我无法以编程方式获取Android外部SD卡中的文件

Question

When I try the code below:

File home = new File(Environment.getExternalStorageDirectory().getAbsolutePath());

File [] f = home.listFiles();

I cannot get the files in the sd card, can anyone explain why? and the path of the external directory is /storage/emulated/0/

Thank you

Answer 1

I think you are using emulator with API-24/25 ..

Unfortunately with both you have no access to storage. I have no idea id that was intended to be change or its just a bug

You can check if you have access to your storage by adb shell ls

You must use Emulator with API-23 or less to be able to access storage.

Answer 2

Include this permission in the manifest file:

<manifest>
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
    ...
    <application>
        ...
        <activity> 
            ...
        </activity>
    </application>
</manifest> 

For API 23+ you need to request the read/write permissions even if they are already in your manifest.

// Storage Permissions
private static final int REQUEST_EXTERNAL_STORAGE = 1;
private static String[] PERMISSIONS_STORAGE = {
        Manifest.permission.READ_EXTERNAL_STORAGE,
        Manifest.permission.WRITE_EXTERNAL_STORAGE
};

/**
 * Checks if the app has permission to write to device storage
 *
 * If the app does not has permission then the user will be prompted to grant permissions
 *
 * @param activity
 */
public static void verifyStoragePermissions(Activity activity) {
    // Check if we have write permission
    int permission = ActivityCompat.checkSelfPermission(activity, Manifest.permission.WRITE_EXTERNAL_STORAGE);

    if (permission != PackageManager.PERMISSION_GRANTED) {
        // We don't have permission so prompt the user
        ActivityCompat.requestPermissions(
                activity,
                PERMISSIONS_STORAGE,
                REQUEST_EXTERNAL_STORAGE
        );
    }
}

And handle the responce , some example:

@Override
public void onRequestPermissionsResult(int requestCode,
        String permissions[], int[] grantResults) {
    switch (requestCode) {
        case MY_PERMISSIONS_REQUEST_READ_CONTACTS: {
            // If request is cancelled, the result arrays are empty.
            if (grantResults.length > 0
                && grantResults[0] == PackageManager.PERMISSION_GRANTED) {

                // permission was granted, yay! Do the
                // contacts-related task you need to do.

            } else {

                // permission denied, boo! Disable the
                // functionality that depends on this permission.
            }
            return;
        }

        // other 'case' lines to check for other
        // permissions this app might request
    }
}

For official documentation about requesting permissions for API 23+, check https://developer.android.com/training/permissions/requesting.html