在API23+以上，不止要在AndroidManifest.xml裡面新增許可權

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

還要在JAVA程式碼中請求許可權：

// 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);
        }
    }
 

在儲存的方法前面呼叫即可