在问题，但我不知道它是怎么了。 有谁知道如何使用文件提供程序进行正确的删除？

更新：我希望这并不过分，我现在发布整个VideoDownloader类：

public class VideoDownloader {

  private final int TIMEOUT_CONNECTION = 5000;//5sec
  private final int TIMEOUT_SOCKET = 30000;//30sec

  private static final String FILE_PROVIDER = "com.orangewise.fileprovider";

  private Context context;
  private String videoURL;
  private String targetFileName;

  private HttpURLConnection connection;
  private InputStream inputStream;
  private FileOutputStream fileOutputStream;

  private boolean downloadFinished;

  public VideoDownloader(Context context, String videoURL, String targetFileName) {
    this.context = context;
    this.videoURL = videoURL;
    this.targetFileName = targetFileName;
    this.downloadFinished = false;
  }

  public boolean isDownloadFinished(){
    return this.downloadFinished;
  }

  public void startDownload(){
    downloadVideoFile(this.context, this.videoURL, this.targetFileName);    
  }

  private void downloadVideoFile(Context context, String videoURL, String targetFileName) {
    URL url = null;
    try {
        url = new URL(videoURL);

        // Open a connection to that URL.
        connection = (HttpURLConnection) url.openConnection();

        connection.setReadTimeout(TIMEOUT_CONNECTION);
        connection.setConnectTimeout(TIMEOUT_SOCKET);r

        inputStream = connection.getInputStream();
        fileOutputStream = context.openFileOutput(targetFileName, Context.MODE_PRIVATE);

        byte[] buffer = new byte[3 * 1024];
        int counter = 0;

        // Read bytes (and store them) until there is nothing more to read(-1)
        do {
            int numread = inputStream.read(buffer);
            if (numread <= 0){
                break;                  
            }
            fileOutputStream.write(buffer, 0, numread);
        } while (true);
        downloadFinished = true;

        // Clean up
        closeStreams();
    } catch (Exception e) {
        Log.d(Constants.ERROR, "ERROR [" + getClass().getName() + "]: Caught exception (" + e + ") when trying to download video: " + e.getMessage());
    }
  }

  public Uri getUriForFile(){
    return getUriForFile(context, targetFileName);
  }

  private Uri getUriForFile(Context context, String fileName){

    File newFile = new File(context.getFilesDir(), fileName);       
    Uri contentUri = FileProvider.getUriForFile(context, FILE_PROVIDER, newFile);

    return contentUri;
  }

  public void cancel(){
    // 1. cancel the connection
    Log.d(Constants.LOG, "DEBUG [" + getClass().getName() + "]: Cancel connection");

    try {
        connection.disconnect();
        closeStreams();

        if(!isDownloadFinished()){              
            // Remove the file if it has not been fully downloaded
            FileProvider fileProvider = new FileProvider();
            Uri fileToDeleteUri = getUriForFile();
            fileProvider.delete(fileToDeleteUri, null, null); // returns 1 if the delete succeeds; otherwise, 0.
        }
        else{
            Log.d(Constants.LOG, "DEBUG [" + getClass().getName() + "]: Leave the file, it has been completely download");
        }
    } 
    catch (Exception e) {
        Log.d(Constants.ERROR, "Exception ( " + e + " ) caught: " +  e.getMessage() + "; ");
    }
}

private void closeStreams() throws IOException{     
    // Close the streams
    try {           
        fileOutputStream.flush();
        fileOutputStream.close();
        inputStream.close();
    } catch (NullPointerException e) {
        Log.d(Constants.ERROR, "Null pointer exception caught: " +  e.getMessage());
    }
    Log.d(Constants.LOG, "DEBUG [" + getClass().getName() + "]: Clean up performed");
  }
} 

另一个更新：这是我的堆栈跟踪：

07-23 17:31:29.114: E/com.organgewise.just4kidstv.LOG(6152): java.lang.NullPointerException
07-23 17:31:29.114: E/com.organgewise.just4kidstv.LOG(6152):    at android.support.v4.content.FileProvider.delete(FileProvider.java:497)
07-23 17:31:29.114: E/com.organgewise.just4kidstv.LOG(6152):    at com.orangewise.just4kidstv.util.VideoDownloader.cancel(VideoDownloader.java:134)
07-23 17:31:29.114: E/com.organgewise.just4kidstv.LOG(6152):    at com.orangewise.just4kidstv.util.VideoDownloadTask.cancel(VideoDownloadTask.java:20)
07-23 17:31:29.114: E/com.organgewise.just4kidstv.LOG(6152):    at com.orangewise.just4kidstv.activities.VideoPlayerActivity.onStop(VideoPlayerActivity.java:64)
07-23 17:31:29.114: E/com.organgewise.just4kidstv.LOG(6152):    at android.app.Instrumentation.callActivityOnStop(Instrumentation.java:1170)
07-23 17:31:29.114: E/com.organgewise.just4kidstv.LOG(6152):    at android.app.Activity.performStop(Activity.java:3873)
07-23 17:31:29.114: E/com.organgewise.just4kidstv.LOG(6152):    at android.app.ActivityThread.performDestroyActivity(ActivityThread.java:2623)
07-23 17:31:29.114: E/com.organgewise.just4kidstv.LOG(6152):    at android.app.ActivityThread.handleDestroyActivity(ActivityThread.java:2694)
07-23 17:31:29.114: E/com.organgewise.just4kidstv.LOG(6152):    at android.app.ActivityThread.access$2100(ActivityThread.java:117)
07-23 17:31:29.114: E/com.organgewise.just4kidstv.LOG(6152):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:968)
07-23 17:31:29.114: E/com.organgewise.just4kidstv.LOG(6152):    at android.os.Handler.dispatchMessage(Handler.java:99)
07-23 17:31:29.114: E/com.organgewise.just4kidstv.LOG(6152):    at android.os.Looper.loop(Looper.java:130)
07-23 17:31:29.114: E/com.organgewise.just4kidstv.LOG(6152):    at android.app.ActivityThread.main(ActivityThread.java:3687)
07-23 17:31:29.114: E/com.organgewise.just4kidstv.LOG(6152):    at java.lang.reflect.Method.invokeNative(Native Method)
07-23 17:31:29.114: E/com.organgewise.just4kidstv.LOG(6152):    at java.lang.reflect.Method.invoke(Method.java:507)
07-23 17:31:29.114: E/com.organgewise.just4kidstv.LOG(6152):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867)
07-23 17:31:29.114: E/com.organgewise.just4kidstv.LOG(6152):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:625)
07-23 17:31:29.114: E/com.organgewise.just4kidstv.LOG(6152):    at dalvik.system.NativeStart.main(Native Method)

Answer 1

检查sdk Extras中随附的v4支持库的FileProvider.java的源，我们发现：

@Override
public int delete(Uri uri, String selection, String[] selectionArgs) {
   // ContentProvider has already checked granted permissions
   final File file = mStrategy.getFileForUri(uri);   /* line 497 */
   return file.delete() ? 1 : 0;
}

因此，mStrategy为空。

进一步搜索，我们发现它仅在一个地方被初始化：

/**
 * After the FileProvider is instantiated, this method is called to provide the system with
 * information about the provider.
 *
 * @param context A {@link Context} for the current component.
 * @param info A {@link ProviderInfo} for the new provider.
 */
@Override
public void attachInfo(Context context, ProviderInfo info) {
    super.attachInfo(context, info);

    // Sanity check our security
    if (info.exported) {
        throw new SecurityException("Provider must not be exported");
    }
    if (!info.grantUriPermissions) {
        throw new SecurityException("Provider must grant uri permissions");
    }

    mStrategy = getPathStrategy(context, info.authority);
}

因此很明显，您的FileProvider 尚未通过调用此方法正确设置。

FileProvider 文档尚不十分清楚，但似乎您不应该简单地执行“ new FileProvider（）”，而应该在清单中进行一些相关的设置。

Answer 2

@克里斯·斯特拉顿是正确的。 您不应使用该构造函数实例化它。 FileProvider是ContentProvider的子类。 由于您在AndroidManifest.xml中声明了它，因此可以使用以下方法获取它的句柄：

context.getContentResolver()

因此，您可以通过执行以下操作来修复NullPointerException：

context.getContentResolver().delete(contentUri, null, null);

您可以检查它返回1以确认它是否起作用。