Architecture Overview §

Android is made up of several layers:

Applications §

This is where your app lives, and is the top layer in the software stack.

Application Framework §

This layer provides high-level services to applications in the form of Java classes. This includes:

• Activity Manager, which controls application lifecycle and Activity stacks
• Content Providers, which allow applications to publish and share data with other applications
• Resource Manager, providing access to resources such as Strings, colors, layouts etc

Binder IPC §

The Binder Inter-Process Communication mechanism allows the Application Framework to cross process boundaries (Android runs all apps with process isolation) and call into the Android system services code. This enables high-level code to interact with the System Server and other remote service components. At the application framework level, this communication is hidden from the developer and things “just work”.

Intents, ContentProviders and Messengers are built ontop of Binder, as they enable communication across processes. When a process sends a message to another process, the kernel allocates space in the destination process’s memory and copies the message data directly from the sending process. It then queues a message which tells the receiver where the message data is.

Each Binder object has a unique reference in memory and this is used for security as it can act as an identifier token - noone is able to create another Binder with the same identifier. For instance, calling PowerManagerService to acquire a WakeLock requires a token - releasing that WakeLock requires the same token. This way, one application or process cannot pretend to be another and interfere.

A good example of this is in the View class, where getWindowToken() is called extensively. This is a Binder token, and is used to ensure that malicious applications aren’t allowed to draw ontop of other applications.

Android Runtime §

This is part of the third layer along with Android libraries. ART uses AOT compilation to translate DEX bytecode down to native instructions in Executable & Linkable Format (ELF)

Android Libraries §

Encompasses Java-based libraries available to Android apps. This includes packages such as:

• android.app
• android.content
• android.database
• android.os
• android.net
• android.text
• android.view
• android.widget
• android.webkit

These by and large are Java wrappers over C++ - not a tonne of logic here.

HAL §

The Hardware Abstraction Layer sits ontop of the kernel, and is responsible for:

• Camera
• Audio
• Graphics
• And more

It defines a standard interface for manufacturers to implement which allows Android to be agnostic about lower-level driver implementations.

Linux Kernel and Libraries §

Onotop of the kernal there’s a set of libraries in C++ such as WebKit, SQLite, SSL etc. The kernel provides abstraction over device hardware and contains drivers, networking, memory management etc.