why aapt2 does not generate the same ID than aapt? and what the purpose of Ids located inside R.java?

Question

With the equivalent command line, why aapt.exe output different ids than aapt2.exe? also how work exacly the r.java? I use aapt and aapt2 to produce the r.java like this :

"aapt.exe" package -f -m -M "AndroidManifest.xml" -I "android.jar" -S "\res" -J "\RJava" --auto-add-overlay

But I still not understand why the value of the generated ids inside the R.java matter? How those ids work exactly

Answer 1

The generated IDs in R.java will not only differ between a run of aapt vs a run of aapt2, but potentially also between consecutive runs of aapt resp. aapt2 themselves.

Purpose of resource IDs

The generation of the R.java file assigns to each resource an ID for referring to it during the app's runtime. As written in B.Burd & J.P.Mueller (2020), Android Application Development All-in-One For Dummies, 3rd Ed., Wiley, p.87:

The hexadecimal values in a R.java file are the jumping-off points for Android’s resource management mechanism. Android uses these numbers for quick and easy loading of the things you store in the res branch.

Structure of resource IDs

The structure of a resource ID is as depicted in Fig.1 (taken from the slides mentioned below):

Fig.1: Structure of a resource ID

• Package type: Mostly 0x7f (application), but also 0x01 (Android framework) and 0x00 (shared resource library) are common.
• Resource type: There is no fixed mapping between types and values (by just looking at an ID value one can’t say if one is dealing with e.g. a string or an integer), but within the same package, all resources of the same type will have the same type identifier.
• Entry: This is simply a running number.

Because of the fields Resource and Entry there's always the chance of changes in the ID assignment by aapt during compile time, even in consectutive runs. In the app's runtime the IDs will be static.

More info

For further details see the Android Dev Docs on accessing your app resources and these interesting slides by M.Kongstad & Z.Jovanovic from their talk on the SonyXperiaDev 2018. For an elaboration on the Doc's note

Caution: You should never modify the R.java file by hand—it is generated by the aapt tool when your project is compiled. Any changes are overridden next time you compile.

you may also take a look on the associated Stackoverflow thread on How to manually generate R.java. In short -- if one manually edits R.java one highly likely ruins the resource references and gets errors at the latest at runtime (the most pesky type of errors ever).