1.概述
上一节我们学习了AMS\ATM的启动流程，这一节主要来学习Launcher的启动流程。
在Android的中，桌面应用Launcher由Launcher演变到Launcher2，再到现在的Launcher3，Google也做了很多改动。
Launcher不支持桌面小工具动画效果，Launcher2添加了动画效果和3D初步效果支持，从Android 4.4 (KK)开始Launcher默认使用Launcher3， Launcher3加入了透明状态栏，增加overview模式，可以调整workspace上页面的前后顺序，可以动态管理屏幕数量，widget列表与app list分开显示等功能。
我们主要研究Launcher3的启动过程。
2.核心源码
/frameworks/base/core/java/com/android/internal/os/ZygoteInit.java
/frameworks/base/core/java/com/android/internal/os/ZygoteServer.java
/frameworks/base/core/java/com/android/internal/os/ZygoteConnection.java
/frameworks/base/core/java/com/android/internal/os/Zygote.java
/frameworks/base/core/java/com/android/internal/os/RuntimeInit.java
/frameworks/base/services/java/com/android/server/SystemServer.java
/frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java
/frameworks/base/services/core/java/com/android/server/am/ProcessList.java
/frameworks/base/services/core/java/com/android/server/wm/ActivityTaskManagerService.java
/frameworks/base/services/core/java/com/android/server/wm/ActivityStartController.java
/frameworks/base/services/core/java/com/android/server/wm/ActivityStarter.java
/frameworks/base/services/core/java/com/android/server/wm/ActivityStack.java
/frameworks/base/services/core/java/com/android/server/wm/ActivityStackSupervisor.java
/frameworks/base/services/core/java/com/android/server/wm/RootActivityContainer.java
/frameworks/base/services/core/java/com/android/server/wm/ClientLifecycleManager.java
/frameworks/base/core/java/android/os/Process.java
/frameworks/base/core/java/android/os/ZygoteProcess.java
/frameworks/base/core/java/android/app/ActivityThread.java
/frameworks/base/core/java/android/app/Activity.java
/frameworks/base/core/java/android/app/ActivityManagerInternal.java
/frameworks/base/core/java/android/app/servertransaction/ClientTransaction.java
/frameworks/base/core/java/android/app/servertransaction/ClientTransaction.aidl
/frameworks/base/core/java/android/app/ClientTransactionHandler.java
/frameworks/base/core/java/android/app/servertransaction/TransactionExecutor.java
/frameworks/base/core/java/android/app/servertransaction/LaunchActivityItem.java
/frameworks/base/core/java/android/app/Instrumentation.java
/frameworks/base/services/core/java/com/android/server/pm/PackageManagerService.java从上面的代码路径可以看出，Android10.0中 Activity的相关功能被放到了wm的目录中，在Android9.0中是在am目录中，Google 最终的目的是把activity 和window融合，在Android10中只是做了简单的代码路径的变更，正在的功能还要到后面的版本才能慢慢融合。
主要代码作用：
- Instrumentation：
负责调用Activity和Application生命周期。
- ActivityTaskManagerService：
负责Activity管理和调度等工作。
ATM是Android10中新增内容
- ActivityManagerService：
负责管理四大组件和进程，包括生命周期和状态切换。
- ActivityTaskManagerInternal：
是由ActivityTaskManagerService对外提供的一个抽象类，真正的实现是在 ActivityTaskManagerService#LocalService
- ActivityThread：
管理应用程序进程中主线程的执行
- ActivityStackSupervisor：
负责所有Activity栈的管理
- TransactionExecutor：
主要作用是执行ClientTransaction
- ClientLifecycleManager：
生命周期的管理调用
3.架构
Android启动流程图：
Launcher启动序列图：
内容较多，例如Zygote的fork流程，realStartActivityLocked启动Activity的中间过程，都没有列出，下一个章节会单独来讲这部分内容
4.源码分析
上一节在AMS启动过程中，我们知道了AMS启动完成前，在systemReady()中会去调用startHomeOnAllDisplays()来启动Launcher，本次就从startHomeOnAllDisplays()函数入口，来看看Launcher是如何被启动起来的。
[ActivityManagerService.java]
public void systemReady(final Runnable goingCallback, TimingsTraceLog
traceLog) {
...
//启动Home Activity
mAtmInternal.startHomeOnAllDisplays(currentUserId, "systemReady");
...
}Launcher的启动由三部分启动：
-
SystemServer完成启动Launcher Activity的调用
-
Zygote()进行Launcher进程的Fork操作
-
进入ActivityThread的main()，完成最终Launcher的onCreate操作
接下来我们分别从源码部分来分析这三个启动过程。
4.1第一阶段SystemServer 启动HomeActivity的调用阶段
调用栈：
[ActivityTaskManagerService.java] startHomeOnAllDisplays()
说明：ActivityTaskManagerInternal是 ActivityTaskManagerService的一个抽象类，正在的实现是在ActivityTaskManagerService的LocalService，所以mAtmInternal.startHomeOnAllDisplays()最终调用的是ActivityTaskManagerService的startHomeOnAllDisplays()方法
源码：
public boolean startHomeOnAllDisplays(int userId, String reason) {
synchronized (mGlobalLock) {
//一路调用到 RootActivityContainer 的startHomeOnDisplay()方法，参考[4.2]
return mRootActivityContainer.startHomeOnAllDisplays(userId, reason);
}
}4.2 [RootActivityContainer.java] startHomeOnDisplay()
说明：在[4.1]中，获取的displayId为DEFAULT_DISPLAY， 首先通过getHomeIntent 来构建一个category为CATEGORY_HOME的Intent，表明是Home Activity；然后通过resolveHomeActivity()从系统所用已安装的引用中，找到一个符合HomeItent的Activity，最终调用startHomeActivity()来启动Activity
源码：
boolean startHomeOnDisplay(int userId, String reason, int displayId, boolean allowInstrumenting,
boolean fromHomeKey) {
...
if (displayId == DEFAULT_DISPLAY) {
//构建一个category为CATEGORY_HOME的Intent，表明是Home Activity，参考[4.2.1]
homeIntent = mService.getHomeIntent();
//通过PKMS从系统所用已安装的引用中，找到一个符合HomeItent的Activity参考[4.2.2]
aInfo = resolveHomeActivity(userId, homeIntent);
}
...
//启动Home Activity，参考[4.3]
mService.getActivityStartController().startHomeActivity(homeIntent, aInfo, myReason,
displayId);
return true;
}4.2.1 [ActivityTaskManagerService.java] getHomeIntent()
说明：构建一个category为CATEGORY_HOME的Intent，表明是Home Activity。
Intent.CATEGORY_HOME = "android.intent.category.HOME"
这个category会在Launcher3的 AndroidManifest.xml中配置，表明是Home Acivity
源码：
Intent getHomeIntent() {
Intent intent = new Intent(mTopAction, mTopData != null ? Uri.parse(mTopData) : null);
intent.setComponent(mTopComponent);
intent.addFlags(Intent.FLAG_DEBUG_TRIAGED_MISSING);
//不是生产模式，add一个CATEGORY_HOME
if (mFactoryTest != FactoryTest.FACTORY_TEST_LOW_LEVEL) {
intent.addCategory(Intent.CATEGORY_HOME);
}
return intent;
}4.2.2 [RootActivityContainer.java] resolveHomeActivity()
说明：通过Binder跨进程通知PackageManagerService从系统所用已安装的引用中，找到一个符合HomeItent的Activity
源码：
ActivityInfo resolveHomeActivity(int userId, Intent homeIntent) {
final int flags = ActivityManagerService.STOCK_PM_FLAGS;
final ComponentName comp = homeIntent.getComponent(); //系统正常启动时，component为null
ActivityInfo aInfo = null;
...
if (comp != null) {
// Factory test.
aInfo = AppGlobals.getPackageManager().getActivityInfo(comp, flags, userId);
} else {
//系统正常启动时，走该流程
final String resolvedType =
homeIntent.resolveTypeIfNeeded(mService.mContext.getContentResolver());
//resolveIntent做了两件事：1.通过queryIntentActivities来查找符合HomeIntent需求Activities
// 2.通过chooseBestActivity找到最符合Intent需求的Activity信息
final ResolveInfo info = AppGlobals.getPackageManager()
.resolveIntent(homeIntent, resolvedType, flags, userId);
if (info != null) {
aInfo = info.activityInfo;
}
}
...
aInfo = new ActivityInfo(aInfo);
aInfo.applicationInfo = mService.getAppInfoForUser(aInfo.applicationInfo, userId);
return aInfo;
}4.3 [ActivityStartController.java ] startHomeActivity()
说明：正在的启动Home Activity入口。obtainStarter() 方法返回的是 ActivityStarter 对象，它负责 Activity 的启动，一系列 setXXX() 方法传入启动所需的各种参数，最后的 execute() 是真正的启动逻辑。另外如果home activity处于顶层的resume activity中，则Home Activity 将被初始化，但不会被恢复。并将保持这种状态，直到有东西再次触发它。我们需要进行另一次恢复。
源码：
void startHomeActivity(Intent intent, ActivityInfo aInfo, String reason, int displayId) {
....
//返回一个 ActivityStarter 对象，它负责 Activity 的启动
//一系列 setXXX() 方法传入启动所需的各种参数，最后的 execute() 是真正的启动逻辑
//最后执行 ActivityStarter的execute方法
mLastHomeActivityStartResult = obtainStarter(intent, "startHomeActivity: " + reason)
.setOutActivity(tmpOutRecord)
.setCallingUid(0)
.setActivityInfo(aInfo)
.setActivityOptions(options.toBundle())
.execute(); //参考[4.3.1]
mLastHomeActivityStartRecord = tmpOutRecord[0];
final ActivityDisplay display =
mService.mRootActivityContainer.getActivityDisplay(displayId);
final ActivityStack homeStack = display != null ? display.getHomeStack() : null;
if (homeStack != null && homeStack.mInResumeTopActivity) {
//如果home activity 处于顶层的resume activity中，则Home Activity 将被初始化，但不会被恢复（以避免递归恢复），
//并将保持这种状态，直到有东西再次触发它。我们需要进行另一次恢复。
mSupervisor.scheduleResumeTopActivities();
}
}4.3.1 [ActivityStarter.java] execute()
说明：在[4.3]中obtainStarter没有调用setMayWait的方法，因此mRequest.mayWait为false，走startActivity流程
源码：
int execute() {
...
if (mRequest.mayWait) {
return startActivityMayWait(...)
} else {
return startActivity(...) //参考[4.3.2]
}
...
}4.3.2 [ActivityStarter.java] startActivity()
说明：延时布局，然后通过startActivityUnchecked()来处理启动标记 flag ，要启动的任务栈等，最后恢复布局
源码：
private int startActivity(final ActivityRecord r, ActivityRecord sourceRecord,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask,
ActivityRecord[] outActivity, boolean restrictedBgActivity) {
...
try {
//延时布局
mService.mWindowManager.deferSurfaceLayout();
//调用 startActivityUnchecked ,一路调用到resumeFocusedStacksTopActivities()，参考[4.3.4]
result = startActivityUnchecked(r, sourceRecord, voiceSession, voiceInteractor,
startFlags, doResume, options, inTask, outActivity, restrictedBgActivity);
} finally {
//恢复布局
mService.mWindowManager.continueSurfaceLayout();
}
...
}4.3.3 [RootActivityContainer.java] resumeFocusedStacksTopActivities()
说明：获取栈顶的Activity，恢复它
源码：
boolean resumeFocusedStacksTopActivities(
ActivityStack targetStack, ActivityRecord target, ActivityOptions targetOptions) {
...
//如果秒表栈就是栈顶Activity，启动resumeTopActivityUncheckedLocked()
if (targetStack != null && (targetStack.isTopStackOnDisplay()
|| getTopDisplayFocusedStack() == targetStack)) {
result = targetStack.resumeTopActivityUncheckedLocked(target, targetOptions);
...
if (!resumedOnDisplay) {
// 获取 栈顶的 ActivityRecord
final ActivityStack focusedStack = display.getFocusedStack();
if (focusedStack != null) {
//最终调用startSpecificActivityLocked(),参考[4.3.4]
focusedStack.resumeTopActivityUncheckedLocked(target, targetOptions);
}
}
}
}4.3.4 [ActivityStackSupervisor.java] startSpecificActivityLocked()
说明：发布消息以启动进程，以避免在ATM锁保持的情况下调用AMS时可能出现死锁,最终调用到ATM的startProcess()
源码：
void startSpecificActivityLocked(ActivityRecord r, boolean andResume, boolean checkConfig) {
...
//发布消息以启动进程，以避免在ATM锁保持的情况下调用AMS时可能出现死锁
//最终调用到AMS的startProcess()，参考[4.3.5]
final Message msg = PooledLambda.obtainMessage(
ActivityManagerInternal::startProcess, mService.mAmInternal, r.processName,
r.info.applicationInfo, knownToBeDead, "activity", r.intent.getComponent());
mService.mH.sendMessage(msg);
...
}4.3.5 [ActivityManagerService.java] startProcess()
说明：一路调用Process start()，最终到ZygoteProcess的attemptUsapSendArgsAndGetResult()，用来fork一个新的Launcher的进程
源码：
public void startProcess(String processName, ApplicationInfo info,boolean knownToBeDead, String hostingType, ComponentName hostingName) {..//同步操作，避免死锁synchronized (ActivityManagerService.this) {//调用startProcessLocked,然后到 Process的start，最终到ZygoteProcess的attemptUsapSendArgsAndGetResult()//用来fork一个新的Launcher的进程，参考[4.3.6]startProcessLocked(processName, info, knownToBeDead, 0 /* intentFlags */,new HostingRecord(hostingType, hostingName),false /* allowWhileBooting */, false /* isolated */,true /* keepIfLarge */);}...}
调用栈如下:
4.3.6 [ZygoteProcess.java] attemptZygoteSendArgsAndGetResult()
说明：通过Socket连接Zygote进程，把之前组装的msg发给Zygote，其中processClass ="android.app.ActivityThread"，通过Zygote进程来Fork出一个新的进程，并执行 "android.app.ActivityThread"的main方法
源码：
private Process.ProcessStartResult attemptZygoteSendArgsAndGetResult(
ZygoteState zygoteState, String msgStr) throws ZygoteStartFailedEx {
try {
//传入的zygoteState为openZygoteSocketIfNeeded()，里面会通过abi来检查是第一个zygote还是第二个
final BufferedWriter zygoteWriter = zygoteState.mZygoteOutputWriter;
final DataInputStream zygoteInputStream = zygoteState.mZygoteInputStream;
zygoteWriter.write(msgStr); //把应用进程的一些参数写给前面连接的zygote进程，包括前面的processClass ="android.app.ActivityThread"
zygoteWriter.flush(); //进入Zygote进程，处于阻塞状态， 参考[4.4]
//从socket中得到zygote创建的应用pid，赋值给 ProcessStartResult的对象
Process.ProcessStartResult result = new Process.ProcessStartResult();
result.pid = zygoteInputStream.readInt();
result.usingWrapper = zygoteInputStream.readBoolean();
if (result.pid < 0) {
throw new ZygoteStartFailedEx("fork() failed");
}
return result;
} catch (IOException ex) {
zygoteState.close();
Log.e(LOG_TAG, "IO Exception while communicating with Zygote - "
+ ex.toString());
throw new ZygoteStartFailedEx(ex);
}
}4.4 第二阶段Zygote fork一个Launcher进程的阶段
说明：Zygote的启动过程我们前面有详细讲到过。SystemServer的AMS服务向启动Home Activity发起一个fork请求，Zygote进程通过Linux的fork函数，孵化出一个新的进程。
由于Zygote进程在启动时会创建Java虚拟机，因此通过fork而创建的Launcher程序进程可以在内部获取一个Java虚拟机的实例拷贝。fork采用copy-on-write机制，有些类如果不做改变，甚至都不用复制，子进程可以和父进程共享这部分数据，从而省去不少内存的占用。
fork过程，参考下图：
Zygote的调用栈如下：
4.4.1 [ZygoteInit.java] main()
说明：Zygote先fork出SystemServer进程，接着进入循环等待，用来接收Socket发来的消息，用来fork出其他应用进程，比如Launcher
源码：
public static void main(String argv[]) {
...
Runnable caller;
....
if (startSystemServer) {
//Zygote Fork出的第一个进程 SystmeServer
Runnable r = forkSystemServer(abiList, zygoteSocketName, zygoteServer);
if (r != null) {
r.run();
return;
}
}
...
//循环等待fork出其他的应用进程，比如Launcher
//最终通过调用processOneCommand()来进行进程的处理，参考[4.4.2]
caller = zygoteServer.runSelectLoop(abiList);
...
if (caller != null) {
caller.run(); //执行返回的Runnable对象，进入子进程
}
}4.4.2 [ZygoteConnection.java] processOneCommand()
说明：通过forkAndSpecialize()来fork出Launcher的子进程，并执行handleChildProc，进入子进程的处理
源码：
Runnable processOneCommand(ZygoteServer zygoteServer) {
int pid = -1;
...
//Fork子进程，得到一个新的pid
/fork子进程,采用copy on write方式，这里执行一次，会返回两次
///pid=0 表示Zygote fork子进程成功
//pid > 0 表示子进程 的真正的PID
pid = Zygote.forkAndSpecialize(parsedArgs.mUid, parsedArgs.mGid, parsedArgs.mGids,
parsedArgs.mRuntimeFlags, rlimits, parsedArgs.mMountExternal, parsedArgs.mSeInfo,
parsedArgs.mNiceName, fdsToClose, fdsToIgnore, parsedArgs.mStartChildZygote,
parsedArgs.mInstructionSet, parsedArgs.mAppDataDir, parsedArgs.mTargetSdkVersion);
...
if (pid == 0) {
// in child, fork成功，第一次返回的pid = 0
...
//参考[4.4.3]
return handleChildProc(parsedArgs, descriptors, childPipeFd,
parsedArgs.mStartChildZygote);
} else {
//in parent
...
childPipeFd = null;
handleParentProc(pid, descriptors, serverPipeFd);
return null;
}
}4.4.3 [ZygoteConnection.java] handleChildProc()
说明：进行子进程的操作，最终获得需要执行的ActivityThread的main()
源码：
private Runnable handleChildProc(ZygoteArguments parsedArgs, FileDescriptor[] descriptors,
FileDescriptor pipeFd, boolean isZygote) {
...
if (parsedArgs.mInvokeWith != null) {
...
throw new IllegalStateException("WrapperInit.execApplication unexpectedly returned");
} else {
if (!isZygote) {
// App进程将会调用到这里，执行目标类的main()方法
return ZygoteInit.zygoteInit(parsedArgs.mTargetSdkVersion,
parsedArgs.mRemainingArgs, null /* classLoader */);
} else {
return ZygoteInit.childZygoteInit(parsedArgs.mTargetSdkVersion,
parsedArgs.mRemainingArgs, null /* classLoader */);
}
}
}zygoteInit 进行一些环境的初始化、启动Binder进程等操作：
public static final Runnable zygoteInit(int targetSdkVersion, String[] argv,
ClassLoader classLoader) {
RuntimeInit.commonInit(); //初始化运行环境
ZygoteInit.nativeZygoteInit(); //启动Binder线程池
//调用程序入口函数
return RuntimeInit.applicationInit(targetSdkVersion, argv, classLoader);
}把之前传来的"android.app.ActivityThread" 传递给findStaticMain：
protected static Runnable applicationInit(int targetSdkVersion, String[] argv,
ClassLoader classLoader) {
...
// startClass: 如果AMS通过socket传递过来的是 ActivityThread
return findStaticMain(args.startClass, args.startArgs, classLoader);
}通过反射，拿到ActivityThread的main()方法：
protected static Runnable findStaticMain(String className, String[] argv,
ClassLoader classLoader) {
Class<?> cl;
try {
cl = Class.forName(className, true, classLoader);
} catch (ClassNotFoundException ex) {
throw new RuntimeException(
"Missing class when invoking static main " + className,
ex);
}
Method m;
try {
m = cl.getMethod("main", new Class[] { String[].class });
} catch (NoSuchMethodException ex) {
throw new RuntimeException(
"Missing static main on " + className, ex);
} catch (SecurityException ex) {
throw new RuntimeException(
"Problem getting static main on " + className, ex);
}
int modifiers = m.getModifiers();
if (! (Modifier.isStatic(modifiers) && Modifier.isPublic(modifiers))) {
throw new RuntimeException(
"Main method is not public and static on " + className);
}
return new MethodAndArgsCaller(m, argv);
}把反射得来的ActivityThread main()入口返回给ZygoteInit的main，通过caller.run()进行调用：
static class MethodAndArgsCaller implements Runnable {
/** method to call */
private final Method mMethod;
/** argument array */
private final String[] mArgs;
public MethodAndArgsCaller(Method method, String[] args) {
mMethod = method;
mArgs = args;
}
//调用ActivityThread的main()
public void run() {
try {
mMethod.invoke(null, new Object[] { mArgs });
} catch (IllegalAccessException ex) {
throw new RuntimeException(ex);
} catch (InvocationTargetException ex) {
Throwable cause = ex.getCause();
if (cause instanceof RuntimeException) {
throw (RuntimeException) cause;
} else if (cause instanceof Error) {
throw (Error) cause;
}
throw new RuntimeException(ex);
}
}
}4.5 第三个阶段，Launcher在自己的进程中进行onCreate等后面的动作
从[4.4]可以看到，Zygote fork出了Launcher的进程，并把接下来的Launcher启动任务交给了ActivityThread来进行，接下来我们就从ActivityThread main()来分析Launcher的创建过程。
调用栈如下：
4.5.1 [ActivityThread.java] main()
说明：主线程处理， 创建ActivityThread对象，调用attach进行处理，最终进入Looper循环
源码：
public static void main(String[] args) {
// 安装选择性的系统调用拦截
AndroidOs.install();
...
//主线程处理
Looper.prepareMainLooper();
...
//之前SystemServer调用attach传入的是true，这里到应用进程传入false就行
ActivityThread thread = new ActivityThread();
thread.attach(false, startSeq);
...
//一直循环，如果退出，说明程序关闭
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}调用ActivityThread的attach进行处理
private void attach(boolean system, long startSeq) {
sCurrentActivityThread = this;
mSystemThread = system;
if (!system) {
//应用进程启动，走该流程
...
RuntimeInit.setApplicationObject(mAppThread.asBinder());
//获取AMS的本地代理类
final IActivityManager mgr = ActivityManager.getService();
try {
//通过Binder调用AMS的attachApplication方法，参考[4.5.2]
mgr.attachApplication(mAppThread, startSeq);
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
...
} else {
//通过system_server启动ActivityThread对象
...
}
// 为 ViewRootImpl 设置配置更新回调，
当系统资源配置（如：系统字体）发生变化时，通知系统配置发生变化
ViewRootImpl.ConfigChangedCallback configChangedCallback
= (Configuration globalConfig) -> {
synchronized (mResourcesManager) {
...
}
};
ViewRootImpl.addConfigCallback(configChangedCallback);
}4.5.2 [ActivityManagerService.java] attachApplication()
说明：清除一些无用的记录，最终调用ActivityStackSupervisor.java的 realStartActivityLocked()，进行Activity的启动
源码：
public final void attachApplication(IApplicationThread thread, long startSeq) {
synchronized (this) {
//通过Binder获取传入的pid信息
int callingPid = Binder.getCallingPid();
final int callingUid = Binder.getCallingUid();
final long origId = Binder.clearCallingIdentity();
attachApplicationLocked(thread, callingPid, callingUid, startSeq);
Binder.restoreCallingIdentity(origId);
}
}private final boolean attachApplicationLocked(IApplicationThread thread,
int pid, int callingUid, long startSeq) {
...
//如果当前的Application记录仍然依附到之前的进程中，则清理掉
if (app.thread != null) {
handleAppDiedLocked(app, true, true);
}·
//mProcessesReady这个变量在AMS的 systemReady 中被赋值为true，
//所以这里的normalMode也为true
boolean normalMode = mProcessesReady || isAllowedWhileBooting(app.info);
...
//上面说到，这里为true，进入StackSupervisor的attachApplication方法
//去真正启动Activity
if (normalMode) {
...
//调用ATM的attachApplication()，最终层层调用到ActivityStackSupervisor.java的 realStartActivityLocked()
//参考[4.5.3]
didSomething = mAtmInternal.attachApplication(app.getWindowProcessController());
...
}
...
return true;
}4.5.3 [ActivityStackSupervisor.java] realStartActivityLocked()
说明：真正准备去启动Activity，通过clientTransaction.addCallback把LaunchActivityItem的obtain作为回调参数加进去，再调用
ClientLifecycleManager.scheduleTransaction()得到
LaunchActivityItem的execute()方法进行最终的执行
参考上面的第三阶段的调用栈流程
调用栈如下：
源码：
boolean realStartActivityLocked(ActivityRecord r, WindowProcessController proc,
boolean andResume, boolean checkConfig) throws RemoteException {
// 直到所有的 onPause() 执行结束才会去启动新的 activity
if (!mRootActivityContainer.allPausedActivitiesComplete()) {
...
return false;
}
try {
// Create activity launch transaction.
// 添加 LaunchActivityItem
final ClientTransaction clientTransaction = ClientTransaction.obtain(
proc.getThread(), r.appToken);
//LaunchActivityItem.obtain(new Intent(r.intent)作为回调参数
clientTransaction.addCallback(LaunchActivityItem.obtain(new Intent(r.intent),
System.identityHashCode(r), r.info,
// TODO: Have this take the merged configuration instead of separate global
// and override configs.
mergedConfiguration.getGlobalConfiguration(),
mergedConfiguration.getOverrideConfiguration(), r.compat,
r.launchedFromPackage, task.voiceInteractor, proc.getReportedProcState(),
r.icicle, r.persistentState, results, newIntents,
dc.isNextTransitionForward(), proc.createProfilerInfoIfNeeded(),
r.assistToken));
...
// 设置生命周期状态
final ActivityLifecycleItem lifecycleItem;
if (andResume) {
lifecycleItem = ResumeActivityItem.obtain(dc.isNextTransitionForward());
} else {
lifecycleItem = PauseActivityItem.obtain();
}
clientTransaction.setLifecycleStateRequest(lifecycleItem);
// Schedule transaction.
// 重点关注：调用 ClientLifecycleManager.scheduleTransaction()，得到上面addCallback的LaunchActivityItem的execute()方法
//参考[4.5.4]
mService.getLifecycleManager().scheduleTransaction(clientTransaction);
} catch (RemoteException e) {
if (r.launchFailed) {
// 第二次启动失败，finish activity
stack.requestFinishActivityLocked(r.appToken, Activity.RESULT_CANCELED, null,
"2nd-crash", false);
return false;
}
// 第一次失败，重启进程并重试
r.launchFailed = true;
proc.removeActivity(r);
throw e;
}
} finally {
endDeferResume();
}
...
return true;
}4.5.4 [TransactionExecutor.java] execute()
说明：执行之前realStartActivityLocked()中的
clientTransaction.addCallback
源码：
[TransactionExecutor.java]
public void execute(ClientTransaction transaction) {
...
// 执行 callBack，参考上面的调用栈，执行回调方法，
//最终调用到ActivityThread的handleLaunchActivity()参考[4.5.5]
executeCallbacks(transaction);
// 执行生命周期状态
executeLifecycleState(transaction);
mPendingActions.clear();
}4.5.5 [ActivityThread.java] handleLaunchActivity()
说明：主要干了两件事，第一件：初始化WindowManagerGlobal；第二件：调用performLaunchActivity方法
源码：
[ActivityThread.java]
public Activity handleLaunchActivity(ActivityClientRecord r,
PendingTransactionActions pendingActions, Intent customIntent) {
...
//初始化WindowManagerGlobal
WindowManagerGlobal.initialize();
...
//调用performLaunchActivity，来处理Activity，参考[4.5.6]
final Activity a = performLaunchActivity(r, customIntent);
..
return a;
}4.5.6 [ActivityThread.java] performLaunchActivity()
说明：获取ComponentName、Context，反射创建Activity，设置Activity的一些内容，比如主题等； 最终调用callActivityOnCreate()来执行Activity的onCreate()方法
源码：
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
// 获取 ComponentName
ComponentName component = r.intent.getComponent();
...
// 获取 Context
ContextImpl appContext = createBaseContextForActivity(r);
Activity activity = null;
try {
// 反射创建 Activity
java.lang.ClassLoader cl = appContext.getClassLoader();
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);
StrictMode.incrementExpectedActivityCount(activity.getClass());
r.intent.setExtrasClassLoader(cl);
r.intent.prepareToEnterProcess();
if (r.state != null) {
r.state.setClassLoader(cl);
}
} catch (Exception e) {
...
}
try {
// 获取 Application
Application app = r.packageInfo.makeApplication(false, mInstrumentation);
if (activity != null) {
...
//Activity的一些处理
activity.attach(appContext, this, getInstrumentation(), r.token,
r.ident, app, r.intent, r.activityInfo, title, r.parent,
r.embeddedID, r.lastNonConfigurationInstances, config,
r.referrer, r.voiceInteractor, window, r.configCallback,
r.assistToken);
if (customIntent != null) {
activity.mIntent = customIntent;
}
...
int theme = r.activityInfo.getThemeResource();
if (theme != 0) {
// 设置主题
activity.setTheme(theme);
}
activity.mCalled = false;
// 执行 onCreate()
if (r.isPersistable()) {
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
} else {
mInstrumentation.callActivityOnCreate(activity, r.state);
}
...
r.activity = activity;
}
//当前状态为ON_CREATE
r.setState(ON_CREATE);
...
} catch (SuperNotCalledException e) {
throw e;
} catch (Exception e) {
...
}
return activity;
}callActivityOnCreate先执行activity onCreate的预处理，再去调用Activity的onCreate，最终完成Create创建后的内容处理
public void callActivityOnCreate(Activity activity, Bundle icicle,
PersistableBundle persistentState) {
prePerformCreate(activity); //activity onCreate的预处理
activity.performCreate(icicle, persistentState);//执行onCreate()
postPerformCreate(activity); //activity onCreate创建后的一些信息处理
}performCreate()主要调用Activity的onCreate()
final void performCreate(Bundle icicle, PersistableBundle persistentState) {
...
if (persistentState != null) {
onCreate(icicle, persistentState);
} else {
onCreate(icicle);
}
...
}至此，看到了我们最熟悉的Activity的onCreate()，Launcher的启动完成，Launcher被真正创建起来。
5.总结
看到onCreate()后，进入到我们最熟悉的Activity的入口，Launcher的启动告一段落。整个Android的启动流程，我们也完整的分析完成。
Launcher的启动经过了三个阶段：
第一个阶段：SystemServer完成启动Launcher Activity的调用
第二个阶段：Zygote()进行Launcher进程的Fork操作
第三个阶段：进入ActivityThread的main()，完成最终Launcher的onCreate操作
