How to backup data on battery plug off in android?

Question

I am developing an application that is writing data to a file. Lets assume while it is writing the data we plug off the battery. What will happen to the file? will it be half-writen (corrupted), empty or same as before we wrote to it? My guess is that it will be corrupted. How to check if it is corrupted when we restart the phone when the file was storing an arraylist of objects? will java throw a corrupted file exception or say that the read arraylist is null or that it is an unknown object?

PS. maybe create another file that will keep the MD5 checksum of the data file? And whenever I write to the file data first I produce its checksum and then when I read from the data file produce a checksum and compare it with the previous. That will indicate whether my data are intact but it wont allow me to roll back to a previous state (pre-corrupted one). I would like a method that would be as lightweight as possible, I am already using the CPU too much by reading/writing changes to my storage on every attribute change of a set of thousands. Probably a database would have been a better idea.

Answer 1

I can't say how Java will read in a corrupted serialized array, but for safety let's assume that there's no error detection.

In that case, you have two easy options:

• Store a checksum of your data inside your data structure, before you serialize it.
• Compute the checksum of the final serialized file.

Either case will work the same way, though the first option might be a bit faster since you compute the checksum before you've written anything to disk (and therefor avoid an extra round of file I/O).

As you mentioned, MD5 would be fine for this. (Even a basic CRC would probably be fine -- you don't need a cryptograhpic hash for this.)

If you want to allow rolling back to a previous version -- I'd just store each version as a separate file and then have a pointer to the most recent one. (If you update the pointer as the last step of your write operation, this will also provide an extra level of protection against corrupt data being input to your app -- though you'll have to prepare for this pointer to be corrupt as well. Since this is essentially a commit step, you could interpret a corrupt pointer as "use the last version".)

And yes, at this point you might want to just use the SQLite functionality built into Android. :)