为什么contentResolver.query（）返回null？

Question

I am refactoring the AWS S3TransferUtilitySample from Java to Kotlin, and hit an exception when I try to upload an image. I traced the error to after the gallery intent returns to my app with an image URI. Within onActivityResult , getContentResolver.query(...) is the problem.

• Java cursor = getContentResolver().query(uri, projection, selection, selectionArgs, null); translates in my code to
• Kotlin cursor = contentResolver.query(uri, projection, selection, selectionArgs, null) .

The query argument values at this point are:

• uri : content://media/external/images/media
• projection : {"_data"}
• selection : "_id=?"
• selectionArgs : {"3812"}
• sortOrder : null

I looked up the documentation for contentResolver and I think I may be unpacking the query arguments wrong. I saw this answer , but nothing there helped. Any suggestions?

Answer 1

It was a permissions error. In the Logcat, I saw

2019-07-30 09:32:57.566 11640-12481/? E/DatabaseUtils: Writing exception to parcel
    java.lang.SecurityException: Permission Denial: reading com.android.providers.media.MediaProvider uri content://media/external/images/media from pid=13823, uid=10149 requires android.permission.READ_EXTERNAL_STORAGE, or grantUriPermission()
        at android.content.ContentProvider.enforceReadPermissionInner(ContentProvider.java:633)
        at android.content.ContentProvider$Transport.enforceReadPermission(ContentProvider.java:503)
        at android.content.ContentProvider$Transport.query(ContentProvider.java:214)
        at android.content.ContentProviderNative.onTransact(ContentProviderNative.java:102)
        at android.os.Binder.execTransact(Binder.java:697)

Then I saw that I had commented out the permissions code beforehand to get a better understanding of what's going on. Mission accomplished smh. I actually just deleted the commented code and wrote it from scratch to drive the lesson home.

Here's what fixed it:

• Add permission to AndroidManifest.xml

<uses-permission android:name=“android.permission.READ_EXTERNAL_STORAGE”/>

• Check for, and grant permission to the app when the user presses the Upload Image button

    buttonUploadImage!!.setOnClickListener {
                if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
                    val permissions = arrayOf(permission.READ_EXTERNAL_STORAGE)

                    val intent = Intent()
                    intent.action = Intent.ACTION_GET_CONTENT
                    intent.type = "image/*"

                    if (checkSelfPermission(permission.READ_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {
                        if (ActivityCompat.shouldShowRequestPermissionRationale(this,
                                permission.READ_EXTERNAL_STORAGE)) {
                            // Explain why the permission is needed
                        } else {
                            ActivityCompat.requestPermissions(this, permissions, PERMISSION_CODE)
                        }
                    } else {
                        // Permission has already been granted
                        startActivityForResult(intent, PERMISSION_CODE)
                    }
                }
            }

---

Resources

• Request App Permissions
• AWS S3TransferUtilitySample