上层获取ActivityManager；

ActivityManager manager = (ActivityManager) getSystemService(Context.ACTIVITY_SERVICE);

熟悉系统服务的话，都知道getSystemService方法最后会调用到SystemServiceRegistry中；

找到添加服务的代码；

registerService(Context.ACTIVITY_SERVICE, ActivityManager.class,
        new CachedServiceFetcher<ActivityManager>() {
    @Override
    public ActivityManager createService(ContextImpl ctx) {
        return new ActivityManager(ctx.getOuterContext(), ctx.mMainThread.getHandler());
    }});

找到ActivityManager；看一眼名词解释；

/**
 * This class gives information about, and interacts
 * with, activities, services, and the containing
 * process.
 *
 * A number of the methods in this class are for
 * debugging or informational purposes and they should
 * not be used to affect any runtime behavior of
 * your app. These methods are called out as such in
 * the method level documentation.
 *
 * Most application developers should not have the need to
 * use this class, most of whose methods are for specialized
 * use cases. However, a few methods are more broadly applicable.
 * For instance, {@link android.app.ActivityManager#isLowRamDevice() isLowRamDevice()}
 * enables your app to detect whether it is running on a low-memory device,
 * and behave accordingly.
 * {@link android.app.ActivityManager#clearApplicationUserData() clearApplicationUserData()}
 * is for apps with reset-data functionality.
 * In some special use cases, where an app interacts with
 * its Task stack, the app may use the
 * {@link android.app.ActivityManager.AppTask} and
 * {@link android.app.ActivityManager.RecentTaskInfo} inner
 * classes. However, in general, the methods in this class should
 * be used for testing and debugging purposes only.
 */

这样差不多对ActivityManager会有一个简要的了解；而后再看看其中的一些方法；

就以名词解释中提到的clearApplicationUserData为例；

/**
 * Permits an application to erase its own data from disk.  This is equivalent to
 * the user choosing to clear the app's data from within the device settings UI.  It
 * erases all dynamic data associated with the app -- its private data and data in its
 * private area on external storage -- but does not remove the installed application
 * itself, nor any OBB files. It also revokes all runtime permissions that the app has acquired,
 * clears all notifications and removes all Uri grants related to this application.
 *
 * @return {@code true} if the application successfully requested that the application's
 *     data be erased; {@code false} otherwise.
 */

public boolean clearApplicationUserData() {
    return clearApplicationUserData(mContext.getPackageName(), null);
}

/**
 * @hide
 */
@RequiresPermission(anyOf={Manifest.permission.CLEAR_APP_USER_DATA,
        Manifest.permission.ACCESS_INSTANT_APPS})
public boolean clearApplicationUserData(String packageName, IPackageDataObserver observer) {
    try {
        return getService().clearApplicationUserData(packageName, false,
                observer, mContext.getUserId());
    } catch (RemoteException e) {
            throw e.rethrowFromSystemServer();
    }
}

实际的调用会通过getService拿到对应的服务；让服务去处理；

/**
 * @hide
 */
public static IActivityManager getService() {
    return IActivityManagerSingleton.get();
}

进入frameworks\base\core\java\android\app目录；找到IActivityManager.aidl；依然先看下名词解释；

/**
 * System private API for talking with the activity manager service.  This
 * provides calls from the application back to the activity manager.
 *
 * {@hide}
 */
interface IActivityManager

在看看获取途径；很明显是个单例模式；

private static final Singleton<IActivityManager> IActivityManagerSingleton =
        new Singleton<IActivityManager>() {
            @Override
            protected IActivityManager create() {
                final IBinder b = ServiceManager.getService(Context.ACTIVITY_SERVICE);
                final IActivityManager am = IActivityManager.Stub.asInterface(b);
                return am;
            }
        };

这里贴一下单例模式的工具类；是hide的内容；

package android.util;

/**
 * Singleton helper class for lazily initialization.
 *
 * Modeled after frameworks/base/include/utils/Singleton.h
 *
 * @hide
 */
public abstract class Singleton<T> {
    private T mInstance;

    protected abstract T create();

    public final T get() {
        synchronized (this) {
            if (mInstance == null) {
                mInstance = create();
            }
            return mInstance;
        }
    }
}

IActivityManager是ServiceManager中的一个服务；众所周知，ServiceManager通过addService方法添加服务；顺藤摸瓜，找到添加Context.ACTIVITY_SERVICE服务的地方；在ActivityManagerService；

public void setSystemProcess() {
    try {
        ServiceManager.addService(Context.ACTIVITY_SERVICE, this, /* allowIsolated= */ true, 
                DUMP_FLAG_PRIORITY_CRITICAL | DUMP_FLAG_PRIORITY_NORMAL | DUMP_FLAG_PROTO);
//...
}

AMS；

public class ActivityManagerService extends IActivityManager.Stub
        implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback

AMS初始化；系统服务的初始化都在SystemServer；

// Activity manager runs the show.
traceBeginAndSlog("StartActivityManager");
mActivityManagerService = mSystemServiceManager.startService(
        ActivityManagerService.Lifecycle.class).getService();
mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
mActivityManagerService.setInstaller(installer);

//...

// Set up the Application instance for the system process and get started.
traceBeginAndSlog("SetSystemProcess");
mActivityManagerService.setSystemProcess();

真正继承SystemService的是内部类Lifecycle；

public static final class Lifecycle extends SystemService

小结

类	维护的地方	获取途径
ActivityManager	SystemServiceRegistry	(ActivityManager) getSystemService(Context.ACTIVITY_SERVICE);
IActivityManager	ServiceManager	final IBinder b = ServiceManager.getService(Context.ACTIVITY_SERVICE); final IActivityManager am = IActivityManager.Stub.asInterface(b);
Lifecycle	SystemServiceManager	SystemServiceManager的startService

Lifecycle是AMS的内部类，通过Lifecycle的getService可以拿到AMS
AMS继承了IActivityManager.Stub

Android P ActivityManagerService（一）初识

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