Main Thread Review

The MainThread, the UIThread, in android.

These services can create an ActivityThread by ActivityThread.systemMain() and get the system context by ActivityThread#systemMain() .

frameworks/base/cmds/svc/src/com/android/commands/svc/NfcCommand.java

frameworks/base/cmds/svc/src/com/android/commands/svc/UsbCommand.java

frameworks/base/cmds/telecom/src/com/android/commands/telecom/Telecom.java 

frameworks/base/services/java/com/android/server/SystemServer.java

Here we focus on SystemServer , as it create the ActivityThread and prepare the main Looper in the process of launching an application and creating the associated main thread, in the SystemServer#run().

So the main thread of an application, and its main looper are created by the SystemServer, within the same process of the SystemServer, by ActivityThread.systemMain() and Looper.prepareMainLooper() .

This is the source of the main looper. The main looper is stored using ThreadLocal , as a local variable in the associated thread with type Looper.
The main thread mUiThread in an activity is specified in Activity#attach(), called by ActivityThread, who manages lifecycle of activities. (While there is a mMainThread variable with ActivityThread type declared in Activity, do not get confused for the name, as the main thread mUiThread is a variable with Thread type.)