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:

1. Name: Give your build a name (LOCAL_MODULE := <build_name>).
2. Local Variables: Clear local variables with CLEAR_VARS (include $(CLEAR_VARS)).
3. Files: Determine which files your application depends upon (LOCAL_SRC_FILES := main.c).
4. Tags: Define tags, as necessary (LOCAL_MODULE_TAGS := eng development).
5. Libraries: Define whether your application links with other libraries (LOCAL_SHARED_LIBRARIES := cutils).
6. 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.

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: