1、概述
我们从上一篇文章知道在SystemServer进程启动之后，就会启动很多系统服务，我们看到源码部分，创建系统服务管理者之后，通过调用startBootstrapServices、startBootstrapServices、startCoreService三个函数。启动了系统的一些关键服务，也就是其他服务需要依赖的服务最早执行，还有核心服务、其他服务。
.....
// Create the system service manager.
//创建系统服务管理者
mSystemServiceManager = new SystemServiceManager(mSystemContext);
mSystemServiceManager.setRuntimeRestarted(mRuntimeRestart);
LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
.....
// Start services.启动服务
try {
traceBeginAndSlog("StartServices");
//启动系统关键服务
startBootstrapServices();
//核心服务
startCoreServices();
//其他服务
startOtherServices();
SystemServerInitThreadPool.shutdown();
} catch (Throwable ex) {
Slog.e("System", "******************************************");
Slog.e("System", "************ Failure starting system services", ex);
throw ex;
} finally {
traceEnd();
}
2、ActivityManageService的创建和启动
下面我们看到启动关键服务的startBootstrapServices的方法里。
private void startBootstrapServices() {
.....
//0、启动安装器服务
Installer installer = mSystemServiceManager.startService(Installer.class);
.....
//1、启动ActivityManagerService服务
mActivityManagerService = mSystemServiceManager.startService(
ActivityManagerService.Lifecycle.class).getService();
//2、为AMS设置系统服务管理者
mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
//3、为AMS设置安装器
mActivityManagerService.setInstaller(installer);
....
//4、初始化PowerManagement相关
mActivityManagerService.initPowerManagement();
......
//5、设置系统进程
mActivityManagerService.setSystemProcess();
..........
mActivityManagerService.setUsageStatsManager(
LocalServices.getService(UsageStatsManagerInternal.class));
}
2.1、SystemServiceManager.startService
SystemServiceManager中的startService方法，创建并启动了ActivityManagerService服务。主要通过是调用了SystemServiceManager的startService函数，传入了Lifecycle.class，并通过getService返回ActivityManagerService实例
mActivityManagerService = mSystemServiceManager.startService(
ActivityManagerService.Lifecycle.class).getService();
我们来看具体的startService方法
public <T extends SystemService> T startService(Class<T> serviceClass) {
try {
final String name = serviceClass.getName();
Slog.i(TAG, "Starting " + name);
Trace.traceBegin(Trace.TRACE_TAG_SYSTEM_SERVER, "StartService " + name);
// Create the service.
if (!SystemService.class.isAssignableFrom(serviceClass)) {
throw new RuntimeException("Failed to create " + name
+ ": service must extend " + SystemService.class.getName());
}
final T service;
.............
//通过反射获取构造函数，并创建实例。
Constructor<T> constructor = serviceClass.getConstructor(Context.class);
service = constructor.newInstance(mContext);
................
//调用startService并传入Lifecycle实例
startService(service);
return service;
} finally {
Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER);
}
}
public void startService(@NonNull final SystemService service) {
// Register it.
mServices.add(service);
// Start it.
long time = SystemClock.elapsedRealtime();
try {
service.onStart();
} catch (RuntimeException ex) {
throw new RuntimeException("Failed to start service " + service.getClass().getName()
+ ": onStart threw an exception", ex);
}
warnIfTooLong(SystemClock.elapsedRealtime() - time, service, "onStart");
}
通过反射获取Lifecycle构造函数，并创建Lifecycle实例。接着调用startService方法，传入Lifecycle实例。并返回当前Lifecycle实例。在startService中又调用了Lifecycle实例的onStart方法。我们再看看Lifecycle类
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();
}
@Override
public void onCleanupUser(int userId) {
mService.mBatteryStatsService.onCleanupUser(userId);
}
public ActivityManagerService getService() {
return mService;
}
}
我们看到在Lifecycle 的构造函数中创建了ActivityManagerService实例，接着在onStart的时候，就调用了ActivityManagerService的start函数，其实这里也就是通过一种静态代理的方式创建ActivityManagerService实例并调用ActivityManagerService的start方法，启动ActivityManagerService服务。接着在Lifecycle 中暴露getService方法，返回了ActivityManagerService实例。所以再一开始的
mActivityManagerService = mSystemServiceManager.startService(
ActivityManagerService.Lifecycle.class).getService();
我们就拿到了ActivityManagerService的实例，因此下面我们来分析ActivityManagerService创建实例的构造函数和start函数
2.2、ActivityManagerService的构造函数
public ActivityManagerService(Context systemContext) {
LockGuard.installLock(this, LockGuard.INDEX_ACTIVITY);
mInjector = new Injector();
mContext = systemContext;
mFactoryTest = FactoryTest.getMode();
mSystemThread = ActivityThread.currentActivityThread();
mUiContext = mSystemThread.getSystemUiContext();
Slog.i(TAG, "Memory class: " + ActivityManager.staticGetMemoryClass());
mPermissionReviewRequired = mContext.getResources().getBoolean(
com.android.internal.R.bool.config_permissionReviewRequired);
//1、创建名为ActivityManagerService的前台线程，并start启动
mHandlerThread = new ServiceThread(TAG,
THREAD_PRIORITY_FOREGROUND, false /*allowIo*/);
mHandlerThread.start();
//2、创建ActivityManagerService线程的Handler
mHandler = new MainHandler(mHandlerThread.getLooper());
//3、创建UiThread线程，并初始化该线程的Handler
mUiHandler = mInjector.getUiHandler(this);
mConstants = new ActivityManagerConstants(this, mHandler);
/* static; one-time init here */
if (sKillHandler == null) {
sKillThread = new ServiceThread(TAG + ":kill",
THREAD_PRIORITY_BACKGROUND, true /* allowIo */);
sKillThread.start();
sKillHandler = new KillHandler(sKillThread.getLooper());
}
//4、创建运行在前台的广播接收器，10S超时
mFgBroadcastQueue = new BroadcastQueue(this, mHandler,
"foreground", BROADCAST_FG_TIMEOUT, false);
//5、后台广播接收器，60S超时
mBgBroadcastQueue = new BroadcastQueue(this, mHandler,
"background", BROADCAST_BG_TIMEOUT, true);
mBroadcastQueues[0] = mFgBroadcastQueue;
mBroadcastQueues[1] = mBgBroadcastQueue;
mServices = new ActiveServices(this);
mProviderMap = new ProviderMap(this);
mAppErrors = new AppErrors(mUiContext, this);
// TODO: Move creation of battery stats service outside of activity manager service.
//6、创建system目录
File dataDir = Environment.getDataDirectory();
File systemDir = new File(dataDir, "system");
systemDir.mkdirs();
//7、启动电池服务
mBatteryStatsService = new BatteryStatsService(systemContext, systemDir, mHandler);
mBatteryStatsService.getActiveStatistics().readLocked();
mBatteryStatsService.scheduleWriteToDisk();
mOnBattery = DEBUG_POWER ? true
: mBatteryStatsService.getActiveStatistics().getIsOnBattery();
mBatteryStatsService.getActiveStatistics().setCallback(this);
//8、创建进程统计服务，统计信息保存在systemDir目录的procstats文件内
mProcessStats = new ProcessStatsService(this, new File(systemDir, "procstats"));
..................
//9、创建StackSupervisor实例
mStackSupervisor = createStackSupervisor();
mStackSupervisor.onConfigurationChanged(mTempConfig);
mKeyguardController = mStackSupervisor.mKeyguardController;
mCompatModePackages = new CompatModePackages(this, systemDir, mHandler);
mIntentFirewall = new IntentFirewall(new IntentFirewallInterface(), mHandler);
mTaskChangeNotificationController =
new TaskChangeNotificationController(this, mStackSupervisor, mHandler);
mActivityStarter = new ActivityStarter(this, mStackSupervisor);
mRecentTasks = new RecentTasks(this, mStackSupervisor);
//10、创建CPU统计线程
mProcessCpuThread = new Thread("CpuTracker") {
@Override
public void run() {
synchronized (mProcessCpuTracker) {
mProcessCpuInitLatch.countDown();
//11、统计CPU使用情况
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) {
}
//12、更新CPU状态
updateCpuStatsNow();
} catch (Exception e) {
Slog.e(TAG, "Unexpected exception collecting process stats", e);
}
}
}
};
Watchdog.getInstance().addMonitor(this);
Watchdog.getInstance().addThread(mHandler);
}
2.3、ActivityManagerService的start函数
private void start() {
//移除所有进程组
removeAllProcessGroups();
//CPU统计的线程启动
mProcessCpuThread.start();
//启动电池服务
mBatteryStatsService.publish();
mAppOpsService.publish(mContext);
Slog.d("AppOps", "AppOpsService published");
//添加到本地服务LocalServices中
LocalServices.addService(ActivityManagerInternal.class, new LocalService());
try {
mProcessCpuInitLatch.await();
} catch (InterruptedException e) {
Slog.wtf(TAG, "Interrupted wait during start", e);
Thread.currentThread().interrupt();
throw new IllegalStateException("Interrupted wait during start");
}
}
3、将ActivityManagerService注册服务到ServiceManager
以上ActivityManagerService就被创建和启动了，回到startBootstrapServices方法里，我们知道在startBootstrapServices方法里创建启动ActivityManagerService之后，又通过ActivityManagerService的实例调用了其setSystemProcess方法，
public void setSystemProcess() {
try {
//注册各种服务到ServiceManager中
//3.1、将ActivityManagerService注册到ServiceManager中
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 | MATCH_SYSTEM_ONLY);
//详见3.2
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);
}
}
以上主要是把各种服务注册到ServiceManager中，我们看到最核心的ActivityManagerService服务此时也注册到了ServiceManager中，注册服务的名字为activity，这里很关键，作为其他进程和SytemServer进程通信的桥梁。例如我们的APP进程就是可以使用ServiceManager通过服务的名字获取到ActivityManagerService作为通信媒介，而我们看到ActivityManagerService本身又继承了Binder。
public class ActivityManagerService extends IActivityManager.Stub
public static abstract class Stub extends android.os.Binder implements android.app.IActivityManager
这样APP进程就可以通过获取到的ActivityManagerService实现跨进程通信。例如最常见的我们在APP进程启动一个Actvity。
int result = ActivityManager.getService()
.startActivity(whoThread, who.getBasePackageName(), intent,
intent.resolveTypeIfNeeded(who.getContentResolver()),
token, target != null ? target.mEmbeddedID : null,
requestCode, 0, null, options);
public static IActivityManager getService() {
return IActivityManagerSingleton.get();
}
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;
}
}
就是首先通过ServiceManager获取到我们的ActivityManagerService，然后转换为IActivityManager 对象。通过调用IActivityManager 接口的startActivity函数，最终实现跨进程通信，就调用到了ActivityManagerService的startActivity函数。
@Override
public final int startActivity(IApplicationThread caller, String callingPackage,
Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
int startFlags, ProfilerInfo profilerInfo, Bundle bOptions) {
return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
resultWho, requestCode, startFlags, profilerInfo, bOptions,
UserHandle.getCallingUserId());
}
详细Activity的启动，我们后续在介绍，在这里我们知道，当创建启动ActivityManagerService之后，会通过一个名字注册到ServiceManager中，其他进程使用ServiceManager便可以通过名字获取到注册的服务，实现跨进程通信。
4、mActivityManagerService.systemReady--SystemServer进程准备好了
在启动了系统的关键和核心服务之后，接着又通过startOtherServices启动一些其他的服务。这样我们把需要的系统服务启动好了，我们在startOtherServices函数的最后，看到 mActivityManagerService.systemReady这个函数。
// We now tell the activity manager it is okay to run third party
// code. It will call back into us once it has gotten to the state
// where third party code can really run (but before it has actually
// started launching the initial applications), for us to complete our
// initialization.
mActivityManagerService.systemReady(() -> {
}
从注释上说明了我们可以告诉活动管理器此时SystemServer进程已经启动好了，此时可以运行第三方的程序代码了，比如我们的APP程序。我们来看看systemReady函数内和回调函数中做了那些操作？
public void systemReady(final Runnable goingCallback, TimingsTraceLog traceLog) {
...............
startHomeActivityLocked(currentUserId, "systemReady");
...............
}
boolean startHomeActivityLocked(int userId, String reason) {
if (mFactoryTest == FactoryTest.FACTORY_TEST_LOW_LEVEL
&& mTopAction == null) {
// We are running in factory test mode, but unable to find
// the factory test app, so just sit around displaying the
// error message and don't try to start anything.
return false;
}
//创建Intent对象
Intent intent = getHomeIntent();
ActivityInfo aInfo = resolveActivityInfo(intent, STOCK_PM_FLAGS, userId);
if (aInfo != null) {
intent.setComponent(new ComponentName(aInfo.applicationInfo.packageName, aInfo.name));
// Don't do this if the home app is currently being
// instrumented.
aInfo = new ActivityInfo(aInfo);
aInfo.applicationInfo = getAppInfoForUser(aInfo.applicationInfo, userId);
ProcessRecord app = getProcessRecordLocked(aInfo.processName,
aInfo.applicationInfo.uid, true);
if (app == null || app.instr == null) {
intent.setFlags(intent.getFlags() | Intent.FLAG_ACTIVITY_NEW_TASK);
final int resolvedUserId = UserHandle.getUserId(aInfo.applicationInfo.uid);
// For ANR debugging to verify if the user activity is the one that actually
// launched.
final String myReason = reason + ":" + userId + ":" + resolvedUserId;
//启动HomeActivtity。
mActivityStarter.startHomeActivityLocked(intent, aInfo, myReason);
}
} else {
Slog.wtf(TAG, "No home screen found for " + intent, new Throwable());
}
return true;
}
调用startHomeActivityLocked。启动Home类型的Activity，并且传入的reason参数是systemReady，也就说明了此时SystemServer已经准备好了，可以启动第一个Home类型的Activity了。在startHomeActivityLocked函数里我们首先调用getHomeIntent函数创建Intent对象，用于启动Home类型的Activity。我们看getHomeIntent函数。
Intent getHomeIntent() {
// public static final String ACTION_MAIN = "android.intent.action.MAIN";
Intent intent = new Intent(mTopAction, mTopData != null ? Uri.parse(mTopData) : null);
intent.setComponent(mTopComponent);
intent.addFlags(Intent.FLAG_DEBUG_TRIAGED_MISSING);
if (mFactoryTest != FactoryTest.FACTORY_TEST_LOW_LEVEL) {
// public static final String CATEGORY_HOME = "android.intent.category.HOME";
intent.addCategory(Intent.CATEGORY_HOME);
}
return intent;
}
我们看到Intent的action为 "android.intent.action.MAIN"，category是android.intent.category.HOME。Launcher和FallbackHome这两个Activity都能匹配到。
<activity android:name=".FallbackHome"
android:excludeFromRecents="true"
android:label=""
android:screenOrientation="nosensor"
android:theme="@style/FallbackHome">
<intent-filter android:priority="-1000">
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.HOME" />
<category android:name="android.intent.category.DEFAULT" />
</intent-filter>
</activity>
<activity
android:name="com.android.launcher2.Launcher"
android:launchMode="singleTask"
android:clearTaskOnLaunch="true"
android:stateNotNeeded="true"
android:resumeWhilePausing="true"
android:theme="@style/Theme"
android:windowSoftInputMode="adjustPan"
android:screenOrientation="nosensor">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.HOME" />
<category android:name="android.intent.category.DEFAULT" />
<category android:name="android.intent.category.MONKEY"/>
</intent-filter>
</activity>
而在FallbackHome中我们看到android:priority="-1000"，优先级很高，因此可以猜到第一个Activity启动的就是FallbackHome。当然我们也可以通过编译系统源码，打印日志的方式。通过打印我们也可以看到
行 1604: 04-06 21:46:42.667 1767 1767 D MyTest : FallbackHome onCreate:
行 1608: 04-06 21:46:42.672 1767 1767 D MyTest : FallbackHome onResume:
行 2400: 04-06 21:46:16.118 1767 1767 D MyTest : FallbackHome onReceive:
行 2402: 04-06 21:46:16.119 1767 1767 D MyTest : User unlocked and real home found; let's go!
行 2407: 04-06 21:46:16.166 1767 1767 D MyTest : FallbackHome onPause:
行 2445: 04-06 21:46:16.600 2064 2064 D MyTest : Launcher2 onCreate:
行 2573: 04-06 21:46:17.576 1767 1767 D MyTest : FallbackHome onDestroy:
首先会启动我们的FallbackHome ，在FallbackHome 解锁之后就会启动我们的Launcher2 ，也就是我们平时习惯说的系统桌面。
5、小结
在ActivityManagerService中做了很多很的工作，总共代码有2W多行，没法每个函数都分析清楚。我们大概小结ActivityManagerService的启动流程
- 在SystemServer中创建系统服务器管理者SystemServiceManager，并通过它启动三种类型的服务：关键服务、核心服务器、其他服务
- 在启动关键服务器中创建启动了ActivityManagerService服务
- 将ActivityManagerService注册到ServiceManager中，名字为activity。其他进程就可以通过名字检索到ActivityManagerService，而ActivityManagerService本身继承了Binder，从而用于实现跨进程通信。
- 在三种类型的服务中启动了很多很多具体的服务，在启动这些服务之后，通过ActivityManagerService的systemReady，表示SystemServer进程准备好了，此时可以运行第三方的代码，比如我的APP应用程序。
- SystemReady之后，就开始启动我们的第一个Activity：FallbackHome ，在FallbackHome 被解锁之后，接着就会启动我们的Launcher2 ，也就是我们的桌面Activity。
下一篇我们将介绍第一个Activity-FallbackHome 做了什么？Launcher2 是怎么启动的，桌面程序进程是怎么被启动的。
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