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我正在使用 DownloadManager 下载文件并保存在内部存储上我的应用程序上下文文件夹中的下载文件夹内的文件夹中，这在 Android 6 中运行良好，但在 android 4 中路径返回 null。此代码适用于 android 6 但不适用于 android 4

File p = getApplicationContext().getExternalFilesDir(Environment.DIRECTORY_DOWNLOADS);

我对此有所改变并且可以工作

File p = getApplicationContext().getExternalFilesDir(Environment.DIRECTORY_DOWNLOADS);
if(p == null){
    p = getApplicationContext().getDir(Environment.DIRECTORY_DOWNLOADS, MODE_PRIVATE);
}

我的问题出在 DownloadManager 中，当我使用以下代码时，会出现 NullPointerReference

req.setDestinationInExternalFilesDir(getApplicationContext(), Environment.DIRECTORY_DOWNLOADS + "/viewer", fileName);

谁能帮我？

编辑

这是堆栈跟踪

FATAL EXCEPTION: main
java.lang.NullPointerException: file
at android.net.Uri.fromFile(Uri.java:441)
at android.app.DownloadManager$Request.setDestinationFromBase(DownloadManager.java:508)
at android.app.DownloadManager$Request.setDestinationInExternalFilesDir(DownloadManager.java:470)
at br.com.digitaldoc.ddv4.activities.MainActivity$7.onClick(MainActivity.java:405)
at android.support.v7.app.AlertController$AlertParams$3.onItemClick(AlertController.java:958)
at android.widget.AdapterView.performItemClick(AdapterView.java:298)
at android.widget.AbsListView.performItemClick(AbsListView.java:1088)
at android.widget.AbsListView$PerformClick.run(AbsListView.java:2861)
at android.widget.AbsListView$1.run(AbsListView.java:3535)
at android.os.Handler.handleCallback(Handler.java:615)
at android.os.Handler.dispatchMessage(Handler.java:92)
at android.os.Looper.loop(Looper.java:137)
at android.app.ActivityThread.main(ActivityThread.java:4931)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:511)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:791)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:558)
at dalvik.system.NativeStart.main(Native Method)

而这一行 MainActivity.java:405 是

req.setDestinationInExternalFilesDir(getApplicationContext(), Environment.DIRECTORY_DOWNLOADS + "/viewer", fileName);
4

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