AMS（ActivityManagerService）主要負(fù)責(zé)四大組件的啟動(dòng)、切換、調(diào)度以及應(yīng)用進(jìn)程的管理和調(diào)度工作。

Android系統(tǒng)啟動(dòng)流程：首先會(huì)從ROM中預(yù)定義的地方加載引導(dǎo)程序BootLoader到RAM中，并執(zhí)行BootLoader程序啟動(dòng)Linux Kernel，然后啟動(dòng)用戶級(jí)別的第一個(gè)進(jìn)程init進(jìn)程。init進(jìn)程會(huì)解析init.rc腳本做一些初始化工作，包括掛載系統(tǒng)文件、創(chuàng)建工作目錄以及啟動(dòng)系統(tǒng)服務(wù)進(jìn)程等，其中系統(tǒng)服務(wù)進(jìn)程包括Zygote、ServiceManager、Media等。在Zygote中會(huì)進(jìn)一步啟動(dòng)system_server進(jìn)程，然后system_server進(jìn)程中會(huì)啟動(dòng)AMS、WMS、PMS等服務(wù)，這些服務(wù)啟動(dòng)后，AMS就會(huì)打開Launcher應(yīng)用的home Activity，最終就看到了手機(jī)桌面。

AMS是在SystemServer中被添加的

//SystemServer.java
/**
 * The main entry point from zygote.
 */
public static void main(String[] args) {
  //new SystemServer()中創(chuàng)建了ActivityThread
  new SystemServer().run();
}

private void run() {
  //創(chuàng)建主線程Looper
  Looper.prepareMainLooper();
  //創(chuàng)建系統(tǒng)context（new ContextImpl）和加載系統(tǒng)資源
  createSystemContext();
  //啟動(dòng)引導(dǎo)服務(wù)
  startBootstrapServices();
  //啟動(dòng)核心服務(wù)
  startCoreServices();
  //啟動(dòng)其它服務(wù)
  startOtherServices();
  //loop循環(huán)
  Looper.loop();
}

SystemServer類的main中初始化SystemServer對(duì)象，調(diào)用run方法：

1）createSystemContext：創(chuàng)建系統(tǒng)上下文
2）startBootstrapServices：?jiǎn)?dòng)引導(dǎo)服務(wù)
3）startCoreServices：?jiǎn)?dòng)核心服務(wù)
4）startOtherServices：?jiǎn)?dòng)其它服務(wù)
5）Looper.loop：loop循環(huán)

在startBootstrapServices中啟動(dòng)了AMS

private void startBootstrapServices() {
  //啟動(dòng)AMS
  mActivityManagerService = ActivityManagerService.Lifecycle.startService(
                        mSystemServiceManager, atm);
  mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
  mActivityManagerService.setInstaller(installer);
  
  //③將AMS注冊(cè)到ServiceManager中
  mActivityManagerService.setSystemProcess();
}

public static final class Lifecycle extends SystemService {
  private final ActivityManagerService mService;
  private static ActivityTaskManagerService sAtm;

  //ActivityManagerService.Lifecycle這個(gè)類的構(gòu)造方法
  public Lifecycle(Context context) {
    super(context);
    //①創(chuàng)建AMS實(shí)例
    mService = new ActivityManagerService(context, sAtm);
  }
 
  public static ActivityManagerService startService(
    SystemServiceManager ssm, ActivityTaskManagerService atm) {
    sAtm = atm;
    //通過(guò)LifeCycle的getService返回AMS對(duì)象
    return ssm.startService(ActivityManagerService.Lifecycle.class).getService();
  }
  
  @Override
  public void onStart() {
    //②啟動(dòng)AMS
    mService.start();
  }
  
  public ActivityManagerService getService() {
    return mService;
  }
}

public <T extends SystemService> T startService(Class<T> serviceClass) {
  //通過(guò)反射創(chuàng)建AMS對(duì)象，會(huì)調(diào)到上面ActivityManagerService.Lifecycle構(gòu)造方法
  Constructor<T> constructor = serviceClass.getConstructor(Context.class);
  service = constructor.newInstance(mContext);
  startService(service);
  return service;
}

// Services that should receive lifecycle events.
private final ArrayList<SystemService> mServices = new ArrayList<SystemService>();

public void startService(@NonNull final SystemService service) {
  // Register it.
  mServices.add(service);
  //開啟服務(wù)
  service.onStart();
}

①創(chuàng)建AMS實(shí)例：new ActivityManagerService

public ActivityManagerService(Context systemContext, ActivityTaskManagerService atm) {
  mInjector = new Injector();
  //獲取系統(tǒng)上下文環(huán)境
  mContext = systemContext;
  //獲取當(dāng)前ActivityThread線程
  mSystemThread = ActivityThread.currentActivityThread();
  //賦值mUiContext
  mUiContext = mSystemThread.getSystemUiContext();
  
  //創(chuàng)建Handler線程，用來(lái)處理handler消息
  mHandlerThread = new ServiceThread(TAG,
                THREAD_PRIORITY_FOREGROUND, false /*allowIo*/);
  mHandlerThread.start();
  mHandler = new MainHandler(mHandlerThread.getLooper());
  //處理ui相關(guān)msg的Handler
  mUiHandler = mInjector.getUiHandler(this);
  
  //初始化前臺(tái)廣播的隊(duì)列，設(shè)置10s超時(shí)
  mFgBroadcastQueue = new BroadcastQueue(this, mHandler,
                "foreground", foreConstants, false);
  //初始化后臺(tái)廣播的隊(duì)列，設(shè)置60s超時(shí)
  mBgBroadcastQueue = new BroadcastQueue(this, mHandler,
                "background", backConstants, true);
  //初始化離線廣播的隊(duì)列，設(shè)置60s超時(shí)
  mOffloadBroadcastQueue = new BroadcastQueue(this, mHandler,
                 "offload", offloadConstants, true);
  mBroadcastQueues[0] = mFgBroadcastQueue;
  mBroadcastQueues[1] = mBgBroadcastQueue;
  mBroadcastQueues[2] = mOffloadBroadcastQueue;
  
  //初始化管理Service的ActiveServices對(duì)象
  mServices = new ActiveServices(this);
  mProviderMap = new ProviderMap(this);
  mPackageWatchdog = PackageWatchdog.getInstance(mUiContext);
  mAppErrors = new AppErrors(mUiContext, this, mPackageWatchdog);
  //創(chuàng)建電池狀態(tài)服務(wù)
   mBatteryStatsService = new BatteryStatsService(systemContext, systemDir,
                BackgroundThread.get().getHandler());
  mBatteryStatsService.getActiveStatistics().readLocked();
  mBatteryStatsService.scheduleWriteToDisk();
  mOnBattery = DEBUG_POWER ? true
    : mBatteryStatsService.getActiveStatistics().getIsOnBattery();
  mBatteryStatsService.getActiveStatistics().setCallback(this);
  mOomAdjProfiler.batteryPowerChanged(mOnBattery);
  
  //啟動(dòng)一個(gè)線程跟進(jìn)CPU當(dāng)前狀態(tài)信息
  mProcessCpuThread = new Thread("CpuTracker"){
    @Override
    public void run() {
      synchronized (mProcessCpuTracker) {
        mProcessCpuInitLatch.countDown();
        mProcessCpuTracker.init();
      }
      while (true) {
        try {
          try {
            synchronized(this) {
              final long now = SystemClock.uptimeMillis();
              long nextCpuDelay = (mLastCpuTime.get()+MONITOR_CPU_MAX_TIME)-now;
              long nextWriteDelay = (mLastWriteTime+BATTERY_STATS_TIME)-now;
              //Slog.i(TAG, "Cpu delay=" + nextCpuDelay
              //        + ", write delay=" + nextWriteDelay);
              if (nextWriteDelay < nextCpuDelay) {
                nextCpuDelay = nextWriteDelay;
              }
              if (nextCpuDelay > 0) {
                mProcessCpuMutexFree.set(true);
                this.wait(nextCpuDelay);
              }
            }
          } catch (InterruptedException e) {
          }
          updateCpuStatsNow();
        } catch (Exception e) {
          Slog.e(TAG, "Unexpected exception collecting process stats", e);
        }
      }
    }
  }
  //看門狗，監(jiān)聽進(jìn)程
  Watchdog.getInstance().addMonitor(this);
  Watchdog.getInstance().addThread(mHandler);
}

②啟動(dòng)AMS

private void start() {
  //移除所有的進(jìn)程組
    removeAllProcessGroups();
  //啟動(dòng)Cpu進(jìn)程
  mProcessCpuThread.start();
  //啟動(dòng)電池狀態(tài)服務(wù)
  mBatteryStatsService.publish();
  mAppOpsService.publish(mContext);
  //創(chuàng)建本地服務(wù)并注冊(cè)，將創(chuàng)建的本地服務(wù)放入本地服務(wù)集合完成注冊(cè)
  LocalServices.addService(ActivityManagerInternal.class, new LocalService());
  
  //等待mProcessCpuThread完成初始化后，釋放鎖
  try {
    mProcessCpuInitLatch.await();
  } catch (InterruptedException e) {
    Slog.wtf(TAG, "Interrupted wait during start", e);
    Thread.currentThread().interrupt();
    throw new IllegalStateException("Interrupted wait during start");
  }
}

1）removeAllProcessGroups：移除所有的進(jìn)程組
2）mProcessCpuThread.start()：?jiǎn)?dòng)CpuTracker線程
3）mBatteryStatsService.publish()：?jiǎn)?dòng)電池狀態(tài)服務(wù)
4）LocalServices.addService(ActivityManagerInternal.class, new LocalService())：創(chuàng)建本地服務(wù)并放入本地服務(wù)集合

③將AMS注冊(cè)到ServiceManager中 setSystemProcess

public void setSystemProcess() {
  try {
        //將AMS注冊(cè)到ServiceManager中
    ServiceManager.addService(Context.ACTIVITY_SERVICE, this, /* allowIsolated= */ true,
                              DUMP_FLAG_PRIORITY_CRITICAL | DUMP_FLAG_PRIORITY_NORMAL | DUMP_FLAG_PROTO);
    //注冊(cè)進(jìn)程狀態(tài)服務(wù)
    ServiceManager.addService(ProcessStats.SERVICE_NAME, mProcessStats);
    //注冊(cè)內(nèi)存Binder
    ServiceManager.addService("meminfo", new MemBinder(this), /* allowIsolated= */ false,
                              DUMP_FLAG_PRIORITY_HIGH);
    //注冊(cè)圖像信息Binder
    ServiceManager.addService("gfxinfo", new GraphicsBinder(this));
    //注冊(cè)數(shù)據(jù)庫(kù)Binder 
    ServiceManager.addService("dbinfo", new DbBinder(this));
    if (MONITOR_CPU_USAGE) {
      //注冊(cè)監(jiān)控CPU使用狀態(tài)Binder
      ServiceManager.addService("cpuinfo", new CpuBinder(this),
                                /* allowIsolated= */ false, DUMP_FLAG_PRIORITY_CRITICAL);
    }
    //注冊(cè)權(quán)限控制Binder
    ServiceManager.addService("permission", new PermissionController(this));
    //注冊(cè)進(jìn)程服務(wù)Binder
    ServiceManager.addService("processinfo", new ProcessInfoService(this));
        //查詢并處理ApplicationInfo
    ApplicationInfo info = mContext.getPackageManager().getApplicationInfo(
      "android", STOCK_PM_FLAGS | MATCH_SYSTEM_ONLY);
    //將Application信息配置到ActivityThread中
    mSystemThread.installSystemApplicationInfo(info, getClass().getClassLoader());

    synchronized (this) {
      //創(chuàng)建并處理ProcessRecord
      ProcessRecord app = mProcessList.newProcessRecordLocked(info, info.processName,
                                                              false,
                                                              0,
                                                              new HostingRecord("system"));
      app.setPersistent(true);
      app.pid = MY_PID;
      app.getWindowProcessController().setPid(MY_PID);
      app.maxAdj = ProcessList.SYSTEM_ADJ;
      app.makeActive(mSystemThread.getApplicationThread(), mProcessStats);
      mPidsSelfLocked.put(app);
      mProcessList.updateLruProcessLocked(app, false, null);
      updateOomAdjLocked(OomAdjuster.OOM_ADJ_REASON_NONE);
    }
  } catch (PackageManager.NameNotFoundException e) {
    throw new RuntimeException(
      "Unable to find android system package", e);
  }

  // Start watching app ops after we and the package manager are up and running.
  mAppOpsService.startWatchingMode(AppOpsManager.OP_RUN_IN_BACKGROUND, null,
          new IAppOpsCallback.Stub() {
              @Override public void opChanged(int op, int uid, String packageName) {
                   if (op == AppOpsManager.OP_RUN_IN_BACKGROUND && packageName != null) {
                       if (mAppOpsService.checkOperation(op, uid, packageName)
                             != AppOpsManager.MODE_ALLOWED) {
                         runInBackgroundDisabled(uid);
                       }
                   }
              }
          });
}

1）添加了各種服務(wù)，如AMS
2）mSystemThread.installSystemApplicationInfo：最終調(diào)到LoadedApk，這是應(yīng)用的基本信息

接著看startOtherServices

private void startOtherServices() {
  //AMS與WMS關(guān)聯(lián)
  mActivityManagerService.setWindowManager(wm);
  //安裝ContentProvider
  mActivityManagerService.installSystemProviders();
  //所有的服務(wù)已經(jīng)準(zhǔn)備就緒
  mActivityManagerService.systemReady(() -> {
    //啟動(dòng)系統(tǒng)UI
    startSystemUi(context, windowManagerF);
    //執(zhí)行一系列服務(wù)的systemReady
    networkStatsF.systemReady();
    //...
  }
}

installSystemProviders

public final void installSystemProviders() {
    List<ProviderInfo> providers;
    synchronized (this) {
        //找到名為"system"的進(jìn)程，就是setSystemProcess中創(chuàng)建的ProcessRecord對(duì)象
        ProcessRecord app = mProcessList.mProcessNames.get("system", SYSTEM_UID);
        providers = generateApplicationProvidersLocked(app);
        if (providers != null) {
            for (int i=providers.size()-1; i>=0; i--) {
                ProviderInfo pi = (ProviderInfo)providers.get(i);
                if ((pi.applicationInfo.flags&ApplicationInfo.FLAG_SYSTEM) == 0) {
                    //移除非系統(tǒng)provider
                    Slog.w(TAG, "Not installing system proc provider " + pi.name
                            + ": not system .apk");
                    providers.remove(i);
                }
            }
        }
    }
    if (providers != null) {
        //安全所有的系統(tǒng)provider
        mSystemThread.installSystemProviders(providers);
    }

    synchronized (this) {
        mSystemProvidersInstalled = true;
    }
    mConstants.start(mContext.getContentResolver());
    //創(chuàng)建核心Settings Observer，用于監(jiān)聽Settings的改變
    mCoreSettingsObserver = new CoreSettingsObserver(this);
    mActivityTaskManager.installSystemProviders();
    //開發(fā)者權(quán)限
    mDevelopmentSettingsObserver = new DevelopmentSettingsObserver();
    SettingsToPropertiesMapper.start(mContext.getContentResolver());
    mOomAdjuster.initSettings();

    // Now that the settings provider is published we can consider sending
    // in a rescue party.
    RescueParty.onSettingsProviderPublished(mContext);

    //mUsageStatsService.monitorPackages();
}

1）installSystemProviders：安裝系統(tǒng)Provider
2）new CoreSettingsObserver(this)：創(chuàng)建CoreSettingsObserver，用于監(jiān)聽系統(tǒng)設(shè)置的改變

systemReady

public void systemReady(final Runnable goingCallback, TimingsTraceLog traceLog) {
  //啟動(dòng)Launcher界面
  mAtmInternal.startHomeOnAllDisplays(currentUserId, "systemReady");
}

小結(jié)：

1、new SystemServer()創(chuàng)建了ActivityThread
2、SystemServer的run方法中：
1）通過(guò)createSystemContext方法創(chuàng)建系統(tǒng)上下文對(duì)象
2）在startBootstrapServices方法中調(diào)用構(gòu)造方法對(duì)AMS進(jìn)行初始化，start方法啟動(dòng)AMS
3）調(diào)用setSystemProcess將AMS注冊(cè)到ServiceManager中
4）在startOtherServices中安裝了系統(tǒng)Provider
5）調(diào)用systemReady啟動(dòng)Launcher界面