Replacing bits of the Android system code

If you've been googling around trying to figure out why your Android device doesn't seem to want to load your custom-built framework.jar (or other file in /system/framework), you may have come across this reply from Android framework engineer Dianne Hackborn:

You need to flash the entire device with your own build.  You can't just selectively replace pieces of a user build.

That didn't sound right to me, since I'd done exactly that on the HTC Dream and was just now having trouble doing it on a Nexus One.  Then again, I was running Cupcake on the old phone, so maybe this was an effect of a newer version of Android?

Nah, I was just missing a couple steps.

The first clue that something was different was that /system/framework was littered with a bunch of extra .odex files, one for each jar/apk file.  These files are optimized versions of the classes.dex files that would otherwise live inside each jar/apk file, generated at first boot by dexopt.  There's a great article over at AddictiveTips that gives an overview of why those files exist.

One nasty misconception I got from reading that article, though, is that the .odex files inside /system/framework are re-generated on boot if they're missing.  This is not the case!  Or at the very least, it's not as simple as that.  If you do like I did - e.g. deleting framework.odex and replacing framework.jar with your rebuilt version - you may be greeted by an unbootable phone.  The reason for this is that the .odex files also store dependency information, the unfortunate result of which is that if you touch one, you probably have to regenerate them all.  This process is scriptable, sure, but it seems easy to get wrong.

So, rather than mucking about with generating .odex files on the phone, I decided to just track down a deodexed ROM, flash it, and then ignore .odex entirely from that point forward.  Luckily, it didn't take me too long to find a custom deodexed ROM of Android 2.3.4 for the Nexus One.  Turns out that deodexing is something that ROM makers do all the time, probably for similar reasons to mine.

Once you've flashed a deodexed ROM, the .odex files are gone, and replacing any of the files in /system/framework is as simple as replacing the file with your custom-built copy (making sure to build with the right device-specific setup and backing up the original files first).