Phase 2 — AOSP Architecture & Source Code

Duration: 3–4 weeks | Level: Intermediate

Understand the layered architecture of Android before diving into any single component.

🗂️ Android System Architecture (Layers)

┌──────────────────────────────────────────┐
│            Applications                  │  ← APKs (your apps)
├──────────────────────────────────────────┤
│         Application Framework            │  ← Java APIs: AMS, WMS, PMS, etc.
├──────────────────────────────────────────┤
│    Native Libraries  │  Android Runtime  │  ← C/C++ libs + ART/Dalvik
├──────────────────────────────────────────┤
│         Hardware Abstraction Layer       │  ← HAL: camera, audio, sensors
├──────────────────────────────────────────┤
│              Linux Kernel                │  ← Drivers, memory, power
└──────────────────────────────────────────┘

Android is built in layers. Each layer only communicates with the layer directly below it through well-defined interfaces.

🏗️ Build System: Soong + Make

Android's build system has two components:

Android.mk (legacy Make)

LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE    := hello_world
LOCAL_SRC_FILES := hello.cpp
include $(BUILD_EXECUTABLE)

Android.bp (Soong — modern)

cc_binary {
    name: "hello_world",
    srcs: ["hello.cpp"],
    shared_libs: ["liblog"],
}

Soong uses Blueprint language. Most new modules use .bp. Key build rules:

• cc_binary, cc_library_shared, cc_library_static — C/C++ targets
• java_library, android_app — Java/app targets
• hidl_interface, aidl_interface — IPC interface definitions

📦 Key Directories Deep Dive

frameworks/base/

The heart of the Android Framework. Read these:

frameworks/base/
├── core/java/android/         # Public Android APIs
├── services/core/java/        # System services (AMS, WMS, PMS)
├── core/jni/                  # JNI bridge to native code
└── cmds/                      # System command-line tools

system/core/

Low-level system daemons:

system/core/
├── init/       # Android init process (PID 1)
├── adb/        # ADB daemon
├── fastboot/   # Fastboot protocol
└── libutils/   # Base utility libraries

hardware/interfaces/

HIDL/AIDL HAL interface definitions:

hardware/interfaces/
├── audio/
├── camera/
├── sensors/
└── wifi/

🔍 Reading AOSP Source Code

How to find anything fast

# Find all files with a class name
find . -name "ActivityManagerService.java"
 
# grep with context
grep -rn "startActivity" frameworks/base/services/core/ --include="*.java" | head -30
 
# Use cs.android.com (best for navigation)

Key source files to read first

🧩 Android Runtime (ART)

ART replaced Dalvik in Android 5.0+.

• AOT compilation: On install, .dex bytecode → native machine code via dex2oat
• JIT compilation: At runtime, hot paths get JIT compiled
• Garbage Collection: Concurrent, generational GC
• Profile-guided optimization (PGO): After first run, frequently-used paths get AOT compiled

Key ART files: art/runtime/, art/compiler/

🛠️ Hands-On Tasks

1. Add a log line to SystemServer.java and rebuild framework only:

   mmm frameworks/base/services
   adb sync system
   adb reboot

2. Add a new cc_binary hello world using Soong (Android.bp), build and push to emulator.

3. Explore ActivityThread.java — find where onCreate() is called.

4. Read SystemServer.java — list all services started in startOtherServices().

✅ Phase 2 Checklist

• Can explain all 5 layers of Android architecture
• Understand Soong build system (Android.bp)
• Can build a single module (mmm)
• Know top-level dirs: frameworks/, system/, hardware/, art/
• Read SystemServer.java and listed all started services
• Read ActivityThread.java — understand app main loop
• Added a custom Android.bp module and built it
• Can use cs.android.com for code navigation
• Understand what ART does vs Dalvik