The android app has launched

Android Application Launch explained: from Zygote to your Activity.onCreate()

Apr 30, 2018 · 6 min read

This article explains how Android launches your application when a user clicks an app icon. The Android system does a lot of heavy lifting behind the curtains to make your launch activity visible to a user. This article covers this process in detail by highlighting the important phases and call sequences.

Android applications are unique in two ways:

1. Multiple Entry points: Android apps are composed of different components and they can invoke the components owned by other apps. These components roughly correspond to multiple entry points for any application. Hence, they differ from traditional applications which have a single entry point like main() method.
2. Own Little World: Every Android application lives in its own world, it runs in a separate process, it has its own Dalvik VM instance and is assigned a unique user ID.

When does Android process start?

An Android process is started whenever it is required.

Any time a user or some other sys t em component requests a component (could be a service, an activity or an intent receiver) that belongs to your application be executed, the Android system spins off a new process for your app if it’s not already running. Generally processes keep running until killed by the system. Application processes are created on demand and a lot of things happen before you see your application’s launch activity up and running.

Every app runs in its own process: By default, every Android app runs in its own Android process which is nothing but a Linux process which gets one execution thread to start with. For example, when you click on a hyper-link in your e-mail, a web page opens in a browser window. Your mail client and the browser are two separate apps and they run in their two separate individual processes. The click event causes Android platform to launch a new process so that it can instantiate the browser activity in the context of its own process. The same holds good for any other component in an application.

Zygote : Spawning new life, new process

Let’s step back for a moment and have a quick look on system start-up process. Like the most Linux based systems, at startup, the boot loader loads the kernel and starts the init process. The init then spawns the low level Linux processes called “daemons” e.g. android debug daemon, USB daemon etc. These daemons typically handle the low level hardware interfaces including radio interface.

Init process then starts a very interesting process called ‘ Zygote ’.

As the name implies it’s the very beginning for the rest of the Android application. This is the process which initializes a very first instance of Dalvik virtual machine. It also pre-loads all common classes used by Android application framework and various apps installed on a system. It thus prepares itself to be replicated. It stats listening on a socket interface for future requests to spawn off new virtual machines (VM)for managing new application processes. On receiving a new request, it forks itself to create a new process which gets a pre-initialized VM instance.

After zygote, init starts the runtime process.

The zygote then forks to start a well managed process called system server. System server starts all core platform services e.g activity manager service and hardware services in its own context.

At this point the full stack is ready to launch the first app process — Home app which displays the home screen also known as Launcher application.

When a user clicks an app icon in Launcher …

The click event gets translated into startActivity(intent) and it is routed to ActivityManagerService via Binder IPC. The ActvityManagerService performs multiple steps

1. The first step is to collect information about the target of the intent object. This is done by using resolveIntent() method on PackageManager object. PackageManager.MATCH_DEFAULT_ONLY and PackageManager.GET_SHARED_LIBRARY_FILES flags are used by default.
2. The target information is saved back into the intent object to avoid re-doing this step.
3. Next important step is to check if user has enough privileges to invoke the target component of the intent. This is done by calling grantUriPermissionLocked() method.
4. If user has enough permissions, ActivityManagerService checks if the target activity requires to be launched in a new task. The task creation depends on Intent flags such as FLAG_ACTIVITY_NEW_TASK and other flags such as FLAG_ACTIVITY_CLEAR_TOP.
5. Now, it’s the time to check if the ProcessRecord already exists for the process.If the ProcessRecord is null, the ActivityManager has to create a new process to instantiate the target component.

As you saw above many things happen behind the scene when a user clicks on an icon and a new application gets launched. Here is the full picture :

There are three distinct phases of process launch :

1. Process Creation
2. Binding Application
3. Launching Activity / Starting Service / Invoking intent receiver …

ActivityManagerService creates a new process by invoking startProcessLocked() method which sends arguments to Zygote process over the socket connection. Zygote forks itself and calls ZygoteInit.main() which then instantiates ActivityThread object and returns a process id of a newly created process.

Every process gets one thread by default. The main thread has a Looper instance to handle messages from a message queue and it calls Looper.loop() in its every iteration of run() method. It’s the job of a Looper to pop off the messages from message queue and invoke the corresponding methods to handle them. ActivityThread then starts the message loop by calling Looper.prepareLoop() and Looper.loop() subsequently.

The following sequence captures the call sequence in detail —

The next step is to attach this newly created process to a specific application. This is done by calling bindApplication() on the thread object. This method sends BIND_APPLICATION message to the message queue. This message is retrieved by the Handler object which then invokes handleMessage() method to trigger the message specific action — handleBindApplication(). This method invokes makeApplication() method which loads app specific classes into memory.

This call sequence is depicted in following figure.

Launching an Activity:

After the previous step, the system contains the process responsible for the application with application classes loaded in process’s private memory. The call sequence to launch an activity is common between a newly created process and an existing process.

The actual process of launching starts in realStartActivity() method which calls sheduleLaunchActivity() on the application thread object. This method sends LAUNCH_ACTIVITY message to the message queue. The message is handled by handleLaunchActivity() method as shown below.

Assuming that user clicks on Video Browser application. the call sequence to launch the activity is as shown in the figure.

The Activity starts its managed lifecycle with onCreate() method call. The activity comes to foreground with onRestart() call and starts interacting with the user with onStart() call.

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This article was initially published as two part series (Part 1 and Part 2) on my blog ./ mult-core-dump in 2010. These articles have been cited in multiple documents including the very famous Introduction to the Android Graphics Pipeline

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Android Activity Launch Mode

Jan 13, 2017 · 5 min read

Launch mode is an instruction for Android OS which specifies how the activity should be launched. It instructs how any new activity should be associated with the current task. Before moving further you first need to understand about very important topics-

Tasks

A task is a collection of activities that users interact with when performing a certain job. In general an application contains number of activities. Normally when user launch an application a new task will be created and the first activity instance is called root of the task.

When us e r launches an app from home icon, it navigates through different screens so different activities placed on the top of one another. This collection of activities is known as tasks.

Back Stack

Activities are arranged with the order in which each activity is opened. This maintained stack called Back Stack. When you start a new activity using startActivity(), it “pushes” a new activity onto your task, and put the previous Activity in the back stack.

Once you press back button then “pops” the top most activity and remove it from the back stack and taking you back to the previous activity.

Above figure explains representation of how each new activity in a task adds an item to the back stack. When the user presses the Back button, the current activity is destroyed and the previous activity resumes. For more details you can also refer .

There are four launch modes for activity. They are:

In the AndroidManifest you can use “launchMode” attribute inside the element to declare the activity’s launch mode like-

Now let’s look at the differences between launch modes.

1. standard

This is the default launch mode of an activity (If not specified). It creates a new instance of an activity in the task from which it was started. Multiple instances of the activity can be created and multiple instances can be added to the same or different tasks. In other words you can create the same activity multiple times in the same task as well as in different tasks.

Example:

Suppose you have A, B, C and D activities and your activity B has “launch mode = standard”. Now you again launching activity B –

State of Activity Stack before launch B

State of Activity Stack after launch B

2. singleTop

In this launch mode if an instance of activity already exists at the top of the current task, a new instance will not be created and Android system will route the intent information through onNewIntent(). If an instance is not present on top of task then new instance will be created.

Using this launch mode you can create multiple instance of the same activity in the same task or in different tasks only if the same instance does not already exist at the top of stack.

Example:

Case 1:

Suppose you have A, B and C activities and your activity D has “launch mode = singleTop”. Now you launching activity D —

State of Activity Stack before launch D

State of Activity Stack after launch D activity

A -> B -> C -> D (Here D launch as usual)

Case 2:

Suppose you have A, B, C and D activities and your activity D has “launch mode = singleTop”. Now you again launching activity D —

State of Activity Stack before launch D

State of Activity Stack after launch D activity

A -> B -> C -> D (Here old instance gets called and intent data route through onNewIntent() callback)

3. singleTask

In this launch mode a new task will always be created and a new instance will be pushed to the task as the root one. If an instance of activity exists on the separate task, a new instance will not be created and Android system routes the intent information through onNewIntent() method. At a time only one instance of activity will exist.

Example:

Case 1:

Suppose you have A, B and C activities and your activity D has “launch mode = singleTask”. Now you launching activity D —

State of Activity Stack before launch D

State of Activity Stack after launch D activity

A -> B -> C -> D (Here D launch as usual)

Case 2:

Suppose you have A, B, C and D activities and your activity B has “launch mode = singleTask”. Now you again launching activity B-

State of Activity Stack before launch D

State of Activity Stack after launch B activity

A -> B (Here old instance gets called and intent data route through onNewIntent() callback)

Also notice that C and D activities get destroyed here.

4. singleInstance

This is very special launch mode and only used in the applications that has only one activity. It is similar to singleTask except that no other activities will be created in the same task. Any other activity started from here will create in a new task.

Example:

Case 1:

Suppose you have A, B and C activities and your activity D has “launch mode = singleInstance”. Now you launching activity D —

State of Activity Stack before launch D

State of Activity Stack after launch D activity

Task2 — D (here D will be in different task)

Now if you continue this and start E and D then Stack will look like-

Task1 — A -> B -> C -> E

Case 2:

Suppose you have A, B, C activities in one task and activity D is in another task with “launch mode = singleInstance”. Now you again launching activity D-

State of Activity Stack before launch D

State of Activity Stack after launch B activity

Task2 — D (Here old instance gets called and intent data route through onNewIntent() callback)

Intent Flags

Android also provides Activity flags by which you can change the default behavior of Activity association with Tasks while starting it via startActivity() method. These flags values can be pass through Intent extra data.

FLAG_ACTIVITY_NEW_TASK

This flag works similar to “launchMode = singleTask”.

FLAG_ACTIVITY_CLEAR_TASK

This flag will cause any existing task that would be associated with the activity to be cleared before the activity is started. The activity becomes the new root of an otherwise empty task, and any old activities are finished.

FLAG_ACTIVITY_SINGLE_TOP

This flag works similar to “launchMode = singleTop”.

FLAG_ACTIVITY_CLEAR_TOP

If set, and the activity being launched is already running in the current task, then instead of launching a new instance of that activity, all of the other activities on top of it will be closed and this Intent will be delivered to the (now on top) old activity as a new Intent.

There are quite a lot on flags. You could find more about it at Intent.

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