File exists and IS directory, but listFiles() returns null

问题

The documentation for File.listFiles() suggests that null will ONLY be returned in the case that the file calling it is not a directory.

I have the following:

String dir = "/storage/emulated/0";
File f = new File(dir);
Log.v("Files",f.exists()+"");
Log.v("Files",f.isDirectory()+"");
Log.v("Files",f.listFiles()+"");

The log reads:

true
true
null

For some reason, listFiles() is returning null even though the File is recognized as a valid directory. I'm not super familiar with Android file hierarchy behavior, so I would guess the problem lies in there.

For reference, I'm debugging on my Moto X, and results are the same whether the phone is plugged in to my computer or not - so I don't think it has to do with mounting when plugged in.

回答1:

For those with this problem, add this to AndroidManifest.xml:

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

Problem solved :D

EDIT: If this does not work just make sure if path is correct

回答2:

For android version 23 or higher, you need to give run time permission programmatically in onresume method as below,

public final String[] EXTERNAL_PERMS = {Manifest.permission.WRITE_EXTERNAL_STORAGE, Manifest.permission.READ_EXTERNAL_STORAGE
};

public final int EXTERNAL_REQUEST = 138;

requestForPermission();

public boolean requestForPermission() {

    boolean isPermissionOn = true;
    final int version = Build.VERSION.SDK_INT;
    if (version >= 23) {
        if (!canAccessExternalSd()) {
            isPermissionOn = false;
            requestPermissions(EXTERNAL_PERMS, EXTERNAL_REQUEST);
        }
    }

    return isPermissionOn;
}

public boolean canAccessExternalSd() {
    return (hasPermission(android.Manifest.permission.WRITE_EXTERNAL_STORAGE));
}

private boolean hasPermission(String perm) {
    return (PackageManager.PERMISSION_GRANTED == ContextCompat.checkSelfPermission(this, perm));

}

回答3:

For those with this problem you to give read permission, add this to AndroidManifest.xml:

android:name="android.permission.READ_EXTERNAL_STORAGE"

and you should also grant permission from

Settings > Apps > "Your app name" > Permissions > storage

回答4:

From the File 'listFiles' method documentation: Returns null if this abstract pathname does not denote a directory, or if an I/O error occurs.

So perhaps an I/O error occurred. Check the permissions on that directory, does the user running the java program have the permission to read that directory? If not, there is your answer. If they do, then perhaps the directory is empty.

BTW you should not use +"" to convert something to a string, it's a bad practice since it's slower than String.valueOf. I would use apache's StringUtils to join that array into a string, but that requires adding that jar to your classpath.

回答5:

In addition to other answers and comments, I'd suggest you check whether you need

<application android:requestLegacyExternalStorage="true"

in your manifest. It seems that some recent apis need it.

Anyway, zapl's comment is rather short but quite insightful. You can "ls -ld" the directory on the device (via "adb shell" or some other shells). If you have "r" permission on the directory, you can call listFiles(). Otherwise, it returns null. Note that you can access files under the unreadable directory if you know the file names and have "x" permission on the directory. You can know who you are by "whoami" and "groups" commands.

回答6:

Just add the permission in manifest uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" and your problem will be resolved.

回答7:

My problem was that for some reason the android:sharedUserId="android.uid.system" attribute was not allowing the app to read /sdcard/ on a specific device.

As I needed to do only a quick test, I removed the sharedUserId attribute, tested and added it back.

回答8:

In the manifest file at the <uses-permission .... > section, add this line android:name="android.permission.READ_EXTERNAL_STORAGE". I think this may solve your problem.