原文：https://github.com/android/platform_system_core/blob/master/adb/OVERVIEW.TXT）

Implementation notes regarding ADB.

ADB实现注解

1. General Overview:

1概要

The Android Debug Bridge (ADB) is used to:

ADB在下面情况下使用：

• keep track of all Android devices and emulators instances connected to or running on a given host developer machine
• 对全部连接到开发机器上的android真机和模拟器进行跟踪管理
• implement various control commands (e.g. "adb shell", "adb pull", etc..) for the benefit of clients (command-line users, or helper programs like DDMS). These commands are what is called a ‘service‘ in ADB.
• 实现了大量的控制命令（比方: "adb shell", "adb pull",等等）来方便用户使用（包含命令行用户和助手类程序如ddms),这些命令往往被我们叫做adb中的一个‘服务’。

As a whole, everything works through the following components:

总而言之。全部的事情都是环绕着下面这几个模块进行的：

1.1 The ADB server

1.1 ADBserver

This is a background process that runs on the host machine. Its purpose if to sense the USB ports to know when devices are attached/removed,as well as when emulator instances start/stop.

这是在主机设备(PC/开发机器)上执行的一个后台进程。它的目的是嗅探何时有设备在主机的usb口上挂载/移除，以及模拟器何时开启/关闭。

It thus maintains a list of "connected devices" and assigns a ‘state‘ to each one of them: OFFLINE, BOOTLOADER, RECOVERY or ONLINE (more on this below).

因此它会维护着一个"已连接设备"列表，而且为每一个设备指定一个‘状态’：OFFLINE, BOOTLOADER, RECOVERY 或 ONLINE (下文会详述）。

The ADB server is really one giant multiplexing loop whose purpose is to orchestrate the exchange of data (packets, really) between clients, services and devices.

1.2 The ADB daemon (adbd)

The ‘adbd‘ program runs as a background process within an Android device or emulated system. Its purpose is to connect to the ADB server (through USB for devices, through TCP for emulators) and provide a few services for clients that run on the host.

adbd是一个在android真实机器或者模拟器上执行的后台伺服程序。

它的目的是为了连接pc端的adbserver（真实机器用usb，模拟器用tcp协议（译者注：事实上真实机器也能够用tcp来连接。这篇文章没有及时更新过来））而且为在主机pc上执行的adbclient应用提供一些服务。

The ADB server considers that a device is ONLINE when it has successfully connected to the adbd program within it. Otherwise, the device is OFFLINE, meaning that the ADB server detected a new device/emulator, but could not connect to the adbd daemon.

当adbserver成功连接上android机器上的adbd伺服程序的时候就会觉得该设备已经online，否者就会觉得该设备是offline，指的是adbserver有检測到一个新的设备连接上来，可是却没有成功连接上该设备的的adbd。

the BOOTLOADER and RECOVERY states correspond to alternate states of devices when they are in the bootloader or recovery mode.

BOOTLOADER和RECOVERY着两个状态分别代表android设备处于bootloader或者recovery模式下的相应的可选状态。

1.3. The ADB command-line client

The ‘adb‘ command-line program is used to run adb commands from a shell or a script. It first tries to locate the ADB server on the host machine, and will start one automatically if none is found.

adb命令行client是给shell或者脚本调用来跑各种adb命令的。它首先会尝试找到主机pc上执行的adbserver，假设没有找到的话就会自己主动启动一个adbserver。

then, the client sends its service requests to the ADB server. It doesn‘t need to know.

然后该adb命令行client会往adbserver发送服务请求。而这些对于adbserver来说是无需知道的。

1.4. Services

There are essentially two kinds of services that a client can talk to.

本质上一个adb命令行client会和两类服务进行通信。

Host Services: these services run within the ADB Server and thus do not need to communicate with a device at all. A typical example is "adb devices" which is used to return the list of currently known devices and their state. They are a few couple other services though.

主机服务：这些服务是在adbserver自身内部执行的所以根本不须要和不论什么的android设备进行交互。一个典型的命令就是列出当前连接的全部android设备和状态的命令“adb devices”。

当然另一些其它的服务了。