嵌入式Android系统开发详解

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"嵌入式Android开发详解" "Embedded Android" 是一本由Karim Yaghmour编写的关于深入理解和定制Android系统的书籍。这本书旨在帮助开发者掌握如何对Android的源代码进行修改，以适应嵌入式设备的需求。以下是书中涵盖的一些关键知识点： 1. **Android历史**：书中介绍了Android自诞生以来的发展历程，包括它如何从一个开源项目演变成全球最广泛使用的移动操作系统之一。 2. **特性与特性**：讨论了Android的主要功能和特点，这些特性使得Android系统在各种设备上得以广泛应用，如智能手机、平板电脑和智能电视等。 3. **开发模型**：探讨了Android的开放源代码开发模式，包括AOSP（Android Open Source Project）的工作流程、版本发布和维护策略。 4. **与传统开源项目的差异**：对比了Android与其他开源项目在管理、更新和社区参与上的不同之处。 5. **版本发布、路线图与新特性**：分析了Android版本的包含内容、未来发展方向以及新版本的发布计划。 6. **生态系统**：详述了Android生态系统，包括OEM厂商、应用开发者、用户和第三方市场的相互作用。 7. **开放手持设备联盟（OHA）**：解释了OHA的角色及其对Android发展的影响，以及成员之间的合作方式。 8. **获取Android**：指南如何获取和使用Android源代码，以及相关的法律框架，如代码许可和品牌使用规定。 9. **Google的Android应用**：讨论了Google自家的应用和服务如何与开源Android系统相结合，以及在非Google认证设备上的使用情况。 10. **替代应用市场**：提到了除了Google Play之外的其他应用分发渠道，如亚马逊应用商店和其他第三方市场。 11. **Oracle与Google的法律纠纷**：简要回顾了Oracle起诉Google关于Java API使用的问题，这对Android生态系统产生的影响。 12. **硬件与合规性要求**：介绍了Android的硬件兼容性要求，包括合规性定义文档（CDD）和合规性测试套件（CTS），以确保设备与系统的兼容性。 13. **开发环境与工具**：详细讲解了设置Android开发环境、使用Android Studio等开发工具的方法，以及如何进行原生（Native）开发。 14. **内部原理介绍**：从应用开发者的角度出发，深入解析Android的内部机制，如Android的概念、框架结构、应用程序开发工具等。 15. **系统架构**：涵盖了Android的整体架构，包括Linux内核的作用、Wakelocks（唤醒锁）用于电源管理、Low Memory Killer（低内存杀手）处理内存紧张的情况，以及Binder作为进程间通信（IPC）的核心组件。 16. **匿名共享内存（ashmem）**：描述了ashmem机制，允许不同进程之间高效地共享内存。 17. **Alarm服务**：解释了Android如何通过Alarm Manager来调度事件和任务。 18. **日志系统（Logger）**：讨论了Android的日志系统，它是调试和监控应用的重要工具。除此之外，书中还涉及到了更多关于Android系统的底层组件和技术，如电源管理、性能优化、安全性等方面的知识，为读者提供了全面而深入的Android开发经验。对于想要从事Android嵌入式开发或者深度定制Android系统的开发者来说，这是一本不可多得的参考资料。

Differences With "Classic" Open Source Projects

Android's open source nature is one of its most trumpeted features. Indeed, as we've

just seen, many of the software engineering benefits that derive from being open source

apply to Android.

Despite its licensing, however, Android is unlike most open source projects in that its

development is mostly done behind closed doors. The vast majority of open source

projects, for example, have public mailing lists and forums where the main developers

can be found interacting with each other, and public source repositories providing

access to the main development branch's tip. No such thing can be found for Android.

This is best summarized by Andy Rubin himself: “Open source is different than a com-

munity-driven project. Android is light on community-driven, somewhat heavy on

open source.”

Whether we like it or not, Android is mainly developed within Google by the Android

development team and the public is not privy to either internal discussions or the tip

of the development branch. Instead, Google makes code-drops every time a new version

of Android ships on a new device, which is usually every 6 months. For instance, a few

days after the Samsung Nexus S was released in December 2010, the code for the new

version of the Android it was running, 2.3/Gingerbread, was made publicly available

at http://android.git.kernel.org/.

Obviously there is a certain amount of discomfort within the open source community

with the contined use of the term "open source" within the context of project whose

development model contradicts the standard modus operandi typically associated with

open source projects, especially given Android's popularity. The open source com-

munity has not historically been well served by projects that had adopted a similar

development model.

Political issues asside, though, Android's development model means that as a developer

your ability to make contributions to Android is limited. Indeed, unless you become

part of the Android development team at Google, you will not be able to make contri-

butions to the tip of the development branch. Also, save for a handful of exceptions,

you will unlikely be able to discuss your enhancements one-on-one with the core de-

velopment team members. However, you are still free to submit enhancements and

fixes to the AOSP code dumps made available at http://android.git.kernel.org/.

The worst side-effect of Google's approach is that you have absolutely no way to get

inside information about the platform decisions being made by the Android develop-

ment team. If new features are added within the AOSP, for example, or if modifications

are made to core components, you will find out how such changes are made and how

they impact changes you might have made to a previous version only by analyzing the

next code dump. Furthermore, you will have no way to find out the underlying re-

quirement or restriction or issue that justified the modification or inclusion. Had this

Development Model | 5

been a true open source project, a public mailing list archive would exist where all this

information, or pointers to it, would be available.

That being said, it's important to remember how significant a contribution Google is

making by distributing Android under an open source license. Despite its awkward

development model from an open source community perspective, it remains that Goo-

gle's work on Android is a godsend for a large number of developers. Plus, it has ac-

complished one thing no other open source projects was ever able to: create a massively

successful Linux distribution. It would, therefore, be hard to fault Android's develop-

ment team for their work.

Furthermore, it can easily be argued that from a business and go-to-market perspective

a community-driven process would definitely knock the wind out of any product an-

nouncements Google would attempt to release, making it impossible to create "buzz"

around press announcements and the like since every new feature would be developed

in the open. That is to say nothing of the non-deterministic nature of community-driven

processes that can see a group of people take years to agree on the best way to implement

a given feature set. And, simply based on track record, Android's success has definitely

benefited from Google's ability to rapidly move it forward and generate press interest

based on releases of cool new products.

Feature Inclusion, Roadmaps, and New Releases

In brief, there is no publicly available roadmap for features and capabilities in future

Android releases. At best, Google will announce ahead of time the name and approx-

imate release date of the next version. Usually, you can expect a new Android release

to be made in time for the Google I/O conference, which is typically held in May, and

another release by year end. What will be in that release, though, is anyone's guess.

Typically, however, Google will choose a single manufacturer to work with on the next

Android release. During that period, Google will work very closely with that single

manufacturer's engineers to ready up the next Android version to work on a targeted

upcoming flagship device. During that period, the manufacturer's team is reported to

have access to the tip of the development branch. Once the device is put on the market,

the corresponding source code dump is made to the public repositories. For the next

release, they choose another manufacturer and start over.

There is one notable exception to that cycle: Android 3.x/Honeycomb. In that specific

case, Google has not released the source code to the corresponding flagship product,

the Motorola Xoom, and all indications suggest that that code may never be publicly

available. The rationale in this case seems to be that the Android development team

essentially forked the Android code-base at some point in time in order to start working

on getting a tablet-ready version of Android out ASAP based on market timing prerog-

atives. Hence, in that version, very little regard was made to preserving backward com-

patibility with the phone form-factor. And given that, Google did not wish to make the

code available to avoid fragmentation of its platform. Instead, at the time of this writing,

6 | Chapter 1: Introduction

plans are that both the phone and tablet form factor support will be merged into the

upcoming Android 4.0/Ice-Cream Sandwich release.

Ecosystem

As of June 2011:

• 400,000 Android phones are activated each day, up from 300,000 in December

2010 and 200,000 in August of that same year.

• The Android Market contains arond 200,000 apps. In comparison, the Apple App

Store has 350,000 apps.

• More than a third of all phones sold in the US are based on Android.

Android is clearly on the upswing. In fact, Gartner predicted in April 2011 that Android

would hold about 50% of the smartphone market by 2015. Much as Linux disrupted

the embedded market about a decade ago, Android is poised to make its mark. Not

only will it flip the mobile market on its head, eliminating or sidelining even some of

the strongest players, but in the embedded space it is likely going to become the de-

facto standard UI for a vast majority of user-centric embedded devices.

An entire ecosystem is therefore rapidly building around Android. Silicon and system-

on-chip (SoC) manufacturers such as ARM, TI, Qualcomm, Freescale, NVidia and TI

have added Android support for their products, and handset and tablet manufacturers

such as Motorola, Samsung, HTC, Sony-Ericsson, LG, Archos, DELL, ASUS, etc. ship

an ever-increasing number of Android-equipped devices. This ecosystem also includes

an increasing number of diverse players, such as Amazon, Verizon, Sprint and

Barnes&Nobles, creating their own application markets.

Grass-roots communities and projects are also starting to sprout around Android, even

though it is developed behind closed doors. Some of those efforts follow in the footsteps

of phone modders, which essentially rely on hacking the binaries provided by the man-

ufacturers to create their own modifications or variants, while others have a more open

source tint to them, relying on the Android sources to create their own forks or addi-

tions. The xda-developers.com online forum, for instance, is traditionally frequented

by modders, while the cyanogenmod.com site hosts an Android fork which modifies

the Android sources to provide additional features and enhancements. Other Android

forks include Replicant (http://replicant.us), which aims at replacing as many of the

Android components with Free Software as possible, and MIUI (http://en.miui.com/),

which provides some cool UI hacks.

A Word on the Open Handset Alliance

As I mentioned earlier, the OHA was the initial vehicle through which Android was

first announced. It describes itself on its website as "... a group of 82 technology and

mobile companies who have come together to accelerate innovation in mobile and offer

Ecosystem | 7

consumers a richer, less expensive, and better mobile experience. Together we have

developed Android™, the first complete, open, and free mobile platform."

Beyond the initial announcement, however, it is unclear what role the OHA plays. For

example, an attendee at the "Fireside Chat with the Android Team" at Google I/O 2010

asked the panel what privileges were confered to him as a developer for belonging to a

company which is part of the OHA. After asking around the panel, the speaker essen-

tially answered that they didn't know because they aren't the OHA. Hence, it would

appear that OHA membership benefits are not clear to the Android development team

itself.

The role of the OHA is further blurred by the fact that it does not seem to be a full-time

organization with board members and permanent staff. Instead, it's just an "alliance."

In addition, there is no mention of the OHA within any of Google's Android an-

nouncements, nor do any new Android announcements emanate from the OHA. In

sum, one would be tempted to speculate that Google likely put the OHA together

mainly as a marketing front to show how much industry support there was for Android,

but that in practice it has little to no bearing on Android's development.

Getting "Android"

There are two main pieces required to get Android working on your embedded system:

an Android-compatible Linux kernel and the Android Platform.

As of this writing, you cannot use a "vanilla" kernel from kernel.org to run the Platform.

Instead, you need to either use one of the kernels available within the AOSP or patch

a "vanilla" kernel for it to be Android-compatible. Unfortunately, while a few attempts

have been made to merge the Android modifications into the mainline kernel, these

efforts haven't yet succeeded. It is expected that, in the long term, the pending issues

will be resolved and the mainline kernel will support Android "out of the box." For the

time being, however, we must contend with the fact Android-compatible kernels are

essentially forks of the mainline and will continue being so for the foreseable future.

The Android Platform is essentially a custom Linux distribution containing the user-

space packages which make up what is typically called "Android." The releases listed

in Table 1-1 are actually Platform releases. We will discuss the content and architecture

of the Platform in the next chapter. For the moment being, keep in mind that a Platform

release has a similar role to standard Linux distributions such as Ubuntu or Fedora. It's

a self-coherent set of software packages that, once built, provides a specific user expe-

rience with specific tools, interfaces, and developer APIs.

8 | Chapter 1: Introduction

While the proper term to identify the source code corresponding to the

Android distribution running on top of an Android-compatible kernel

is "Android Platform," it is commonly referred to as "the AOSP"—as is

the case in fact throughout this book—even though the Android Open

Source Project proper, which is hosted at http://android.git.kernel.org/,

contains a few more components in addition to the Platform, such as

sample Linux kernel trees and additional packages that would not typ-

ically be downloaded when the Platform is fetched using the usual

repo command.

Hacking Binaries

Lack of access to Android sources hasn't discouraged passionate modders from actually

hacking and customizing Android to their liking. For example, the fact that Android

3.x/Honeycomb isn't available hasn't precluded modders from getting it to run on the

Barnes&Noble Nook. They achieved this by retrieving the executable binaries found

in the emulator image provided as part of the Honeycomb SDK and used those as-is

on the Nook, albeit forfeiting hardware accelaration. The same type of hack has been

used to "root" or update versions of various Android components on actual devices for

which the manufacturer provides no source code.

Legal Framework

Like any other piece of software, Android's use and distribution is constricted by a set

of licenses, intellectual property restrictions, and market realities. Let's look at a few of

these.

Code Licenses

As we discussed earlier, there are two parts to "Android": an Android-compatible Linux

kernel and an AOSP release. Even though it is modified to run the AOSP, the Linux

kernel continues to be subject to the same GNU GPLv2 license that it has always been

under. As such, remember that you are not allowed to distribute any modifications you

make to the kernel under any other license than the GPL. Hence, if you take a kernel

version from android.git.kernel.org and modify it to make it run on your system, you

are allowed to distribute the resulting kernel image in your product only so long as you

abide by the GPL and, therefore, make the sources used to create the image, including

your modifications, available to recipients under the terms of the GPL.

The kernel also includes a notice by Linus Torvalds in its COPYING file in its sources

that clearly identifies that only the kernel is subject to the GPL and that applications

running on top of it are not considered "derived works." Hence, you are free to create

Legal Framework | 9

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嵌入式Android系统开发详解

embedded android

Embedded Android

Embedded System

halcon embedded for android

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zxing-android-embeddecore/contentd

implementation 'com.journeyapps:zxing-android-embedded:4.0.0'报错Unexpected tokens (use ';' to separate expressions on the same line)

embedded browser(3.1.0)

flutter-embedded-linux

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