android - FileProvider 在 GetUriForFile 上引发异常

Question

我按照 android developer reference on FileProviders 上的示例代码进行操作但它不会工作。

我在 Manifest 的文件提供程序定义中设置了路径 files/exports/，并且引用的文件确实存在于该路径中。

在我的 list 中:

<provider android:name="android.support.v4.content.FileProvider" android:authorities="com.company.app.fileprovider" android:exported="false" android:grantUriPermissions="true">
    <meta-data android:name="android.support.FILE_PROVIDER_PATHS" android:resource="@xml/file_paths" />
</provider>

而在xml/file_paths.xml

<?xml version="1.0" encoding="UTF-8" ?>
<paths xmlns:android="http://schemas.android.com/apk/res/android">
    <files-path name="exports" path="exports/"/>
</paths>

然后我尝试使用以下代码获取 content-uri:

Java.IO.File exportFile = 
    new Java.IO.File ("/data/data/com.company.app/files/exports/file.pdf"); 
Android.Net.Uri exportUri = 
    Android.Support.V4.Content.FileProvider.GetUriForFile (this, "com.company.app.fileprovider", exportFile);

但是我得到了这个错误:

Exception of type 'Java.Lang.NullPointerException' was thrown.
  at Android.Runtime.JNIEnv.CallStaticObjectMethod (IntPtr jclass, IntPtr jmethod, Android.Runtime.JValue[] parms) [0x00064] in /Users/builder/data/lanes/monodroid-mlion-monodroid-4.10.2-branch/4b53fbd0/source/monodroid/src/Mono.Android/src/Runtime/JNIEnv.g.cs:1160 
  at Android.Support.V4.Content.FileProvider.GetUriForFile (Android.Content.Context context, System.String authority, Java.IO.File file) [0x00000] in <filename unknown>:0 
  at com.company.App.ImageAndSubtitle.cloudStorageDidDownloadFile (System.Object sender, com.company.App.CloudStorageEventArgs args) [0x001f7] in /app_path/Screens/ImageAndSubtitle.cs:187 
  --- End of managed exception stack trace ---
java.lang.NullPointerException
    at android.support.v4.content.FileProvider.parsePathStrategy(FileProvider.java:243)
    at android.support.v4.content.FileProvider.getPathStrategy(FileProvider.java:217)
    at android.support.v4.content.FileProvider.getUriForFile(FileProvider.java:130)
    at mono.java.lang.RunnableImplementor.n_run(Native Method)
    at mono.java.lang.RunnableImplementor.run(RunnableImplementor.java:29)
    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:4838)
    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:875)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:642)
    at dalvik.system.NativeStart.main(Native Method)

更新

从 Android 源代码中我可以找到它，因为我现在知道 android.support.v4.content.FileProvider.parsePathStrategy(FileProvider.java:243) 中的这个命令将失败并返回 null:

final ProviderInfo info = context.getPackageManager().resolveContentProvider(authority, PackageManager.GET_META_DATA);

但我不知道为什么......

Answer 1

我终于在 https://github.com/commonsguy/cw-omnibus/tree/master/ContentProvider 上找到了关于如何创建 ContentProviders 和 FileProviders 的好示例代码

我的代码中的实际错误是，在 Manifest 文件中，我在 application 标记之外有 provider 标记，但它必须在里面。 list 必须如下所示:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android" android:versionCode="1" android:versionName="1.0" package="com.company.app">
<uses-sdk android:minSdkVersion="13" />

<permission
    android:name="com.company.app.fileprovider.READ"
    android:description="@string/perm_read"
    android:label="@string/perm_read_label"/>
<uses-permission android:name="com.company.app.fileprovider.READ"/>

<application android:label="MyApp">

    <provider 
    android:name="android.support.v4.content.FileProvider" 
    android:authorities="com.company.app.fileprovider" 
    android:exported="false" 
    android:grantUriPermissions="true"
    android:readPermission="com.company.app.fileprovider.READ">
                <meta-data 
                android:name="android.support.FILE_PROVIDER_PATHS" 
                android:resource="@xml/file_paths" />
    </provider>

</application>
<uses-permission android:name="android.permission.INTERNET" />

</manifest>