没错就是这么坑，也不报错花了一下午。
1.AndroidMainfest.xml
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />2.在写入sd卡的activity文件中加入如下代码，并且在onCreate中调用
//动态申请sd卡读写权限private static final int REQUEST_EXTERNAL_STORAGE = 1;private static String[] PERMISSIONS_STORAGE = {"android.permission.READ_EXTERNAL_STORAGE","android.permission.WRITE_EXTERNAL_STORAGE" };public static void verifyStoragePermissions(Activity activity) {try {//检测是否有写的权限int permission = ActivityCompat.checkSelfPermission(activity,"android.permission.WRITE_EXTERNAL_STORAGE");if (permission != PackageManager.PERMISSION_GRANTED) {// 没有写的权限，去申请写的权限，会弹出对话框ActivityCompat.requestPermissions(activity, PERMISSIONS_STORAGE,REQUEST_EXTERNAL_STORAGE);}} catch (Exception e) {e.printStackTrace();}}