Android Porting Book--Android Build System

Android
Build System

In this
document

Understanding
the makefile

Layers

Building
the Android Platform

Building
the Android Kernel

Build
Variants

Android
uses a custom build system to generate tools, binaries, and
documentation. This document provides an overview of Android's build
system and instructions for doing a simple build.

Android's
build system is make based and requires a recent version of GNU Make
(note that Android uses advanced features of GNU Make that may not
yet appear on the GNU Make web site). Before continuing, check your
version of make by running

%
make -v

.
If you don't have version 3.80 or greater, you need to upgrade
your version of make.

Understanding
the makefile

A
makefile defines how to build a particular application. Makefiles
typically include all of the following elements:

Name:
Give your build a name (

LOCAL_MODULE
:= <build_name>

).

Local
Variables: Clear local variables with CLEAR_VARS (

include
$(CLEAR_VARS)

).

Files:
Determine which files your application depends upon
(

LOCAL_SRC_FILES
:= main.c

).

Tags:
Define tags, as necessary (

LOCAL_MODULE_TAGS
:= eng development

).

Libraries:
Define whether your application links with other libraries
(

LOCAL_SHARED_LIBRARIES
:= cutils

).

Template
file: Include a template file to define underlining make tools for
a particular target (

include
$(BUILD_EXECUTABLE)

).

The
following snippet illustrates a typical makefile.

LOCAL_PATH := $(my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := <buil_name>
LOCAL_SRC_FILES := main.c
LOCAL_MODULE_TAGS := eng development
LOCAL_SHARED_LIBRARIES := cutils
include $(BUILD_EXECUTABLE)
(HOST_)EXECUTABLE, (HOST_)JAVA_LIBRARY, (HOST_)PREBUILT, (HOST_)SHARED_LIBRARY,
(HOST_)STATIC_LIBRARY, PACKAGE, JAVADOC, RAW_EXECUTABLE, RAW_STATIC_LIBRARY,
COPY_HEADERS, KEY_CHAR_MAP

The
snippet above includes artificial line breaks to maintain a
print-friendly document.

Layers

The
build hierarchy includes the abstraction layers described in the
table below.

Each
layer relates to the one above it in a one-to-many relationship. For
example, an arch can have more than one board and each board can
have more than one device. You may define an element in a given
layer as a specialization of an element in the same layer, thus
eliminating copying and simplifying maintenance.

Layer	Example	Description
Product	myProduct, myProduct_eu, myProduct_eu_fr, j2, sdk	The product layer defines a complete specification of a shipping product, defining which modules to build and how to configure them. You might offer a device in several different versions based on locale, for example, or on features such as a camera.
Device	myDevice, myDevice_eu, myDevice_eu_lite	The device layer represents the physical layer of plastic on the device. For example, North American devices probably include QWERTY keyboards whereas devices sold in France probably include AZERTY keyboards. Peripherals typically connect to the device layer.
Board	sardine, trout, goldfish	The board layer represents the bare schematics of a product. You may still connect peripherals to the board layer.
Arch	arm (arm5te) (arm6), x86, 68k	The arch layer describes the processor running on your board.

Building
the Android Platform

This
section describes how to build the default version of Android. Once
you are comfortable with a generic build, then you can begin to
modify Android for your own target device.

Device
Code

To
do a generic build of android, source

build/envsetup.sh

,
which contains necessary variable and function definitions, as
described below.

% cd $TOP

% . build/envsetup.sh

# pick a configuration using choosecombo
% choosecombo

% make -j4 PRODUCT-generic-user

You
can also replace user with eng for a debug engineering build:

% make -j4 PRODUCT-generic-eng

These Build
Variants differ
in terms of debug options and packages installed.

Cleaning
Up

Execute

%
m clean

to
clean up the binaries you just created. You can also execute

%
m clobber

to
get rid of the binaries of all combos.

%
m clobber

is
equivalent to removing the

//out/

directory
where all generated files are stored.

Speeding
Up Rebuilds

The
binaries of each combo are stored as distinct sub-directories
of

//out/

,
making it possible to quickly switch between combos without having
to recompile all sources each time.

However,
performing a clean rebuild is necessary if the build system doesn't
catch changes to environment variables or makefiles. If this happens
often, you should define the

USE_CCACHE

environment
variable as shown below:

% export USE_CCACHE=1

Doing
so will force the build system to use the ccache compiler cache
tool, which reduces recompiling all sources.

ccache

binaries
are provided in

//prebuilt/...

and
don't need to get installed on your system.

Troubleshooting

The
following error is likely caused by running an outdated version of
Java.

device Dex: core  UNEXPECTED TOP-LEVEL ERROR:
java.lang.NoSuchMethodError: method java.util.Arrays.hashCode with
signature ([Ljava.lang.Object;)I was not found.
at com.google.util.FixedSizeList.hashCode(FixedSizeList.java:66)
at com.google.rop.code.Rop.hashCode(Rop.java:245)
at java.util.HashMap.hash(libgcj.so.7)
[...]

dx

is
a Java program that uses facilities first made available in Java
version 1.5. Check your version of Java by executing

%
java -version

in
the shell you use to build. You should see something like:

java version "1.5.0_07"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_07-164)
Java HotSpot(TM) Client VM (build 1.5.0_07-87, mixed mode, sharing)

If
you do have Java 1.5 or later and your receive this error, verify
that you have properly updated your

PATH

variable.

Building
the Android Kernel

This
section describes how to build Android's default kernel. Once you
are comfortable with a generic build, then you can begin to modify
Android drivers for your own target device.

To
build the kernel base, switch to the device directory
(

/home/joe/android/device

)
in order to establish variables and run:

% . build/envsetup.sh
% partner_setup generic

Then
switch to the kernel directory

/home/joe/android/kernel

.

Checking
Out a Branch

The
default branch is always

android

.
To check out a different branch, execute the following:

% git checkout --track -b android-mydevice origin/android-mydevice
//Branch android-mydevice set up to track remote branch
% refs/remotes/origin/android-mydevice.
//Switched to a new branch "android-mydevice"

To
simplify code management, give your local branch the same name as
the remote branch it is tracking (as illustrated in the snippet
above). Switch between branches by executing

%
git checkout <branchname>

.

Verifying
Location

Find
out which branches exist (both locally and remotely) and which one
is active (marked with an asterisk) by executing the following:

% git branch -a
android
* android-mydevice
origin/HEAD
origin/android
origin/android-mydevice
origin/android-mychipset

To
only see local branches, omit the

-a

flag.

Building
the Kernel

To
build the kernel, execute:

% make -j4

Build
Variants

When
building for a particular product, it's often useful to have minor
variations on what is ultimately the final release build. These are
the currently-defined build variants:

eng	This is the default flavor. A plain make is the same as make eng . Installs modules tagged with: eng , debug , user , and/or development . Installs non-APK modules that have no tags specified. Installs APKs according to the product definition files, in addition to tagged APKs. ro.secure=0 ro.debuggable=1 ro.kernel.android.checkjni=1 adb is enabled by default.
user	make user This is the flavor intended to be the final release bits. Installs modules tagged with user . Installs non-APK modules that have no tags specified. Installs APKs according to the product definition files; tags are ignored for APK modules. ro.secure=1 ro.debuggable=0 adb is disabled by default.
userdebug	make userdebug The same as user , except: Also installs modules tagged with debug . ro.debuggable=1 adb is enabled by default.

If
you build one flavor and then want to build another, you should
run

make
installclean

between
the two makes to guarantee that you don't pick up files installed by
the previous flavor.

make
clean

will
also suffice, but it takes a lot longer.