前言

一直用，但是具体在代码中是如何实现的并知道。

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

以获得AM为类，进行跟踪framework流程

activity.java

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

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

    }

Activity.java

    @Override
    public Object getSystemService(@ServiceName @NonNull String name) {
        if (getBaseContext() == null) {
            throw new IllegalStateException(
                    "System services not available to Activities before onCreate()");
        }

        if (WINDOW_SERVICE.equals(name)) {
            return mWindowManager;
        } else if (SEARCH_SERVICE.equals(name)) {
            ensureSearchManager();
            return mSearchManager;
        }
        return super.getSystemService(name);
    }

ContextThemeWrapper.java

    @Override public Object getSystemService(String name) {
        if (LAYOUT_INFLATER_SERVICE.equals(name)) {
            if (mInflater == null) {
                mInflater = LayoutInflater.from(getBaseContext()).cloneInContext(this);
            }
            return mInflater;
        }
        return getBaseContext().getSystemService(name);
    }

Context.java

    public abstract Object getSystemService(@ServiceName @NonNull String name);

最终我们发现这个方法是Context中的一个抽象接口。
我们知道Context类是一个abstract类。它的具体实现是在ContextImpl类中。
于是我们在ComtextImpl中找到

    @Override
    public Object getSystemService(String name) {
        return SystemServiceRegistry.getSystemService(this, name);
    }

SystemServiceRegistry.java
顾名思义SystemServiceRegistry这个类是为了实现系统服务注册的。
SystemServiceRegistry 这个类中有一些静态代码块，就是为了注册系统服务的；

    static {
        registerService(Context.ACCESSIBILITY_SERVICE, AccessibilityManager.class,
                new CachedServiceFetcher<AccessibilityManager>() {
            @Override
            public AccessibilityManager createService(ContextImpl ctx) {
                return AccessibilityManager.getInstance(ctx);
            }});

        registerService(Context.CAPTIONING_SERVICE, CaptioningManager.class,
                new CachedServiceFetcher<CaptioningManager>() {
            @Override
            public CaptioningManager createService(ContextImpl ctx) {
                return new CaptioningManager(ctx);
            }});

        registerService(Context.ACCOUNT_SERVICE, AccountManager.class,
                new CachedServiceFetcher<AccountManager>() {
            @Override
            public AccountManager createService(ContextImpl ctx) {
                IBinder b = ServiceManager.getService(Context.ACCOUNT_SERVICE);
                IAccountManager service = IAccountManager.Stub.asInterface(b);
                return new AccountManager(ctx, service);
            }});

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

        registerService(Context.ALARM_SERVICE, AlarmManager.class,
                new CachedServiceFetcher<AlarmManager>() {
            @Override
            public AlarmManager createService(ContextImpl ctx) {
                IBinder b = ServiceManager.getService(Context.ALARM_SERVICE);
                IAlarmManager service = IAlarmManager.Stub.asInterface(b);
                return new AlarmManager(service, ctx);
            }});

        registerService(Context.AUDIO_SERVICE, AudioManager.class,
                new CachedServiceFetcher<AudioManager>() {
            @Override
            public AudioManager createService(ContextImpl ctx) {
                return new AudioManager(ctx);
            }});

        registerService(Context.MEDIA_ROUTER_SERVICE, MediaRouter.class,
                new CachedServiceFetcher<MediaRouter>() {
            @Override
            public MediaRouter createService(ContextImpl ctx) {
                return new MediaRouter(ctx);
            }});

        registerService(Context.BLUETOOTH_SERVICE, BluetoothManager.class,
                new CachedServiceFetcher<BluetoothManager>() {
            @Override
            public BluetoothManager createService(ContextImpl ctx) {
                return new BluetoothManager(ctx);
            }});

        registerService(Context.HDMI_CONTROL_SERVICE, HdmiControlManager.class,
                new StaticServiceFetcher<HdmiControlManager>() {
            @Override
            public HdmiControlManager createService() {
                IBinder b = ServiceManager.getService(Context.HDMI_CONTROL_SERVICE);
                return new HdmiControlManager(IHdmiControlService.Stub.asInterface(b));
            }});

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

        // The clipboard service moved to a new package. If someone asks for the old
        // interface by class then we want to redirect over to the new interface instead
        // (which extends it).
        SYSTEM_SERVICE_NAMES.put(android.text.ClipboardManager.class, Context.CLIPBOARD_SERVICE);

        registerService(Context.CONNECTIVITY_SERVICE, ConnectivityManager.class,
                new StaticOuterContextServiceFetcher<ConnectivityManager>() {
            @Override
            public ConnectivityManager createService(Context context) {
                IBinder b = ServiceManager.getService(Context.CONNECTIVITY_SERVICE);
                IConnectivityManager service = IConnectivityManager.Stub.asInterface(b);
                return new ConnectivityManager(context, service);
            }});

        registerService(Context.COUNTRY_DETECTOR, CountryDetector.class,
                new StaticServiceFetcher<CountryDetector>() {
            @Override
            public CountryDetector createService() {
                IBinder b = ServiceManager.getService(Context.COUNTRY_DETECTOR);
                return new CountryDetector(ICountryDetector.Stub.asInterface(b));
            }});

        registerService(Context.DEVICE_POLICY_SERVICE, DevicePolicyManager.class,
                new CachedServiceFetcher<DevicePolicyManager>() {
            @Override
            public DevicePolicyManager createService(ContextImpl ctx) {
                return DevicePolicyManager.create(ctx, ctx.mMainThread.getHandler());
            }});

        registerService(Context.DOWNLOAD_SERVICE, DownloadManager.class,
                new CachedServiceFetcher<DownloadManager>() {
            @Override
            public DownloadManager createService(ContextImpl ctx) {
                return new DownloadManager(ctx.getContentResolver(), ctx.getPackageName());
            }});

        registerService(Context.BATTERY_SERVICE, BatteryManager.class,
                new StaticServiceFetcher<BatteryManager>() {
            @Override
            public BatteryManager createService() {
                return new BatteryManager();
            }});

        registerService(Context.NFC_SERVICE, NfcManager.class,
                new CachedServiceFetcher<NfcManager>() {
            @Override
            public NfcManager createService(ContextImpl ctx) {
                return new NfcManager(ctx);
            }});

        registerService(Context.DROPBOX_SERVICE, DropBoxManager.class,
                new StaticServiceFetcher<DropBoxManager>() {
            @Override
            public DropBoxManager createService() {
                IBinder b = ServiceManager.getService(Context.DROPBOX_SERVICE);
                IDropBoxManagerService service = IDropBoxManagerService.Stub.asInterface(b);
                if (service == null) {
                    // Don't return a DropBoxManager that will NPE upon use.
                    // This also avoids caching a broken DropBoxManager in
                    // getDropBoxManager during early boot, before the
                    // DROPBOX_SERVICE is registered.
                    return null;
                }
                return new DropBoxManager(service);
            }});

        registerService(Context.INPUT_SERVICE, InputManager.class,
                new StaticServiceFetcher<InputManager>() {
            @Override
            public InputManager createService() {
                return InputManager.getInstance();
            }});

        registerService(Context.DISPLAY_SERVICE, DisplayManager.class,
                new CachedServiceFetcher<DisplayManager>() {
            @Override
            public DisplayManager createService(ContextImpl ctx) {
                return new DisplayManager(ctx.getOuterContext());
            }});

        registerService(Context.INPUT_METHOD_SERVICE, InputMethodManager.class,
                new StaticServiceFetcher<InputMethodManager>() {
            @Override
            public InputMethodManager createService() {
                return InputMethodManager.getInstance();
            }});

        registerService(Context.TEXT_SERVICES_MANAGER_SERVICE, TextServicesManager.class,
                new StaticServiceFetcher<TextServicesManager>() {
            @Override
            public TextServicesManager createService() {
                return TextServicesManager.getInstance();
            }});

        registerService(Context.KEYGUARD_SERVICE, KeyguardManager.class,
                new StaticServiceFetcher<KeyguardManager>() {
            @Override
            public KeyguardManager createService() {
                return new KeyguardManager();
            }});

        registerService(Context.LAYOUT_INFLATER_SERVICE, LayoutInflater.class,
                new CachedServiceFetcher<LayoutInflater>() {
            @Override
            public LayoutInflater createService(ContextImpl ctx) {
                return new PhoneLayoutInflater(ctx.getOuterContext());
            }});

        registerService(Context.LOCATION_SERVICE, LocationManager.class,
                new CachedServiceFetcher<LocationManager>() {
            @Override
            public LocationManager createService(ContextImpl ctx) {
                IBinder b = ServiceManager.getService(Context.LOCATION_SERVICE);
                return new LocationManager(ctx, ILocationManager.Stub.asInterface(b));
            }});

        registerService(Context.NETWORK_POLICY_SERVICE, NetworkPolicyManager.class,
                new CachedServiceFetcher<NetworkPolicyManager>() {
            @Override
            public NetworkPolicyManager createService(ContextImpl ctx) {
                return new NetworkPolicyManager(ctx, INetworkPolicyManager.Stub.asInterface(
                        ServiceManager.getService(Context.NETWORK_POLICY_SERVICE)));
            }});

        registerService(Context.NOTIFICATION_SERVICE, NotificationManager.class,
                new CachedServiceFetcher<NotificationManager>() {
            @Override
            public NotificationManager createService(ContextImpl ctx) {
                final Context outerContext = ctx.getOuterContext();
                return new NotificationManager(
                    new ContextThemeWrapper(outerContext,
                            Resources.selectSystemTheme(0,
                                    outerContext.getApplicationInfo().targetSdkVersion,
                                    com.android.internal.R.style.Theme_Dialog,
                                    com.android.internal.R.style.Theme_Holo_Dialog,
                                    com.android.internal.R.style.Theme_DeviceDefault_Dialog,
                                    com.android.internal.R.style.Theme_DeviceDefault_Light_Dialog)),
                    ctx.mMainThread.getHandler());
            }});

        registerService(Context.NSD_SERVICE, NsdManager.class,
                new CachedServiceFetcher<NsdManager>() {
            @Override
            public NsdManager createService(ContextImpl ctx) {
                IBinder b = ServiceManager.getService(Context.NSD_SERVICE);
                INsdManager service = INsdManager.Stub.asInterface(b);
                return new NsdManager(ctx.getOuterContext(), service);
            }});

        registerService(Context.POWER_SERVICE, PowerManager.class,
                new CachedServiceFetcher<PowerManager>() {
            @Override
            public PowerManager createService(ContextImpl ctx) {
                IBinder b = ServiceManager.getService(Context.POWER_SERVICE);
                IPowerManager service = IPowerManager.Stub.asInterface(b);
                if (service == null) {
                    Log.wtf(TAG, "Failed to get power manager service.");
                }
                return new PowerManager(ctx.getOuterContext(),
                        service, ctx.mMainThread.getHandler());
            }});

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

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

        registerService(Context.STATUS_BAR_SERVICE, StatusBarManager.class,
                new CachedServiceFetcher<StatusBarManager>() {
            @Override
            public StatusBarManager createService(ContextImpl ctx) {
                return new StatusBarManager(ctx.getOuterContext());
            }});

        registerService(Context.STORAGE_SERVICE, StorageManager.class,
                new CachedServiceFetcher<StorageManager>() {
            @Override
            public StorageManager createService(ContextImpl ctx) {
                return new StorageManager(ctx, ctx.mMainThread.getHandler().getLooper());
            }});

        registerService(Context.TELEPHONY_SERVICE, TelephonyManager.class,
                new CachedServiceFetcher<TelephonyManager>() {
            @Override
            public TelephonyManager createService(ContextImpl ctx) {
                return new TelephonyManager(ctx.getOuterContext());
            }});

        registerService(Context.TELEPHONY_SUBSCRIPTION_SERVICE, SubscriptionManager.class,
                new CachedServiceFetcher<SubscriptionManager>() {
            @Override
            public SubscriptionManager createService(ContextImpl ctx) {
                return new SubscriptionManager(ctx.getOuterContext());
            }});

        registerService(Context.CARRIER_CONFIG_SERVICE, CarrierConfigManager.class,
                new CachedServiceFetcher<CarrierConfigManager>() {
            @Override
            public CarrierConfigManager createService(ContextImpl ctx) {
                return new CarrierConfigManager();
            }});

        registerService(Context.TELECOM_SERVICE, TelecomManager.class,
                new CachedServiceFetcher<TelecomManager>() {
            @Override
            public TelecomManager createService(ContextImpl ctx) {
                return new TelecomManager(ctx.getOuterContext());
            }});

        registerService(Context.UI_MODE_SERVICE, UiModeManager.class,
                new CachedServiceFetcher<UiModeManager>() {
            @Override
            public UiModeManager createService(ContextImpl ctx) {
                return new UiModeManager();
            }});

        registerService(Context.USB_SERVICE, UsbManager.class,
                new CachedServiceFetcher<UsbManager>() {
            @Override
            public UsbManager createService(ContextImpl ctx) {
                IBinder b = ServiceManager.getService(Context.USB_SERVICE);
                return new UsbManager(ctx, IUsbManager.Stub.asInterface(b));
            }});

        registerService(Context.SERIAL_SERVICE, SerialManager.class,
                new CachedServiceFetcher<SerialManager>() {
            @Override
            public SerialManager createService(ContextImpl ctx) {
                IBinder b = ServiceManager.getService(Context.SERIAL_SERVICE);
                return new SerialManager(ctx, ISerialManager.Stub.asInterface(b));
            }});

        registerService(Context.VIBRATOR_SERVICE, Vibrator.class,
                new CachedServiceFetcher<Vibrator>() {
            @Override
            public Vibrator createService(ContextImpl ctx) {
                return new SystemVibrator(ctx);
            }});

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

        registerService(Context.WIFI_SERVICE, WifiManager.class,
                new CachedServiceFetcher<WifiManager>() {
            @Override
            public WifiManager createService(ContextImpl ctx) {
                IBinder b = ServiceManager.getService(Context.WIFI_SERVICE);
                IWifiManager service = IWifiManager.Stub.asInterface(b);
                return new WifiManager(ctx.getOuterContext(), service);
            }});

        registerService(Context.WIFI_PASSPOINT_SERVICE, WifiPasspointManager.class,
                new CachedServiceFetcher<WifiPasspointManager>() {
            @Override
            public WifiPasspointManager createService(ContextImpl ctx) {
                IBinder b = ServiceManager.getService(Context.WIFI_PASSPOINT_SERVICE);
                IWifiPasspointManager service = IWifiPasspointManager.Stub.asInterface(b);
                return new WifiPasspointManager(ctx.getOuterContext(), service);
            }});

        registerService(Context.WIFI_P2P_SERVICE, WifiP2pManager.class,
                new StaticServiceFetcher<WifiP2pManager>() {
            @Override
            public WifiP2pManager createService() {
                IBinder b = ServiceManager.getService(Context.WIFI_P2P_SERVICE);
                IWifiP2pManager service = IWifiP2pManager.Stub.asInterface(b);
                return new WifiP2pManager(service);
            }});

        registerService(Context.WIFI_SCANNING_SERVICE, WifiScanner.class,
                new CachedServiceFetcher<WifiScanner>() {
            @Override
            public WifiScanner createService(ContextImpl ctx) {
                IBinder b = ServiceManager.getService(Context.WIFI_SCANNING_SERVICE);
                IWifiScanner service = IWifiScanner.Stub.asInterface(b);
                return new WifiScanner(ctx.getOuterContext(), service);
            }});

        registerService(Context.WIFI_RTT_SERVICE, RttManager.class,
                new CachedServiceFetcher<RttManager>() {
            @Override
            public RttManager createService(ContextImpl ctx) {
                IBinder b = ServiceManager.getService(Context.WIFI_RTT_SERVICE);
                IRttManager service = IRttManager.Stub.asInterface(b);
                return new RttManager(ctx.getOuterContext(), service);
            }});

        registerService(Context.ETHERNET_SERVICE, EthernetManager.class,
                new CachedServiceFetcher<EthernetManager>() {
            @Override
            public EthernetManager createService(ContextImpl ctx) {
                IBinder b = ServiceManager.getService(Context.ETHERNET_SERVICE);
                IEthernetManager service = IEthernetManager.Stub.asInterface(b);
                return new EthernetManager(ctx.getOuterContext(), service);
            }});

        registerService(Context.WINDOW_SERVICE, WindowManager.class,
                new CachedServiceFetcher<WindowManager>() {
            @Override
            public WindowManager createService(ContextImpl ctx) {
                return new WindowManagerImpl(ctx.getDisplay());
            }});

        registerService(Context.USER_SERVICE, UserManager.class,
                new CachedServiceFetcher<UserManager>() {
            @Override
            public UserManager createService(ContextImpl ctx) {
                IBinder b = ServiceManager.getService(Context.USER_SERVICE);
                IUserManager service = IUserManager.Stub.asInterface(b);
                return new UserManager(ctx, service);
            }});

        registerService(Context.APP_OPS_SERVICE, AppOpsManager.class,
                new CachedServiceFetcher<AppOpsManager>() {
            @Override
            public AppOpsManager createService(ContextImpl ctx) {
                IBinder b = ServiceManager.getService(Context.APP_OPS_SERVICE);
                IAppOpsService service = IAppOpsService.Stub.asInterface(b);
                return new AppOpsManager(ctx, service);
            }});


        registerService(Context.CAMERA_SERVICE, CameraManager.class,
                new CachedServiceFetcher<CameraManager>() {
            @Override
            public CameraManager createService(ContextImpl ctx) {
                return new CameraManager(ctx);
            }});

        registerService(Context.LAUNCHER_APPS_SERVICE, LauncherApps.class,
                new CachedServiceFetcher<LauncherApps>() {
            @Override
            public LauncherApps createService(ContextImpl ctx) {
                IBinder b = ServiceManager.getService(Context.LAUNCHER_APPS_SERVICE);
                ILauncherApps service = ILauncherApps.Stub.asInterface(b);
                return new LauncherApps(ctx, service);
            }});

        registerService(Context.RESTRICTIONS_SERVICE, RestrictionsManager.class,
                new CachedServiceFetcher<RestrictionsManager>() {
            @Override
            public RestrictionsManager createService(ContextImpl ctx) {
                IBinder b = ServiceManager.getService(Context.RESTRICTIONS_SERVICE);
                IRestrictionsManager service = IRestrictionsManager.Stub.asInterface(b);
                return new RestrictionsManager(ctx, service);
            }});

        registerService(Context.PRINT_SERVICE, PrintManager.class,
                new CachedServiceFetcher<PrintManager>() {
            @Override
            public PrintManager createService(ContextImpl ctx) {
                IBinder iBinder = ServiceManager.getService(Context.PRINT_SERVICE);
                IPrintManager service = IPrintManager.Stub.asInterface(iBinder);
                return new PrintManager(ctx.getOuterContext(), service, UserHandle.myUserId(),
                        UserHandle.getAppId(Process.myUid()));
            }});

        registerService(Context.CONSUMER_IR_SERVICE, ConsumerIrManager.class,
                new CachedServiceFetcher<ConsumerIrManager>() {
            @Override
            public ConsumerIrManager createService(ContextImpl ctx) {
                return new ConsumerIrManager(ctx);
            }});

        registerService(Context.MEDIA_SESSION_SERVICE, MediaSessionManager.class,
                new CachedServiceFetcher<MediaSessionManager>() {
            @Override
            public MediaSessionManager createService(ContextImpl ctx) {
                return new MediaSessionManager(ctx);
            }});

        registerService(Context.TRUST_SERVICE, TrustManager.class,
                new StaticServiceFetcher<TrustManager>() {
            @Override
            public TrustManager createService() {
                IBinder b = ServiceManager.getService(Context.TRUST_SERVICE);
                return new TrustManager(b);
            }});

        registerService(Context.FINGERPRINT_SERVICE, FingerprintManager.class,
                new CachedServiceFetcher<FingerprintManager>() {
            @Override
            public FingerprintManager createService(ContextImpl ctx) {
                IBinder binder = ServiceManager.getService(Context.FINGERPRINT_SERVICE);
                IFingerprintService service = IFingerprintService.Stub.asInterface(binder);
                return new FingerprintManager(ctx.getOuterContext(), service);
            }});

        registerService(Context.TV_INPUT_SERVICE, TvInputManager.class,
                new StaticServiceFetcher<TvInputManager>() {
            @Override
            public TvInputManager createService() {
                IBinder iBinder = ServiceManager.getService(Context.TV_INPUT_SERVICE);
                ITvInputManager service = ITvInputManager.Stub.asInterface(iBinder);
                return new TvInputManager(service, UserHandle.myUserId());
            }});

        registerService(Context.NETWORK_SCORE_SERVICE, NetworkScoreManager.class,
                new CachedServiceFetcher<NetworkScoreManager>() {
            @Override
            public NetworkScoreManager createService(ContextImpl ctx) {
                return new NetworkScoreManager(ctx);
            }});

        registerService(Context.USAGE_STATS_SERVICE, UsageStatsManager.class,
                new CachedServiceFetcher<UsageStatsManager>() {
            @Override
            public UsageStatsManager createService(ContextImpl ctx) {
                IBinder iBinder = ServiceManager.getService(Context.USAGE_STATS_SERVICE);
                IUsageStatsManager service = IUsageStatsManager.Stub.asInterface(iBinder);
                return new UsageStatsManager(ctx.getOuterContext(), service);
            }});

        registerService(Context.NETWORK_STATS_SERVICE, NetworkStatsManager.class,
                new CachedServiceFetcher<NetworkStatsManager>() {
            @Override
            public NetworkStatsManager createService(ContextImpl ctx) {
                return new NetworkStatsManager(ctx.getOuterContext());
            }});

        registerService(Context.JOB_SCHEDULER_SERVICE, JobScheduler.class,
                new StaticServiceFetcher<JobScheduler>() {
            @Override
            public JobScheduler createService() {
                IBinder b = ServiceManager.getService(Context.JOB_SCHEDULER_SERVICE);
                return new JobSchedulerImpl(IJobScheduler.Stub.asInterface(b));
            }});

        registerService(Context.PERSISTENT_DATA_BLOCK_SERVICE, PersistentDataBlockManager.class,
                new StaticServiceFetcher<PersistentDataBlockManager>() {
            @Override
            public PersistentDataBlockManager createService() {
                IBinder b = ServiceManager.getService(Context.PERSISTENT_DATA_BLOCK_SERVICE);
                IPersistentDataBlockService persistentDataBlockService =
                        IPersistentDataBlockService.Stub.asInterface(b);
                if (persistentDataBlockService != null) {
                    return new PersistentDataBlockManager(persistentDataBlockService);
                } else {
                    // not supported
                    return null;
                }
            }});

        registerService(Context.MEDIA_PROJECTION_SERVICE, MediaProjectionManager.class,
                new CachedServiceFetcher<MediaProjectionManager>() {
            @Override
            public MediaProjectionManager createService(ContextImpl ctx) {
                return new MediaProjectionManager(ctx);
            }});

        registerService(Context.APPWIDGET_SERVICE, AppWidgetManager.class,
                new CachedServiceFetcher<AppWidgetManager>() {
            @Override
            public AppWidgetManager createService(ContextImpl ctx) {
                IBinder b = ServiceManager.getService(Context.APPWIDGET_SERVICE);
                return new AppWidgetManager(ctx, IAppWidgetService.Stub.asInterface(b));
            }});

        registerService(Context.MIDI_SERVICE, MidiManager.class,
                new CachedServiceFetcher<MidiManager>() {
            @Override
            public MidiManager createService(ContextImpl ctx) {
                IBinder b = ServiceManager.getService(Context.MIDI_SERVICE);
                if (b == null) {
                    return null;
                }
                return new MidiManager(IMidiManager.Stub.asInterface(b));
            }});

        registerService(Context.RADIO_SERVICE, RadioManager.class,
                new CachedServiceFetcher<RadioManager>() {
            @Override
            public RadioManager createService(ContextImpl ctx) {
                return new RadioManager(ctx);
            }});

//SPRD: add for heartbeat powerguru service
if (PowerGuru.isEnabled()) {
        registerService(Context.POWERGURU_SERVICE,PowerGuru.class,
            new CachedServiceFetcher<PowerGuru>() {
        public PowerGuru createService(ContextImpl ctx) {
            IBinder b = ServiceManager.getService(Context.POWERGURU_SERVICE);
            IPowerGuru service = IPowerGuru.Stub.asInterface(b);
            return new PowerGuru(service, ctx);
        }});
    }
    }

找到对AM注册的代码块

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

   /** * Override this class when the system service constructor needs a * ContextImpl and should be cached and retained by that context. */
    static abstract class CachedServiceFetcher<T> implements ServiceFetcher<T> {
        private final int mCacheIndex;

        public CachedServiceFetcher() {
            mCacheIndex = sServiceCacheSize++;
        }

        @Override
        @SuppressWarnings("unchecked")
        public final T getService(ContextImpl ctx) {
            final Object[] cache = ctx.mServiceCache;
            synchronized (cache) {
                // Fetch or create the service.
                Object service = cache[mCacheIndex];
                if (service == null) {
                    service = createService(ctx);
                    cache[mCacheIndex] = service;
                }
                return (T)service;
            }
        }

        public abstract T createService(ContextImpl ctx);
    }

            @Override
            public ActivityManager createService(ContextImpl ctx) {
                return new ActivityManager(ctx.getOuterContext(), ctx.mMainThread.getHandler());
            }});

最终调用

new ActivityManager(ctx.getOuterContext(), ctx.mMainThread.getHandler());

总结

上面代码贴的有点乱