Building for the right device

I don't know all the reasons for this, and it may be obvious to more seasoned Android hackers than myself, but:

If you're rebuilding framework.jar for a device, it's important to set up your build environment for that specific device.

Though I'm focusing on framework.jar at the moment, this probably pertains to several other of the core system-level classes.

I'm using a Nexus One (aka HTC Passion), so for me, this meant roughly following the directions discussed in this somewhat outdated, but still useful, post.

Here's the short version (the main steps that resolved things for me):

1. Make sure your Android source tree contains a device/htc/passion folder.
   (The name will be different depending on your device.)
   
2. Change to that folder and run the extract-files.sh script.
   You'll need your device running and connected via USB (with USB debugging enabled, naturally) to do this.
   
3. Set up your build environment by running . ./build/envsetup.sh from the top directory of the source tree.
   This should be familiar if you've spent any time at all working with the Android source.
   
4. Type lunch full_passion-userdebug.
   This is the key step; as far as I can tell, it sets variables and pulls in the right files to build system classes and JARs specific to the Nexus One. (Again, the exact command will vary depending on your device; try print_lunch_menu to see the available options. Also, you may need to clone the relevant git repository under device/<vendor>/<model>.
   
5. Type make to build the new configuration.

This assumes that, like me, you've already built the generic source tree, tried to replace something like framework.jar on the phone, and had it fail mysteriously. (In my case, there was a NullPointerException while loading resources pertaining to the mobile carrier - something that sounded suspiciously related to something device-specific.) Unfortunately, this does require rebuilding everything from the start, into a new subdirectory of out/target/product/. Once it's done, though, building the individual piece you're interested in changing will be quick.

I get the sense that this is a pretty basic step that Android porters deal with all the time, but when I was building for the HTC Dream, the generic build (that is, lunch 1 or lunch generic-eng) was sufficient for replacing services.jar. Maybe it's more that framework.jar has some device-specific components that I wasn't aware of.