Learn Some Framework-5 ActivityManagerService[Initialize]
在之前的章節,我們花了大量時間爲大家介紹ServiceManager與Binder機制,在這一章節開始我們通過對ActivityManagerService這個Android核心的SystemService的學習,來理解系統級的Java Service是如何工作的
我們在介紹Zygote與SystemServer一章時介紹過, startBootstrapServices啓動了Android啓動必不可少的幾個系統級的Service,他們是Installer, AMS, PowerManagerService, LightService, PMS和SensorService, 其中的AMS就是我們今天的主角ActivityManagerService.
AMS的啓動代碼如下:
private void startBootstrapServices() {
...
// Activity manager runs the show.
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.
mActivityManagerService.setSystemProcess();
...
}
可以看出,從Marshmallow開始,AMS其實是被其一個Inner Class Lifecycle叫起,Lifecycle的實現非常簡單:
public static final class Lifecycle extends SystemService {
private final ActivityManagerService mService;
public Lifecycle(Context context) {
super(context);
mService = new ActivityManagerService(context);
}
@Override
public void onStart() {
mService.start();
}
public ActivityManagerService getService() {
return mService;
}
}
這是一個靜態的內部類,於是他的生命週期實現了與AMS的隔離,在它的constructor內,一個新的AMS的實例被構造出來,接着,在它的onStart方法內,AMS被啓動。
我們先來看AMS的constructor:
// Note: This method is invoked on the main thread but may need to attach various
// handlers to other threads. So take care to be explicit about the looper.
public ActivityManagerService(Context systemContext) {
mContext = systemContext;
mFactoryTest = FactoryTest.getMode();
mSystemThread = ActivityThread.currentActivityThread();
Slog.i(TAG, "Memory class: " + ActivityManager.staticGetMemoryClass());
mHandlerThread = new ServiceThread(TAG,
android.os.Process.THREAD_PRIORITY_FOREGROUND, false /*allowIo*/);
mHandlerThread.start();
mHandler = new MainHandler(mHandlerThread.getLooper());
mUiHandler = new UiHandler();
mFgBroadcastQueue = new BroadcastQueue(this, mHandler,
"foreground", BROADCAST_FG_TIMEOUT, false);
mBgBroadcastQueue = new BroadcastQueue(this, mHandler,
"background", BROADCAST_BG_TIMEOUT, true);
mBroadcastQueues[0] = mFgBroadcastQueue;
mBroadcastQueues[1] = mBgBroadcastQueue;
mServices = new ActiveServices(this);
mProviderMap = new ProviderMap(this);
// TODO: Move creation of battery stats service outside of activity manager service.
File dataDir = Environment.getDataDirectory();
File systemDir = new File(dataDir, "system");
systemDir.mkdirs();
mBatteryStatsService = new BatteryStatsService(systemDir, mHandler);
mBatteryStatsService.getActiveStatistics().readLocked();
mBatteryStatsService.scheduleWriteToDisk();
mOnBattery = DEBUG_POWER ? true
: mBatteryStatsService.getActiveStatistics().getIsOnBattery();
mBatteryStatsService.getActiveStatistics().setCallback(this);
mProcessStats = new ProcessStatsService(this, new File(systemDir, "procstats"));
mAppOpsService = new AppOpsService(new File(systemDir, "appops.xml"), mHandler);
mGrantFile = new AtomicFile(new File(systemDir, "urigrants.xml"));
// User 0 is the first and only user that runs at boot.
mStartedUsers.put(UserHandle.USER_OWNER, new UserState(UserHandle.OWNER, true));
mUserLru.add(UserHandle.USER_OWNER);
updateStartedUserArrayLocked();
GL_ES_VERSION = SystemProperties.getInt("ro.opengles.version",
ConfigurationInfo.GL_ES_VERSION_UNDEFINED);
mTrackingAssociations = "1".equals(SystemProperties.get("debug.track-associations"));
mConfiguration.setToDefaults();
mConfiguration.setLocale(Locale.getDefault());
mConfigurationSeq = mConfiguration.seq = 1;
mProcessCpuTracker.init();
mCompatModePackages = new CompatModePackages(this, systemDir, mHandler);
mIntentFirewall = new IntentFirewall(new IntentFirewallInterface(), mHandler);
mRecentTasks = new RecentTasks(this);
mStackSupervisor = new ActivityStackSupervisor(this, mRecentTasks);
mTaskPersister = new TaskPersister(systemDir, mStackSupervisor, mRecentTasks);
mProcessCpuThread = new Thread("CpuTracker") {
@Override
public void run() {
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);
}
}
}
};
Watchdog.getInstance().addMonitor(this);
Watchdog.getInstance().addThread(mHandler);
}
這個方法很簡單,但是涉及到很多AMS的概念,我們逐一介紹:
首先,入參的context的來源,可以發現:
整個過程,其實Context最早在SystemServer中初始化,之後SystemServer在構建SystemServiceManager時,帶入了該Context,接下來就由SystemServiceManager將這個Context傳給了各個系統級的Service線程。
接下來mFactoryTest是工廠模式的讀取,這個涉及到開機的應用是HOME還是工廠測試應用,相關的知識點我們會在後續章節介紹,目前不影響閱讀。
mSystemThread是當前ActivityThread的實例,對於AMS自然就是system_process的ActivityThread,它的構造是在我們上方diagram內ActivityThread#systemMain這個方法內,在這裏面通過調用Activity#attach方法,將當前ActivityThread的實例attach。
mHandlerThread是一個ServiceThread,而ServiceThread繼承至HandlerThread,於是這個實例其實目的就是提供一個Looper給接下來的MainHandler, 這個Handler並沒有在SystemServer的main loop內處理消息,而是利用了AMS專屬的ServiceThread,這樣這個Handler便是AMS獨有的,不會因爲處理其他service的問題被阻塞。
mUiHandler是在UI線程(主線程)的Handler,主要處理一些UI相關的消息。
mFgBroadcastQueue, mBgBroadcastQueue是兩個前後臺廣播存放的queue,我們之後的章節會講到,根據是否是低內存設備,queue的長度會不同,原生的低內存設備存放25條,非低內存設備可以存放300條。
mBroadcastQueues是組織上面兩個queue的地方。
mServices是ActiveService的實例,之後主要負責Service相關的操作。
mProviderMap是ProviderMap的實例,之後主要負責provider相關的操作。
接下來初始化system文件,並初始化電池電量監控,這部分我們在後續章節會有單獨介紹。
mStartedUsers記錄了User0作爲第一個User。
接下來是一些MultiUser的處理。
GL_ES_VERSION是當前opengl的版本。
mIntentFirewall會更安全地處理XML內的Intent。
mRecentTasks記錄了最近的Activity Task棧。
mStackSupervisor是Activity棧的處理者,我們稍後會看到,Activity的切換是透過它完成的。
mProcessCpuThread是記錄CPU狀態的一個線程。
Watchdog相關知識我們會在介紹ANR時再介紹。
接下來便進入了start:
private void start() {
Process.removeAllProcessGroups();
mProcessCpuThread.start();
mBatteryStatsService.publish(mContext);
mAppOpsService.publish(mContext);
Slog.d("AppOps", "AppOpsService published");
LocalServices.addService(ActivityManagerInternal.class, new LocalService());
}
實際上就是啓動了constructor內的一些thread等。
最後就是setSystemProcess這個方法了:
public void setSystemProcess() {
try {
ServiceManager.addService(Context.ACTIVITY_SERVICE, this, true);
ServiceManager.addService(ProcessStats.SERVICE_NAME, mProcessStats);
ServiceManager.addService("meminfo", new MemBinder(this));
ServiceManager.addService("gfxinfo", new GraphicsBinder(this));
ServiceManager.addService("dbinfo", new DbBinder(this));
if (MONITOR_CPU_USAGE) {
ServiceManager.addService("cpuinfo", new CpuBinder(this));
}
ServiceManager.addService("permission", new PermissionController(this));
ServiceManager.addService("processinfo", new ProcessInfoService(this));
ApplicationInfo info = mContext.getPackageManager().getApplicationInfo(
"android", STOCK_PM_FLAGS);
mSystemThread.installSystemApplicationInfo(info, getClass().getClassLoader());
synchronized (this) {
ProcessRecord app = newProcessRecordLocked(info, info.processName, false, 0);
app.persistent = true;
app.pid = MY_PID;
app.maxAdj = ProcessList.SYSTEM_ADJ;
app.makeActive(mSystemThread.getApplicationThread(), mProcessStats);
synchronized (mPidsSelfLocked) {
mPidsSelfLocked.put(app.pid, app);
}
updateLruProcessLocked(app, false, null);
updateOomAdjLocked();
}
} catch (PackageManager.NameNotFoundException e) {
throw new RuntimeException(
"Unable to find android system package", e);
}
}
如同我們之前講過的,其實這裏便是在向ServiceManager註冊一系列的Service,之後Client可以通過getService拿到這些Service的Bn端來實現與Bp端的通信。至此,AMS的初始化就完成了,並不複雜,很多變量我們會在之後的學習中瞭解用意。