最近做项目遇到的难题，调用系统拍照获取不到缩略图，非得关机重启才会生成，所以我们要主动通知系统扫描SD卡生成缩略图，

在Android4.4之前也就是以发送一个Action为“Intent.ACTION_MEDIA_MOUNTED”的广播通知执行扫描。如下：

this.sendBroadcast(new Intent(Intent.ACTION_MEDIA_MOUNTED, Uri.parse("file://" + Environment.getExternalStorageDirectory())));

但在Android4.4中，则会抛出以下异常：

W/ActivityManager(  498): Permission Denial: not allowed to send broadcast android.intent.action.MEDIA_MOUNTED from pid=2269, uid=20016 

那是因为Android4.4中限制了系统应用才有权限使用广播通知系统扫描SD卡。 

解决方式： 

使用MediaScannerConnection执行具体文件或文件夹进行扫描。

1 Intent mediaScanIntent = new Intent(2                     Intent.ACTION_MEDIA_SCANNER_SCAN_FILE);3             String path = "file://"4                             + out.getPath();5             Uri contentUri = Uri.parse(path); //out is your output file6             mediaScanIntent.setData(contentUri);7             this.sendBroadcast(mediaScanIntent);

MediaScannerReceiver 源码中的广播接收的部分代码：

1  @Override   2     public void onReceive(Context context, Intent intent) {   3         String action = intent.getAction();   4         Uri uri = intent.getData();   5         String externalStoragePath = Environment.getExternalStorageDirectory().getPath();   6    7         if (action.equals(Intent.ACTION_BOOT_COMPLETED)) {   8             // scan internal storage   9             scan(context, MediaProvider.INTERNAL_VOLUME);  10         } else {  11             if (uri.getScheme().equals("file")) {  12                 // handle intents related to external storage  13                 String path = uri.getPath();  14                 if (action.equals(Intent.ACTION_MEDIA_MOUNTED) &&   15                         externalStoragePath.equals(path)) {  16                     scan(context, MediaProvider.EXTERNAL_VOLUME);  17                 } else if (action.equals(Intent.ACTION_MEDIA_SCANNER_SCAN_FILE) &&  18                         path != null && path.startsWith(externalStoragePath + "/")) {  19                     scanFile(context, path);  20                 }  21             }  22         }  23     }

 

所以最后的代码是：

1 if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) { 2             Intent mediaScanIntent = new Intent( 3                     Intent.ACTION_MEDIA_SCANNER_SCAN_FILE); 4             String path = "file://" 5                             + out.getPath(); 6             Uri contentUri = Uri.parse(path); //out is your output file 7             mediaScanIntent.setData(contentUri); 8             this.sendBroadcast(mediaScanIntent);10         } else {11             sendBroadcast(new Intent(12                     Intent.ACTION_MEDIA_MOUNTED,13                     Uri.parse("file://"14                             + Environment.getExternalStorageDirectory())));15         }