Open this file in a code editor of your choice. We will move through the ActivityManagerService file and understand its various components.

The Activity Manager extends a class known as ActivityManagerNative, which implements the binder protocol for remote invocation. This code is written manually like we've written in the recipe on using IPC with our system service code. This was done as the AIDL compiler did not exist the time the Activity Manager code was written by Google employees.

In the beginning of the file, there are various control variables for different aspects related to activities. One such variable is:

static final int MAX_ACTIVITIES = 20;

This defines the maximum number of activities that can be in the stack. It means the maximum number of activities that could be running at a time.

Another interesting variable is the duration the system waits for a process to be launched.

static final int PROC_START_TIMEOUT = 10*1000;

SDK developers will know that when using a broadcast receiver, the onReceive() method should finish fast. The timeout is governed by:

static final int BROADCAST_TIMEOUT = 10*1000;

which is around 10 seconds.

Scroll down in the file, there is a handler which does the majority of the work that does not require immediate return. As you may know, Android has the concept of a handler, which basically is a mechanism to execute code in a separate thread and have it return results later. The handler is defined as:

final Handler mHandler = new Handler() {

Some example tasks performed are the ANR dialogs, indicated by case SHOW_NOT_RESPONDING_MSG and the sending of broadcasts to registered receivers.

Other functionality is implemented as methods that return synchronously. An asynchronous return means that the results of a request are returned to the caller at a later time, and not immediately, like a synchronous call.

There is a separate section in the sources for permissions manipulation and verification. The only public entry point for permissions checking is the method:

public int checkPermission(String permission, int pid, int uid)

All permission checks that are executed in the system go through this one method. Hence it serves as a reliable checkpoint to understand what processes are doing with respect to protected data and operations.

Following this, the Activity Manager has a group of methods that manage tasks by manipulating the activity stack. The stack is a data structure defined in the ActivityStack.java file in the same directory as the activity manager service.

The following method is executed when the system has completely booted and is ready to start launching user processes:

public void systemReady(final Runnable goingCallback)

There are separate sections managing services, content providers, and broadcasts. At the start of each section, a comment indicates what type of methods follow. Various API methods such as registerReceiver, startActivity, and startService are implemented by the ActivityManagerService class in the same file.