A closer look at the Android project build system part I

摘自：http://www.androidenea.com/2009/04/closer-look-at-android-project-build.html

 

In this two part post we will take a closer look at the structure of the build system for native functionality. The build system is based on make and resides in the build folder in the project. The first part will look at what is found in this folder while the second part discusses the Android.mk file and options.

envsetup.sh
There is one important file in build directory itself. That is the envsetup.sh file. This shell script contains a lot of functionality that is nice to have when working with Android. To invoke it type

. build/envsetup.sh

in the android root directory. This will allow you to use a number of new commands including:

m - make from the top of the tree
mm - builds all modules in the current directory
cgrep - grep in all c-files
jgrep - grep in all java-files
mgrep - grep in all makefiles
lunch - choose build target

These shell commands are very useful when working with the platform. Whether you are looking for something specific in a source file or you just want to build the current module for a test.

build/target
The target directory contains make configurations for different boards and products. The target directory is where new products can be added as needed by a hardware manufacturer. In the current release there is a generic product description, and sdk product and some other files. The available boards are the emulator, a generic board and the simulator.

The board makefiles describe the hardware platform level and can be shared among a number of products. For example you can build both the generic product and the generic_with_google product to run on in the emulator.

The makefiles for different products are listed in the fileAndroidProducts.h. To add a new product put the makefile in the build/target/product folder and add the product toAndroidProduct.h. It is possible for product makefiles to inherit from each other to reduce the need for copying. The generic_with_google products inherits the generic product that inherits the core product to make up a three level product specification.

build/core
The core folder is the location of the build system files. These files are used to set up the build system and to build all the different modules as configured in the individual Android.mk files. The main.mk file is the entry point to the build system and it is this files that is included in the top level makefile in the project root directory. The build system can also be invoked by the shell command mm with at subset of the makefiles

The include order and hierarchy of the makefiles is rather complicated but an attempt of illustration can be found in this figure.


Most of the build configuration is set in theconfig.mkfile and thedefinitions.mkfile contains the functions to be invoked for the different source types. In the above figure the build system is invoked to build a shared library. The Android makefile uses theBUILD_SHARED_LIBRARYvariable set in theconfig.mkfile. This will set the build chain for building a shared library in a number of steps using several other files.