Android processes

Android startup sequence

1. Bootloader

• Initializes hardware.

• Loads the Linux kernel into memory.

• Passes control to the kernel.

2. Linux Kernel

• Starts low-level drivers.

• Mounts the root filesystem.

• Starts the init process (PID 1).

3. init process (Android Init)

• Reads init.rc scripts.

• Starts key system services, including:

  • servicemanager

  • vold

  • surfaceflinger

  • zygote

4. Zygote

• Preloads system classes/resources

• Preloads Android framework classes

• Preloads common libraries

• Forks to create:

  • System Server (which manages system services)

  • App processes when apps start

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Application startup

1. User Action and intent creation

• The user clicks an app icon.

• The Launcher is just another app.

• It builds an Intent describing which Activity should start.

• It sends this intent to the Activity Manager Service (AMS) using Binder IPC.

2. Activity Manager Service (AMS)

AMS is the system component that manages:

• Intents

• Activity life cycles

• Permissions

• Tasks and back stack

When AMS receives the startActivity(intent) request, it:

• Resolves the Intent

• Checks permissions

• Determines whether the app already has a running process

If the app does not have a process running, AMS goes to the next step.

3. Process creation via Zygote

AMS calls Zygote to ask it to clone itself to create a new process for this application via Process.start() (the new process becomes the application process).

4. Activity Thread + Dalvik VM (ART)

Inside the newly forked process:

• Android runtime starts (Dalvik VM in older Android; now ART)

• It launches the ActivityThread class

ActivityThread is NOT a thread. It's a manager for the app’s main thread responsible for:

• Starting activities

• Running the main (UI) thread

• Managing the Looper (Looper.loop() starts the event loop)

5. AMS communicates with the new process**

After the process is ready, AMS sends in order:

1. BIND_APPLICATION: it tells the new process:

   • Load the app package

   • Initialize classes

   • Prepare resources

2. LAUNCH_ACTIVITY: it tells ActivityThread:

   • Call the lifecycle methods of the Activity.

6. Activity Life Cycle Begins

ActivityThread now calls:

1. onCreate()

2. onStart()

3. onResume()

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Main components

• Activity class: unlike programming paradigms in which apps are launched with a main() method, the Android system initiates code in an Activity instance by invoking specific callback methods that correspond to specific stages of its lifecycle.

Activity lifecycle