As shown in the figure, the read and write permissions of SD card are declared in mainfest file, and the error is still reported:

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

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For Android version 23.0 and above, you should not only set the above permissions, but also authorize the SD card where you have read and write operations. The following is a public class:

import android.Manifest;
import android.app.Activity;
import android.content.pm.PackageManager;
import android.support.v4.app.ActivityCompat;

public class PermisionUtils {

    // 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);
        }
    }
}

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Then call directly where authorization is needed:

verifyStoragePermissions(this);

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this refers to the current Activity. The following results will appear when the program runs:

After clicking ALLOW, the program obtains the SD card read and write permission, even if you put the
verifyStoragePermissions(this); statement is deleted. As long as the application is not uninstalled, the permissions still exist.

Reprints: https://www.2cto.com/kf/201707/653938.html