Android Application Launch Part 2

There are three distinct phases of process launch :

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

Process Creation -

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 finally returns pid of newly created process.

ActivityThread then starts the message loop by calling Looper.prepareLoop() and Looper.loop() subsequently.

The following sequence captures the call sequence in detail -

Application Binding -

The next step is to attach the process to the 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.

Activity Launch -

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.