定制嵌入式Android：移植与扩展指南

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"《嵌入式Android移植、扩展与定制》是一本实践导向的书籍，专为想要将Android应用到嵌入式设备开发人员设计。它深入剖析了Android的架构，引导读者理解其源代码工作原理，以及如何对不同组件进行修改，从而定制出适合特定硬件平台的Android版本。对于那些熟悉嵌入式系统开发，并且对Linux有一定了解的开发者来说，这本书提供了宝贵的指导，帮助他们突破移动设备的限制，实现跨平台的Android适应性。本书首先介绍了Android的开发模型和运行所需的基本硬件环境，包括Linux内核和Dalvik虚拟机的基础知识。作者通过讲解如何在没有硬件的情况下设置和探索Android Open Source Project (AOSP) 的功能模拟器，让读者能够理解和掌握非递归构建系统，以及如何进行自定义修改。接下来，书中详细阐述了如何使用评估板来原型化嵌入式Android系统，包括对用户空间的深入了解，如根文件系统布局、adb工具以及Android命令行操作。特别重要的是第4、6和7章，它们深入探讨了Android构建系统和框架的工作原理，这些信息在官方源码中不易获取，能够显著节省开发者的时间和精力。对于那些负责创建自定义Android镜像、设备或ROM修改的团队，这本书更是不可或缺的资源。例如，Mark Gross，Android/Linux内核架构师，他认为此书在团队开发基于Intel的参考手机时，如果早些时候就有，将会大有裨益。书中提到的"Karim方法"可能是作者独特的工作流程或者解决问题的方法，这可能是书中一个实用的亮点，可以帮助读者更高效地进行定制工作。《嵌入式Android移植、扩展与定制》是一本实用的指南，不仅涵盖了理论知识，而且提供了丰富的实践案例和深入的技术细节，是嵌入式开发者进行Android平台迁移和定制的宝贵参考书籍。"

Preface

Android’s growth is phenomenal. In a very short time span, it has succeeded in becom‐

ing one of the top mobile platforms in the market. Clearly, the unique combination of

open source licensing, aggressive go-to-market, and trendy interface is bearing fruit for

Google’s Android team. Needless to say, the massive user uptake generated by Android

has not gone unnoticed by handset manufacturers, mobile network operators, silicon

manufacturers, and app developers. Products, apps, and devices “for,” “compatible with,”

or “based on” Android seem to be coming out ever so fast.

Beyond its mobile success, however, Android is also attracting the attention of yet an‐

other, unintended crowd: embedded systems developers. While a large number of em‐

bedded devices have little to no human interface, a substantial number of devices that

would traditionally be considered “embedded” do have user interfaces. For a goodly

number of modern machines, in addition to pure technical functionality, developers

creating user-facing devices must also contend with human-computer interaction

(HCI) factors. Therefore, designers must either present users with an experience they

are already familiar with or risk alienating users by requiring them to learn a lesser-

known or entirely new user experience. Before Android, the user interface choices

available to the developers of such devices were fairly limited and limiting.

Clearly, embedded developers prefer to offer users an interface they are already familiar

with. Although that interface might have been window-based in the past—and hence a

lot of embedded devices were based on classic window-centric, desktop-like, or desktop-

based interfaces—Apple’s iOS and Google’s Android have forever democratized the use

of touch-based, iPhone-like graphical interfaces. This shift in user paradigms and ex‐

pectations, combined with Android’s open source licensing, have created a groundswell

of interest about Android within the embedded world.

Unlike Android app developers, however, developers wanting to do any sort of platform

work in Android, including porting or adapting Android to an embedded device, rap‐

idly run into quite a significant problem: the almost total lack of documentation on how

to do that. So, while Google provides app developers with a considerable amount of

online documentation, and while there are a number of books on the topic, such as

O’Reilly’s Learning Android, embedded developers have to contend with the minimal‐

istic set of documents provided by Google at http://source.android.com. In sum, em‐

bedded developers seriously entertaining the use of Android in their systems were es‐

sentially reduced to starting with Android’s source code.

The purpose of this book is to remedy that situation and to enable you to embed Android

in any device. You will, therefore, learn about Android’s architecture, how to navigate

its source code, how to modify its various components, and how to create your own

version for your particular device. In addition, you will learn how Android integrates

into the Linux kernel and understand the commonalities and differences it has with its

Linux roots. For instance, we will discuss how Android leverages Linux’s driver model

to create its very own hardware layer and how to take “legacy” Linux components such

as glibc and BusyBox and package them as part of Android. Along the way, you will

learn day-to-day tips and tricks, such as how to use Android’s repo tool and how to

integrate with or modify Android’s build system.

Learning How to Embed Android

I’ve been involved with open source software since the mid-’90s. I was fortunate enough

to join in before it became recognized as the powerful software movement that it is today

and, therefore, witness its rise firsthand in the early 2000s. I’ve also made my share of

open source contributions and, yes, participated in a couple of, shall we say, colorful

flame wars here and there. Among other things, I also wrote the first edition of O’Reilly’s

Building Embedded Linux Systems.

So when Android—which I knew was Linux-based—started becoming popular, I knew

enough about Linux’s history and embedded Linux to know that it was worth investi‐

gating. Then, I was naively thinking: “I know Linux fairly well and Android is based on

Linux; how hard could it be?” That is, until I actually started to seriously look into and,

most importantly, inside Android. That’s when I realized that Android was very foreign.

Little of what I knew about Linux and the packages it’s commonly used with in embedded

systems applied to Android. Not only that, but the abstractions built in Android were

even weirder still.

So began a very long (and ongoing) quest to figure things out. How does Android work?

How is it different from regular Linux? How can I customize it? How can I use it in an

embedded system? How do I build it? How does its app development API translate into

what I know about Linux’s user-space? etc. And the more I dug into Android, the more

alien it felt and the more questions I had.

The first thing I did was to actually go to http://developer.android.com and http://

source.android.com and print out everything I could get my hands on, save for the actual

developer API reference. I ended up with a stack of about 8 to 10 inches of paper. I read

xii | Preface

through most of it, underlined a lot of the key passages I found, added plenty of notes

in the margins, and created a whole list of questions I couldn’t find answers for. In

parallel, I started exploring the sources made available by Google through the Android

Open Source Project (AOSP). In all honesty, it took me about 6 to 12 months before I

actually started feeling confident enough to navigate within the AOSP.

The book you presently hold is a result of the work I’ve done on Android since starting

to explore it—including the various projects I’ve been involved in, such as helping dif‐

ferent development teams customizing Android for use in their embedded designs. And

I’ve learned enough about Android to say this: By no means is this book exhaustive.

There are a lot of things about Android and its internals that this book doesn’t and can’t

cover. This book should, nevertheless, allow you to jump-start your efforts in molding

Android to fit your needs.

Audience for This Book

This book is primarily geared toward developers who intend to create embedded sys‐

tems based on Android or who would like to take Android and customize it for specific

uses. It’s assumed you know about embedded systems development and have at least a

good handle on how Linux works and how to interact with its command line.

I don’t assume you have any knowledge of Java, and you can get away without knowing

Java for quite a few of the tasks required to customize Android. However, as your work

within Android progresses, you’ll find it necessary to start becoming familiar with Java

to a certain degree. Indeed, many of Android’s key parts are written in Java, and you’ll

therefore need to learn the language in order to properly integrate most additions to

specific parts of the stack.

This book isn’t, however, about either app development or Java programming in any

way. If these are the topics you are interested in, I recommend you look elsewhere. There

are quite a few books on each of these topics already available. This book isn’t about

embedded systems, either, and there are books on that topic, too. Finally, this book isn’t

about embedded Linux, which also has its own books. Still, being familiar with Linux’s

use in embedded systems is something of a plus when it comes to Android. Indeed,

though Android is a departure from all things traditionally known as “embedded Linux,”

many of the techniques typically used for creating embedded Linux systems can guide

and help in the creation of embedded Android systems.

This book will also be helpful to you if you’re interested in understanding Android’s

internals. Indeed, customizing Android for use in embedded systems requires knowing

at least some basics about its internals. So while the discussion isn’t geared toward a

thorough exploration of Android’s sources, the explanations do show how to interact

with the various parts of the Android stack at a fairly intimate level.

Preface | xiii

Organization of the Material

Like many other titles, this book gradually builds in complexity as it goes, with the early

chapters serving as background material for later chapters. If you’re a manager and just

want to grab the essentials, or if you’re wondering which set of chapters you have to

read through before you can start skipping chapters and read material selectively, I

recommend you at least read through the first three chapters. That doesn’t mean that

the rest isn’t relevant, but the content is much more modular after that.

Chapter 1, Introduction, covers the general things you should know about Android’s

use in embedded systems, such as where it comes from, how its development model

and licensing differ from conventional open source projects, and the type of hardware

required to run Android.

Chapter 2, Internals Primer, digs into Android’s internals and exposes you to the main

abstractions it comprises. We start by introducing the app development model that app

developers are accustomed to. Then we dig into the Android-specific kernel modifica‐

tions, how hardware support is added in Android, the Android native user-space, Dal‐

vik, the system server, and the overall system startup.

Chapter 3, AOSP Jump-Start, explains how to get the Android sources from Google,

how to compile them into a functional emulator image, and how to run that image and

shell into it. Using the emulator is an easy way to explore Android’s underpinnings

without requiring actual hardware.

Chapter 4, The Build System, provides a detailed explanation of Android’s build system.

Indeed, unlike most open source projects out there, Android’s build system is nonre‐

cursive. This chapter explains the architecture of Android’s build system, how it’s typ‐

ically used within the AOSP, and how to add your own modifications to the AOSP.

Chapter 5, Hardware Primer, introduces you to the types of hardware for which Android

is designed. This includes covering the System-on-Chips (SoCs) typically used with

Android, the memory layout of typical Android systems, the typical development setup

to use with Android, and a couple of evaluation boards you can easily use for prototyping

embedded Android systems.

Chapter 6, Native User-Space, covers the root filesystem layout, the adb tool, Android’s

command line, and its custom init.

Chapter 7, Android Framework, discusses how the Android Framework is kick-started,

the utilities and commands used to interact with it, and the support daemons required

for it to operate properly.

Appendix A, Legacy User-Space, explains how to get a legacy stack of “embedded Linux”

software to coexist with Android’s user-space.

xiv | Preface

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KevinWu2019

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