How can I lock files in android using java code?

Question

I want to lock files that is on the sdcard of android. I need to lock them so no one except my software can delete copy move or send them over bluetooth or any other way. How can I do it?

Answer 1

You cannot do this on any platform (let alone Android) using a regular SD card. Sorry!

Answer 2

There is no way to lock a file in Android; even if there was, the files would only be locked when your application was running which wouldn't stop other apps accessing the file when the user hadn't started your app.

SD Cards are usually formatted with FAT (or FAT32) and this is the case with Android. FAT offers very limited file permissions so you can't restrict access to the files this way. Again, even if you could do something like this, this wouldn't offer any protection if the user took the SD Card out of their phone and put it their PC.

If you want to keep your application's files private you need to put them in the phone's internal storage in the area reserved for you application. Each application runs with a different User ID and each application's data directory is protected using UNIX-style file permissions which prevents other applications from reading them. So while this will stop other applications having access to your files it wouldn't stop a reasonably determined user with root access to their phone copying them off.

Have a look at the Context.openFileInput(), Content.openFileOutput() and Context.getFilesDir() methods for details on how to store files in the phone's internal storage.

Answer 3

I think you want to encrypt the file. While the file can still be moved around, it will be useless to anything but your application. People will be able to copy, move, delete but the contents won't be compromised.