Embedded Systems Series Part 9: Android System Architecture

Introduction to Android Architecture

Android is not just a smartphone OS—it powers embedded devices from automotive infotainment to industrial HMIs. Understanding AOSP (Android Open Source Project) internals is essential for embedded Android development.

AOSP Layer Overview

+-----------------------------------------+
│         Applications (APKs)             │  → Java/Kotlin
+-----------------------------------------+
│       Android Framework (Java)          │  → System APIs
+-----------------------------------------+
│    Native Libraries + Android Runtime   │  → C/C++, ART
+-----------------------------------------+
│       HAL (Hardware Abstraction)        │  → HIDL/AIDL
+-----------------------------------------+
│         Linux Kernel + Drivers          │  → Kernel modules
+-----------------------------------------+

Init Process & Boot

Android boot sequence: Bootloader → Kernel → init (PID 1) → Zygote → System Server → Apps.

flowchart TD
    BL["Bootloader"] --> KERNEL["Linux Kernel"]
    KERNEL --> INIT["init\n(PID 1)"]
    INIT --> PROP["Property Service"]
    INIT --> ZYGO["Zygote\n(App Runtime)"]
    ZYGO --> SS["System Server\n(AMS, WMS, PMS)"]
    SS --> LAUNCHER["Launcher\n(Home Screen)"]
    INIT --> SERV["Native Services\n(SurfaceFlinger, AudioFlinger)"]
    ZYGO --> APP["App Process\n(fork from Zygote)"]
                        

# Android init process location
/system/bin/init         # First userspace process

# Key init.rc files
/system/etc/init/        # Core system services
/vendor/etc/init/        # Vendor-specific services
/init.rc                 # Main configuration

# Init service definition
service surfaceflinger /system/bin/surfaceflinger
    class core animation
    user system
    group graphics drmrpc readproc
    onrestart restart zygote
    writepid /dev/stune/foreground/tasks

System Services

System Server hosts critical services: ActivityManager, PackageManager, WindowManager, etc.

# View running services
adb shell service list

# Key system services
activity              # Activity lifecycle management
package               # App installation/packages
window                # Window management
input                 # Input events
SurfaceFlinger        # Display composition (native)
AudioFlinger          # Audio mixing (native)

# Service Manager registration (Java)
ServiceManager.addService("my_service", myBinder);

Binder IPC Mechanism

Binder is Android's IPC mechanism—efficient, type-safe, and secure. It enables communication between apps and system services.

// AIDL interface definition (IMyService.aidl)
interface IMyService {
    int getValue();
    void setValue(int value);
}

// Client usage
IMyService service = IMyService.Stub.asInterface(
    ServiceManager.getService("my_service")
);
int value = service.getValue();

Android Runtime (ART)

ART replaced Dalvik—provides Ahead-of-Time (AOT) compilation for better performance.

# Zygote - parent of all Android apps
/system/bin/app_process64   # 64-bit Zygote
/system/bin/app_process32   # 32-bit Zygote

# Zygote preloads classes and resources
# Apps fork from Zygote (fast startup)

# ART compilation
dex2oat --dex-file=app.apk --oat-file=app.oat

# Runtime files
/system/framework/         # Framework JARs
/data/dalvik-cache/        # Compiled OAT files

SELinux in Android

# SELinux mode
getenforce                # Enforcing or Permissive
adb shell setenforce 0    # Permissive (debugging only!)

# SELinux policy files
/system/etc/selinux/      # System policies
/vendor/etc/selinux/      # Vendor policies

# Check denials
adb logcat | grep avc     # Access Vector Cache denials

# audit2allow - generate policy from denials
audit2allow -i audit.log

Android Partitions

Android Build System

# AOSP source setup
repo init -u https://android.googlesource.com/platform/manifest
repo sync -j8

# Build commands
source build/envsetup.sh
lunch aosp_arm64-eng       # Select target
make -j$(nproc)            # Build everything

# Build specific module
make SurfaceFlinger
make bootimage             # Kernel + ramdisk

# Output
out/target/product/*/system.img
out/target/product/*/boot.img

Conclusion & What's Next

You've learned Android's architecture—from the Linux kernel through HAL, native libraries, ART, and the Java framework. This foundation is essential for Android system development.

In Part 10, we'll explore Android HAL & Native Development—creating HAL interfaces and native code with NDK/JNI.