前文简要介绍了Android应用程序的Activity的启动过程。在Android系统中，应用程序是由Activity组成的，因此，应用程序的启动过程实际上就是应用程序中的默认Activity的启动过程，本文将详细分析应用程序框架层的源代码，了解Android应用程序的启动过程。

在上一篇文章Android应用程序的Activity启动过程简要介绍和学习计划中，我们举例子说明了启动Android应用程序中的Activity的两种情景，其中，在手机屏幕中点击应用程序图标的情景就会引发Android应用程序中的默认Activity的启动，从而把应用程序启动起来。这种启动方式的特点是会启动一个新的进程来加载相应的Activity。这里，我们继续以这个例子为例来说明Android应用程序的启动过程，即MainActivity的启动过程。

MainActivity的启动过程如下图所示：

点击查看大图

下面详细分析每一步是如何实现的。

Step 1. Launcher.startActivitySafely

在Android系统中，应用程序是由Launcher启动起来的，其实，Launcher本身也是一个应用程序，其它的应用程序安装后，就会Launcher的界面上出现一个相应的图标，点击这个图标时，Launcher就会对应的应用程序启动起来。

Launcher的源代码工程在packages/apps/Launcher2目录下，负责启动其它应用程序的源代码实现在src/com/android/launcher2/Launcher.java文件中：

/*** Default launcher application.*/public final class Launcher extends Activityimplements View.OnClickListener, OnLongClickListener, LauncherModel.Callbacks, AllAppsView.Watcher {....../*** Launches the intent referred by the clicked shortcut.** @param v The view representing the clicked shortcut.*/public void onClick(View v) {Object tag = v.getTag();if (tag instanceof ShortcutInfo) {// Open shortcutfinal Intent intent = ((ShortcutInfo) tag).intent;int[] pos = new int[2];v.getLocationOnScreen(pos);intent.setSourceBounds(new Rect(pos[0], pos[1],pos[0] + v.getWidth(), pos[1] + v.getHeight()));startActivitySafely(intent, tag);} else if (tag instanceof FolderInfo) {......} else if (v == mHandleView) {......}}void startActivitySafely(Intent intent, Object tag) {intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);try {startActivity(intent);} catch (ActivityNotFoundException e) {......} catch (SecurityException e) {......}}......} 回忆一下前面一篇文章 Android应用程序的Activity启动过程简要介绍和学习计划说到的应用程序Activity，它的默认Activity是MainActivity，这里是AndroidManifest.xml文件中配置的：

<activity android:name=".MainActivity" android:label="@string/app_name"> <intent-filter> <action android:name="android.intent.action.MAIN" /> <category android:name="android.intent.category.LAUNCHER" /> </intent-filter> </activity> 因此，这里的intent包含的信息为：action = "android.intent.action.Main"，category="android.intent.category.LAUNCHER", cmp="shy.luo.activity/.MainActivity"，表示它要启动的Activity为shy.luo.activity.MainActivity。Intent.FLAG_ACTIVITY_NEW_TASK表示要在一个新的Task中启动这个Activity，注意，Task是Android系统中的概念，它不同于进程Process的概念。简单地说，一个Task是一系列Activity的集合，这个集合是以堆栈的形式来组织的，遵循后进先出的原则。事实上，Task是一个非常复杂的概念，有兴趣的读者可以到官网 http://developer.android.com/guide/topics/manifest/activity-element.html查看相关的资料。这里，我们只要知道，这个MainActivity要在一个新的Task中启动就可以了。

Step 2. Activity.startActivity

在Step 1中，我们看到，Launcher继承于Activity类，而Activity类实现了startActivity函数，因此，这里就调用了Activity.startActivity函数，它实现在frameworks/base/core/java/android/app/Activity.java文件中：

public class Activity extends ContextThemeWrapperimplements LayoutInflater.Factory,Window.Callback, KeyEvent.Callback,OnCreateContextMenuListener, ComponentCallbacks {......@Overridepublic void startActivity(Intent intent) {startActivityForResult(intent, -1);}......} 这个函数实现很简单，它调用startActivityForResult来进一步处理，第二个参数传入-1表示不需要这个Actvity结束后的返回结果。

Step 3.Activity.startActivityForResult

这个函数也是实现在frameworks/base/core/java/android/app/Activity.java文件中：

public class Activity extends ContextThemeWrapperimplements LayoutInflater.Factory,Window.Callback, KeyEvent.Callback,OnCreateContextMenuListener, ComponentCallbacks {......public void startActivityForResult(Intent intent, int requestCode) {if (mParent == null) {Instrumentation.ActivityResult ar =mInstrumentation.execStartActivity(this, mMainThread.getApplicationThread(), mToken, this,intent, requestCode);......} else {......}......} 这里的mInstrumentation是Activity类的成员变量，它的类型是Intrumentation，定义在frameworks/base/core/java/android/app/Instrumentation.java文件中，它用来监控应用程序和系统的交互。

这里的mMainThread也是Activity类的成员变量，它的类型是ActivityThread，它代表的是应用程序的主线程，我们在Android系统在新进程中启动自定义服务过程（startService）的原理分析一文中已经介绍过了。这里通过mMainThread.getApplicationThread获得它里面的ApplicationThread成员变量，它是一个Binder对象，后面我们会看到，ActivityManagerService会使用它来和ActivityThread来进行进程间通信。这里我们需注意的是，这里的mMainThread代表的是Launcher应用程序运行的进程。

这里的mToken也是Activity类的成员变量，它是一个Binder对象的远程接口。

Step 4. Instrumentation.execStartActivity
这个函数定义在frameworks/base/core/java/android/app/Instrumentation.java文件中：

public class Instrumentation {......public ActivityResult execStartActivity(Context who, IBinder contextThread, IBinder token, Activity target,Intent intent, int requestCode) {IApplicationThread whoThread = (IApplicationThread) contextThread;if (mActivityMonitors != null) {......}try {int result = ActivityManagerNative.getDefault().startActivity(whoThread, intent,intent.resolveTypeIfNeeded(who.getContentResolver()),null, 0, token, target != null ? target.mEmbeddedID : null,requestCode, false, false);......} catch (RemoteException e) {}return null;}......} 这里的ActivityManagerNative.getDefault返回ActivityManagerService的远程接口，即ActivityManagerProxy接口，具体可以参考 Android系统在新进程中启动自定义服务过程（startService）的原理分析一文。

这里的intent.resolveTypeIfNeeded返回这个intent的MIME类型，在这个例子中，没有AndroidManifest.xml设置MainActivity的MIME类型，因此，这里返回null。

这里的target不为null，但是target.mEmbddedID为null，我们不用关注。

Step 5. ActivityManagerProxy.startActivity

这个函数定义在frameworks/base/core/java/android/app/ActivityManagerNative.java文件中：

class ActivityManagerProxy implements IActivityManager{......public int startActivity(IApplicationThread caller, Intent intent,String resolvedType, Uri[] grantedUriPermissions, int grantedMode,IBinder resultTo, String resultWho,int requestCode, boolean onlyIfNeeded,boolean debug) throws RemoteException {Parcel data = Parcel.obtain();Parcel reply = Parcel.obtain();data.writeInterfaceToken(IActivityManager.descriptor);data.writeStrongBinder(caller != null ? caller.asBinder() : null);intent.writeToParcel(data, 0);data.writeString(resolvedType);data.writeTypedArray(grantedUriPermissions, 0);data.writeInt(grantedMode);data.writeStrongBinder(resultTo);data.writeString(resultWho);data.writeInt(requestCode);data.writeInt(onlyIfNeeded ? 1 : 0);data.writeInt(debug ? 1 : 0);mRemote.transact(START_ACTIVITY_TRANSACTION, data, reply, 0);reply.readException();int result = reply.readInt();reply.recycle();data.recycle();return result;}......} 这里的参数比较多，我们先整理一下。从上面的调用可以知道，这里的参数resolvedType、grantedUriPermissions和resultWho均为null；参数caller为ApplicationThread类型的Binder实体；参数resultTo为一个Binder实体的远程接口，我们先不关注它；参数grantedMode为0，我们也先不关注它；参数requestCode为-1；参数onlyIfNeeded和debug均空false。

Step 6. ActivityManagerService.startActivity

上一步Step 5通过Binder驱动程序就进入到ActivityManagerService的startActivity函数来了，它定义在frameworks/base/services/java/com/android/server/am/ActivityManagerService.java文件中：

public final class ActivityManagerService extends ActivityManagerNativeimplements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {......public final int startActivity(IApplicationThread caller,Intent intent, String resolvedType, Uri[] grantedUriPermissions,int grantedMode, IBinder resultTo,String resultWho, int requestCode, boolean onlyIfNeeded,boolean debug) {return mMainStack.startActivityMayWait(caller, intent, resolvedType,grantedUriPermissions, grantedMode, resultTo, resultWho,requestCode, onlyIfNeeded, debug, null, null);}......} 这里只是简单地将操作转发给成员变量mMainStack的startActivityMayWait函数，这里的mMainStack的类型为ActivityStack。

Step 7.ActivityStack.startActivityMayWait

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {......final int startActivityMayWait(IApplicationThread caller,Intent intent, String resolvedType, Uri[] grantedUriPermissions,int grantedMode, IBinder resultTo,String resultWho, int requestCode, boolean onlyIfNeeded,boolean debug, WaitResult outResult, Configuration config) {......boolean componentSpecified = intent.getComponent() != null;// Don't modify the client's object!intent = new Intent(intent);// Collect information about the target of the Intent.ActivityInfo aInfo;try {ResolveInfo rInfo =AppGlobals.getPackageManager().resolveIntent(intent, resolvedType,PackageManager.MATCH_DEFAULT_ONLY| ActivityManagerService.STOCK_PM_FLAGS);aInfo = rInfo != null ? rInfo.activityInfo : null;} catch (RemoteException e) {......}if (aInfo != null) {// Store the found target back into the intent, because now that// we have it we never want to do this again. For example, if the// user navigates back to this point in the history, we should// always restart the exact same activity.intent.setComponent(new ComponentName(aInfo.applicationInfo.packageName, aInfo.name));......}synchronized (mService) {int callingPid;int callingUid;if (caller == null) {......} else {callingPid = callingUid = -1;}mConfigWillChange = config != null&& mService.mConfiguration.diff(config) != 0;......if (mMainStack && aInfo != null &&(aInfo.applicationInfo.flags&ApplicationInfo.FLAG_CANT_SAVE_STATE) != 0) { ......}int res = startActivityLocked(caller, intent, resolvedType,grantedUriPermissions, grantedMode, aInfo,resultTo, resultWho, requestCode, callingPid, callingUid,onlyIfNeeded, componentSpecified);if (mConfigWillChange && mMainStack) {......}......if (outResult != null) {......}return res;}}......} 注意，从Step 6传下来的参数outResult和config均为null，此外，表达式(aInfo.applicationInfo.flags&ApplicationInfo.FLAG_CANT_SAVE_STATE) != 0为false，因此，这里忽略了无关代码。

下面语句对参数intent的内容进行解析，得到MainActivity的相关信息，保存在aInfo变量中：

ActivityInfo aInfo; try {ResolveInfo rInfo =AppGlobals.getPackageManager().resolveIntent(intent, resolvedType,PackageManager.MATCH_DEFAULT_ONLY| ActivityManagerService.STOCK_PM_FLAGS);aInfo = rInfo != null ? rInfo.activityInfo : null; } catch (RemoteException e) {...... } 解析之后，得到的aInfo.applicationInfo.packageName的值为"shy.luo.activity"，aInfo.name的值为"shy.luo.activity.MainActivity"，这是在这个实例的配置文件AndroidManifest.xml里面配置的。

此外，函数开始的地方调用intent.getComponent()函数的返回值不为null，因此，这里的componentSpecified变量为true。

接下去就调用startActivityLocked进一步处理了。

Step 8. ActivityStack.startActivityLocked

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {......final int startActivityLocked(IApplicationThread caller, Intent intent, String resolvedType, Uri[] grantedUriPermissions, int grantedMode, ActivityInfo aInfo, IBinder resultTo, String resultWho, int requestCode, int callingPid, int callingUid, boolean onlyIfNeeded, boolean componentSpecified) { int err = START_SUCCESS;ProcessRecord callerApp = null;if (caller != null) {callerApp = mService.getRecordForAppLocked(caller);if (callerApp != null) {callingPid = callerApp.pid;callingUid = callerApp.info.uid;} else {......}}......ActivityRecord sourceRecord = null;ActivityRecord resultRecord = null;if (resultTo != null) {int index = indexOfTokenLocked(resultTo);......if (index >= 0) {sourceRecord = (ActivityRecord)mHistory.get(index);if (requestCode >= 0 && !sourceRecord.finishing) {......}}}int launchFlags = intent.getFlags();if ((launchFlags&Intent.FLAG_ACTIVITY_FORWARD_RESULT) != 0&& sourceRecord != null) {......}if (err == START_SUCCESS && intent.getComponent() == null) {......}if (err == START_SUCCESS && aInfo == null) {......}if (err != START_SUCCESS) {......}......ActivityRecord r = new ActivityRecord(mService, this, callerApp, callingUid,intent, resolvedType, aInfo, mService.mConfiguration,resultRecord, resultWho, requestCode, componentSpecified);......return startActivityUncheckedLocked(r, sourceRecord,grantedUriPermissions, grantedMode, onlyIfNeeded, true);}......} 从传进来的参数caller得到调用者的进程信息，并保存在callerApp变量中，这里就是Launcher应用程序的进程信息了。

前面说过，参数resultTo是Launcher这个Activity里面的一个Binder对象，通过它可以获得Launcher这个Activity的相关信息，保存在sourceRecord变量中。
再接下来，创建即将要启动的Activity的相关信息，并保存在r变量中：

ActivityRecord r = new ActivityRecord(mService, this, callerApp, callingUid,intent, resolvedType, aInfo, mService.mConfiguration,resultRecord, resultWho, requestCode, componentSpecified); 接着调用startActivityUncheckedLocked函数进行下一步操作。

Step 9. ActivityStack.startActivityUncheckedLocked

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {......final int startActivityUncheckedLocked(ActivityRecord r,ActivityRecord sourceRecord, Uri[] grantedUriPermissions,int grantedMode, boolean onlyIfNeeded, boolean doResume) {final Intent intent = r.intent;final int callingUid = r.launchedFromUid;int launchFlags = intent.getFlags();// We'll invoke onUserLeaving before onPause only if the launching// activity did not explicitly state that this is an automated launch.mUserLeaving = (launchFlags&Intent.FLAG_ACTIVITY_NO_USER_ACTION) == 0;......ActivityRecord notTop = (launchFlags&Intent.FLAG_ACTIVITY_PREVIOUS_IS_TOP)!= 0 ? r : null;// If the onlyIfNeeded flag is set, then we can do this if the activity// being launched is the same as the one making the call... or, as// a special case, if we do not know the caller then we count the// current top activity as the caller.if (onlyIfNeeded) {......}if (sourceRecord == null) {......} else if (sourceRecord.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE) {......} else if (r.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE|| r.launchMode == ActivityInfo.LAUNCH_SINGLE_TASK) {......}if (r.resultTo != null && (launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) != 0) {......}boolean addingToTask = false;if (((launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) != 0 &&(launchFlags&Intent.FLAG_ACTIVITY_MULTIPLE_TASK) == 0)|| r.launchMode == ActivityInfo.LAUNCH_SINGLE_TASK|| r.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE) {// If bring to front is requested, and no result is requested, and// we can find a task that was started with this same// component, then instead of launching bring that one to the front.if (r.resultTo == null) {// See if there is a task to bring to the front. If this is// a SINGLE_INSTANCE activity, there can be one and only one// instance of it in the history, and it is always in its own// unique task, so we do a special search.ActivityRecord taskTop = r.launchMode != ActivityInfo.LAUNCH_SINGLE_INSTANCE? findTaskLocked(intent, r.info): findActivityLocked(intent, r.info);if (taskTop != null) {......}}}......if (r.packageName != null) {// If the activity being launched is the same as the one currently// at the top, then we need to check if it should only be launched// once.ActivityRecord top = topRunningNonDelayedActivityLocked(notTop);if (top != null && r.resultTo == null) {if (top.realActivity.equals(r.realActivity)) {......}}} else {......}boolean newTask = false;// Should this be considered a new task?if (r.resultTo == null && !addingToTask&& (launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) != 0) {// todo: should do better management of integers.mService.mCurTask++;if (mService.mCurTask <= 0) {mService.mCurTask = 1;}r.task = new TaskRecord(mService.mCurTask, r.info, intent,(r.info.flags&ActivityInfo.FLAG_CLEAR_TASK_ON_LAUNCH) != 0);......newTask = true;if (mMainStack) {mService.addRecentTaskLocked(r.task);}} else if (sourceRecord != null) {......} else {......}......startActivityLocked(r, newTask, doResume);return START_SUCCESS;}......} 函数首先获得intent的标志值，保存在launchFlags变量中。

这个intent的标志值的位Intent.FLAG_ACTIVITY_NO_USER_ACTION没有置位，因此 ，成员变量mUserLeaving的值为true。

这个intent的标志值的位Intent.FLAG_ACTIVITY_PREVIOUS_IS_TOP也没有置位，因此，变量notTop的值为null。

由于在这个例子的AndroidManifest.xml文件中，MainActivity没有配置launchMode属值，因此，这里的r.launchMode为默认值0，表示以标准（Standard，或者称为ActivityInfo.LAUNCH_MULTIPLE）的方式来启动这个Activity。Activity的启动方式有四种，其余三种分别是ActivityInfo.LAUNCH_SINGLE_INSTANCE、ActivityInfo.LAUNCH_SINGLE_TASK和ActivityInfo.LAUNCH_SINGLE_TOP，具体可以参考官方网站http://developer.android.com/reference/android/content/pm/ActivityInfo.html。

传进来的参数r.resultTo为null，表示Launcher不需要等这个即将要启动的MainActivity的执行结果。

由于这个intent的标志值的位Intent.FLAG_ACTIVITY_NEW_TASK被置位，而且Intent.FLAG_ACTIVITY_MULTIPLE_TASK没有置位，因此，下面的if语句会被执行：

if (((launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) != 0 &&(launchFlags&Intent.FLAG_ACTIVITY_MULTIPLE_TASK) == 0)|| r.launchMode == ActivityInfo.LAUNCH_SINGLE_TASK|| r.launchMode == ActivityInfo.LAUNCH_SINGLE_INSTANCE) {// If bring to front is requested, and no result is requested, and// we can find a task that was started with this same// component, then instead of launching bring that one to the front.if (r.resultTo == null) {// See if there is a task to bring to the front. If this is// a SINGLE_INSTANCE activity, there can be one and only one// instance of it in the history, and it is always in its own// unique task, so we do a special search.ActivityRecord taskTop = r.launchMode != ActivityInfo.LAUNCH_SINGLE_INSTANCE? findTaskLocked(intent, r.info): findActivityLocked(intent, r.info);if (taskTop != null) {......}} } 这段代码的逻辑是查看一下，当前有没有Task可以用来执行这个Activity。由于r.launchMode的值不为ActivityInfo.LAUNCH_SINGLE_INSTANCE，因此，它通过findTaskLocked函数来查找存不存这样的Task，这里返回的结果是null，即taskTop为null，因此，需要创建一个新的Task来启动这个Activity。

接着往下看：

if (r.packageName != null) {// If the activity being launched is the same as the one currently// at the top, then we need to check if it should only be launched// once.ActivityRecord top = topRunningNonDelayedActivityLocked(notTop);if (top != null && r.resultTo == null) {if (top.realActivity.equals(r.realActivity)) {......}} } 这段代码的逻辑是看一下，当前在堆栈顶端的Activity是否就是即将要启动的Activity，有些情况下，如果即将要启动的Activity就在堆栈的顶端，那么，就不会重新启动这个Activity的别一个实例了，具体可以参考官方网站 http://developer.android.com/reference/android/content/pm/ActivityInfo.html。现在处理堆栈顶端的Activity是Launcher，与我们即将要启动的MainActivity不是同一个Activity，因此，这里不用进一步处理上述介绍的情况。

执行到这里，我们知道，要在一个新的Task里面来启动这个Activity了，于是新创建一个Task：

if (r.resultTo == null && !addingToTask&& (launchFlags&Intent.FLAG_ACTIVITY_NEW_TASK) != 0) {// todo: should do better management of integers.mService.mCurTask++;if (mService.mCurTask <= 0) {mService.mCurTask = 1;}r.task = new TaskRecord(mService.mCurTask, r.info, intent,(r.info.flags&ActivityInfo.FLAG_CLEAR_TASK_ON_LAUNCH) != 0);......newTask = true;if (mMainStack) {mService.addRecentTaskLocked(r.task);} } 新建的Task保存在r.task域中，同时，添加到mService中去，这里的mService就是ActivityManagerService了。

最后就进入startActivityLocked(r, newTask, doResume)进一步处理了。这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {......private final void startActivityLocked(ActivityRecord r, boolean newTask,boolean doResume) {final int NH = mHistory.size();int addPos = -1;if (!newTask) {......}// Place a new activity at top of stack, so it is next to interact// with the user.if (addPos < 0) {addPos = NH;}// If we are not placing the new activity frontmost, we do not want// to deliver the onUserLeaving callback to the actual frontmost// activityif (addPos < NH) {......}// Slot the activity into the history stack and proceedmHistory.add(addPos, r);r.inHistory = true;r.frontOfTask = newTask;r.task.numActivities++;if (NH > 0) {// We want to show the starting preview window if we are// switching to a new task, or the next activity's process is// not currently running.......} else {// If this is the first activity, don't do any fancy animations,// because there is nothing for it to animate on top of.......}......if (doResume) {resumeTopActivityLocked(null);}}......} 这里的NH表示当前系统中历史任务的个数，这里肯定是大于0，因为Launcher已经跑起来了。当NH>0时，并且现在要切换新任务时，要做一些任务切的界面操作，这段代码我们就不看了，这里不会影响到下面启Activity的过程，有兴趣的读取可以自己研究一下。

这里传进来的参数doResume为true，于是调用resumeTopActivityLocked进一步操作。

Step 10. Activity.resumeTopActivityLocked

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {....../*** Ensure that the top activity in the stack is resumed.** @param prev The previously resumed activity, for when in the process* of pausing; can be null to call from elsewhere.** @return Returns true if something is being resumed, or false if* nothing happened.*/final boolean resumeTopActivityLocked(ActivityRecord prev) {// Find the first activity that is not finishing.ActivityRecord next = topRunningActivityLocked(null);// Remember how we'll process this pause/resume situation, and ensure// that the state is reset however we wind up proceeding.final boolean userLeaving = mUserLeaving;mUserLeaving = false;if (next == null) {......}next.delayedResume = false;// If the top activity is the resumed one, nothing to do.if (mResumedActivity == next && next.state == ActivityState.RESUMED) {......}// If we are sleeping, and there is no resumed activity, and the top// activity is paused, well that is the state we want.if ((mService.mSleeping || mService.mShuttingDown)&& mLastPausedActivity == next && next.state == ActivityState.PAUSED) {......}......// If we are currently pausing an activity, then don't do anything// until that is done.if (mPausingActivity != null) {......}......// We need to start pausing the current activity so the top one// can be resumed...if (mResumedActivity != null) {......startPausingLocked(userLeaving, false);return true;}......}......} 函数先通过调用topRunningActivityLocked函数获得堆栈顶端的Activity，这里就是MainActivity了，这是在上面的Step 9设置好的，保存在next变量中。

接下来把mUserLeaving的保存在本地变量userLeaving中，然后重新设置为false，在上面的Step 9中，mUserLeaving的值为true，因此，这里的userLeaving为true。

这里的mResumedActivity为Launcher，因为Launcher是当前正被执行的Activity。

当我们处理休眠状态时，mLastPausedActivity保存堆栈顶端的Activity，因为当前不是休眠状态，所以mLastPausedActivity为null。

有了这些信息之后，下面的语句就容易理解了：

// If the top activity is the resumed one, nothing to do. if (mResumedActivity == next && next.state == ActivityState.RESUMED) {...... } // If we are sleeping, and there is no resumed activity, and the top // activity is paused, well that is the state we want. if ((mService.mSleeping || mService.mShuttingDown)&& mLastPausedActivity == next && next.state == ActivityState.PAUSED) {...... } 它首先看要启动的Activity是否就是当前处理Resumed状态的Activity，如果是的话，那就什么都不用做，直接返回就可以了；否则再看一下系统当前是否休眠状态，如果是的话，再看看要启动的Activity是否就是当前处于堆栈顶端的Activity，如果是的话，也是什么都不用做。

上面两个条件都不满足，因此，在继续往下执行之前，首先要把当处于Resumed状态的Activity推入Paused状态，然后才可以启动新的Activity。但是在将当前这个Resumed状态的Activity推入Paused状态之前，首先要看一下当前是否有Activity正在进入Pausing状态，如果有的话，当前这个Resumed状态的Activity就要稍后才能进入Paused状态了，这样就保证了所有需要进入Paused状态的Activity串行处理。

这里没有处于Pausing状态的Activity，即mPausingActivity为null，而且mResumedActivity也不为null，于是就调用startPausingLocked函数把Launcher推入Paused状态去了。

Step 11. ActivityStack.startPausingLocked

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {......private final void startPausingLocked(boolean userLeaving, boolean uiSleeping) {if (mPausingActivity != null) {......}ActivityRecord prev = mResumedActivity;if (prev == null) {......}......mResumedActivity = null;mPausingActivity = prev;mLastPausedActivity = prev;prev.state = ActivityState.PAUSING;......if (prev.app != null && prev.app.thread != null) {......try {......prev.app.thread.schedulePauseActivity(prev, prev.finishing, userLeaving,prev.configChangeFlags);......} catch (Exception e) {......}} else {......}......}......}

函数首先把mResumedActivity保存在本地变量prev中。在上一步Step 10中，说到mResumedActivity就是Launcher，因此，这里把Launcher进程中的ApplicationThread对象取出来，通过它来通知Launcher这个Activity它要进入Paused状态了。当然，这里的prev.app.thread是一个ApplicationThread对象的远程接口，通过调用这个远程接口的schedulePauseActivity来通知Launcher进入Paused状态。

参数prev.finishing表示prev所代表的Activity是否正在等待结束的Activity列表中，由于Laucher这个Activity还没结束，所以这里为false；参数prev.configChangeFlags表示哪些config发生了变化，这里我们不关心它的值。

Step 12.ApplicationThreadProxy.schedulePauseActivity

这个函数定义在frameworks/base/core/java/android/app/ApplicationThreadNative.java文件中：

class ApplicationThreadProxy implements IApplicationThread {......public final void schedulePauseActivity(IBinder token, boolean finished,boolean userLeaving, int configChanges) throws RemoteException {Parcel data = Parcel.obtain();data.writeInterfaceToken(IApplicationThread.descriptor);data.writeStrongBinder(token);data.writeInt(finished ? 1 : 0);data.writeInt(userLeaving ? 1 :0);data.writeInt(configChanges);mRemote.transact(SCHEDULE_PAUSE_ACTIVITY_TRANSACTION, data, null,IBinder.FLAG_ONEWAY);data.recycle();}......}

这个函数通过Binder进程间通信机制进入到ApplicationThread.schedulePauseActivity函数中。

Step 13. ApplicationThread.schedulePauseActivity

这个函数定义在frameworks/base/core/java/android/app/ActivityThread.java文件中，它是ActivityThread的内部类：

public final class ActivityThread {......private final class ApplicationThread extends ApplicationThreadNative {......public final void schedulePauseActivity(IBinder token, boolean finished,boolean userLeaving, int configChanges) {queueOrSendMessage(finished ? H.PAUSE_ACTIVITY_FINISHING : H.PAUSE_ACTIVITY,token,(userLeaving ? 1 : 0),configChanges);}......}......} 这里调用的函数queueOrSendMessage是ActivityThread类的成员函数。

上面说到，这里的finished值为false，因此，queueOrSendMessage的第一个参数值为H.PAUSE_ACTIVITY，表示要暂停token所代表的Activity，即Launcher。

Step 14. ActivityThread.queueOrSendMessage

这个函数定义在frameworks/base/core/java/android/app/ActivityThread.java文件中：

public final class ActivityThread {......private final void queueOrSendMessage(int what, Object obj, int arg1) {queueOrSendMessage(what, obj, arg1, 0);}private final void queueOrSendMessage(int what, Object obj, int arg1, int arg2) {synchronized (this) {......Message msg = Message.obtain();msg.what = what;msg.obj = obj;msg.arg1 = arg1;msg.arg2 = arg2;mH.sendMessage(msg);}}......} 这里首先将相关信息组装成一个msg，然后通过mH成员变量发送出去，mH的类型是H，继承于Handler类，是ActivityThread的内部类，因此，这个消息最后由H.handleMessage来处理。

Step 15. H.handleMessage

这个函数定义在frameworks/base/core/java/android/app/ActivityThread.java文件中：

public final class ActivityThread {......private final class H extends Handler {......public void handleMessage(Message msg) {......switch (msg.what) {......case PAUSE_ACTIVITY:handlePauseActivity((IBinder)msg.obj, false, msg.arg1 != 0, msg.arg2);maybeSnapshot();break;......}......}......}

这里调用ActivityThread.handlePauseActivity进一步操作，msg.obj是一个ActivityRecord对象的引用，它代表的是Launcher这个Activity。
Step 16.ActivityThread.handlePauseActivity

这个函数定义在frameworks/base/core/java/android/app/ActivityThread.java文件中：

public final class ActivityThread {......private final void handlePauseActivity(IBinder token, boolean finished,boolean userLeaving, int configChanges) {ActivityClientRecord r = mActivities.get(token);if (r != null) {//Slog.v(TAG, "userLeaving=" + userLeaving + " handling pause of " + r);if (userLeaving) {performUserLeavingActivity(r);}r.activity.mConfigChangeFlags |= configChanges;Bundle state = performPauseActivity(token, finished, true);// Make sure any pending writes are now committed.QueuedWork.waitToFinish();// Tell the activity manager we have paused.try {ActivityManagerNative.getDefault().activityPaused(token, state);} catch (RemoteException ex) {}}}......} 函数首先将Binder引用token转换成ActivityRecord的远程接口ActivityClientRecord，然后做了三个事情：1. 如果userLeaving为true，则通过调用performUserLeavingActivity函数来调用Activity.onUserLeaveHint通知Activity，用户要离开它了；2. 调用performPauseActivity函数来调用Activity.onPause函数，我们知道，在Activity的生命周期中，当它要让位于其它的Activity时，系统就会调用它的onPause函数；3. 它通知ActivityManagerService，这个Activity已经进入Paused状态了，ActivityManagerService现在可以完成未竟的事情，即启动MainActivity了。

Step 17. ActivityManagerProxy.activityPaused

这个函数定义在frameworks/base/core/java/android/app/ActivityManagerNative.java文件中：

class ActivityManagerProxy implements IActivityManager{......public void activityPaused(IBinder token, Bundle state) throws RemoteException{Parcel data = Parcel.obtain();Parcel reply = Parcel.obtain();data.writeInterfaceToken(IActivityManager.descriptor);data.writeStrongBinder(token);data.writeBundle(state);mRemote.transact(ACTIVITY_PAUSED_TRANSACTION, data, reply, 0);reply.readException();data.recycle();reply.recycle();}......} 这里通过Binder进程间通信机制就进入到ActivityManagerService.activityPaused函数中去了。

Step 18.ActivityManagerService.activityPaused

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityManagerService.java文件中：

public final class ActivityManagerService extends ActivityManagerNativeimplements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {......public final void activityPaused(IBinder token, Bundle icicle) {......final long origId = Binder.clearCallingIdentity();mMainStack.activityPaused(token, icicle, false);......}......} 这里，又再次进入到ActivityStack类中，执行activityPaused函数。

Step 19.ActivityStack.activityPaused

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {......final void activityPaused(IBinder token, Bundle icicle, boolean timeout) {......ActivityRecord r = null;synchronized (mService) {int index = indexOfTokenLocked(token);if (index >= 0) {r = (ActivityRecord)mHistory.get(index);if (!timeout) {r.icicle = icicle;r.haveState = true;}mHandler.removeMessages(PAUSE_TIMEOUT_MSG, r);if (mPausingActivity == r) {r.state = ActivityState.PAUSED;completePauseLocked();} else {......}}}}......} 这里通过参数token在mHistory列表中得到ActivityRecord，从上面我们知道，这个ActivityRecord代表的是Launcher这个Activity，而我们在Step 11中，把Launcher这个Activity的信息保存在mPausingActivity中，因此，这里mPausingActivity等于r，于是，执行completePauseLocked操作。

Step 20.ActivityStack.completePauseLocked

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {......private final void completePauseLocked() {ActivityRecord prev = mPausingActivity;......if (prev != null) {......mPausingActivity = null;}if (!mService.mSleeping && !mService.mShuttingDown) {resumeTopActivityLocked(prev);} else {......}......}......} 函数首先把mPausingActivity变量清空，因为现在不需要它了，然后调用resumeTopActivityLokced进一步操作，它传入的参数即为代表Launcher这个Activity的ActivityRecord。

Step 21.ActivityStack.resumeTopActivityLokced
这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {......final boolean resumeTopActivityLocked(ActivityRecord prev) {......// Find the first activity that is not finishing.ActivityRecord next = topRunningActivityLocked(null);// Remember how we'll process this pause/resume situation, and ensure// that the state is reset however we wind up proceeding.final boolean userLeaving = mUserLeaving;mUserLeaving = false;......next.delayedResume = false;// If the top activity is the resumed one, nothing to do.if (mResumedActivity == next && next.state == ActivityState.RESUMED) {......return false;}// If we are sleeping, and there is no resumed activity, and the top// activity is paused, well that is the state we want.if ((mService.mSleeping || mService.mShuttingDown)&& mLastPausedActivity == next && next.state == ActivityState.PAUSED) {......return false;}.......// We need to start pausing the current activity so the top one// can be resumed...if (mResumedActivity != null) {......return true;}......if (next.app != null && next.app.thread != null) {......} else {......startSpecificActivityLocked(next, true, true);}return true;}......} 通过上面的Step 9，我们知道，当前在堆栈顶端的Activity为我们即将要启动的MainActivity，这里通过调用topRunningActivityLocked将它取回来，保存在next变量中。之前最后一个Resumed状态的Activity，即Launcher，到了这里已经处于Paused状态了，因此，mResumedActivity为null。最后一个处于Paused状态的Activity为Launcher，因此，这里的mLastPausedActivity就为Launcher。前面我们为MainActivity创建了ActivityRecord后，它的app域一直保持为null。有了这些信息后，上面这段代码就容易理解了，它最终调用startSpecificActivityLocked进行下一步操作。

Step 22. ActivityStack.startSpecificActivityLocked
这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {......private final void startSpecificActivityLocked(ActivityRecord r,boolean andResume, boolean checkConfig) {// Is this activity's application already running?ProcessRecord app = mService.getProcessRecordLocked(r.processName,r.info.applicationInfo.uid);......if (app != null && app.thread != null) {try {realStartActivityLocked(r, app, andResume, checkConfig);return;} catch (RemoteException e) {......}}mService.startProcessLocked(r.processName, r.info.applicationInfo, true, 0,"activity", r.intent.getComponent(), false);}......} 注意，这里由于是第一次启动应用程序的Activity，所以下面语句：

ProcessRecord app = mService.getProcessRecordLocked(r.processName,r.info.applicationInfo.uid); 取回来的app为null。在Activity应用程序中的AndroidManifest.xml配置文件中，我们没有指定Application标签的process属性，系统就会默认使用package的名称，这里就是"shy.luo.activity"了。每一个应用程序都有自己的uid，因此，这里uid + process的组合就可以为每一个应用程序创建一个ProcessRecord。当然，我们可以配置两个应用程序具有相同的uid和package，或者在AndroidManifest.xml配置文件的application标签或者activity标签中显式指定相同的process属性值，这样，不同的应用程序也可以在同一个进程中启动。

函数最终执行ActivityManagerService.startProcessLocked函数进行下一步操作。

Step 23.ActivityManagerService.startProcessLocked

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityManagerService.java文件中：

public final class ActivityManagerService extends ActivityManagerNativeimplements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {......final ProcessRecord startProcessLocked(String processName,ApplicationInfo info, boolean knownToBeDead, int intentFlags,String hostingType, ComponentName hostingName, boolean allowWhileBooting) {ProcessRecord app = getProcessRecordLocked(processName, info.uid);......String hostingNameStr = hostingName != null? hostingName.flattenToShortString() : null;......if (app == null) {app = new ProcessRecordLocked(null, info, processName);mProcessNames.put(processName, info.uid, app);} else {// If this is a new package in the process, add the package to the listapp.addPackage(info.packageName);}......startProcessLocked(app, hostingType, hostingNameStr);return (app.pid != 0) ? app : null;}......} 这里再次检查是否已经有以process + uid命名的进程存在，在我们这个情景中，返回值app为null，因此，后面会创建一个ProcessRecord，并存保存在成员变量mProcessNames中，最后，调用另一个startProcessLocked函数进一步操作：

public final class ActivityManagerService extends ActivityManagerNativeimplements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {......private final void startProcessLocked(ProcessRecord app,String hostingType, String hostingNameStr) {......try {int uid = app.info.uid;int[] gids = null;try {gids = mContext.getPackageManager().getPackageGids(app.info.packageName);} catch (PackageManager.NameNotFoundException e) {......}......int debugFlags = 0;......int pid = Process.start("android.app.ActivityThread",mSimpleProcessManagement ? app.processName : null, uid, uid,gids, debugFlags, null);......} catch (RuntimeException e) {......}}......} 这里主要是调用Process.start接口来创建一个新的进程，新的进程会导入android.app.ActivityThread类，并且执行它的main函数，这就是为什么我们前面说每一个应用程序都有一个ActivityThread实例来对应的原因。

Step 24. ActivityThread.main

这个函数定义在frameworks/base/core/java/android/app/ActivityThread.java文件中：

public final class ActivityThread {......private final void attach(boolean system) {......mSystemThread = system;if (!system) {......IActivityManager mgr = ActivityManagerNative.getDefault();try {mgr.attachApplication(mAppThread);} catch (RemoteException ex) {}} else {......}}......public static final void main(String[] args) {.......ActivityThread thread = new ActivityThread();thread.attach(false);......Looper.loop();.......thread.detach();......}} 这个函数在进程中创建一个ActivityThread实例，然后调用它的attach函数，接着就进入消息循环了，直到最后进程退出。

函数attach最终调用了ActivityManagerService的远程接口ActivityManagerProxy的attachApplication函数，传入的参数是mAppThread，这是一个ApplicationThread类型的Binder对象，它的作用是用来进行进程间通信的。

Step 25.ActivityManagerProxy.attachApplication

这个函数定义在frameworks/base/core/java/android/app/ActivityManagerNative.java文件中：

class ActivityManagerProxy implements IActivityManager{......public void attachApplication(IApplicationThread app) throws RemoteException{Parcel data = Parcel.obtain();Parcel reply = Parcel.obtain();data.writeInterfaceToken(IActivityManager.descriptor);data.writeStrongBinder(app.asBinder());mRemote.transact(ATTACH_APPLICATION_TRANSACTION, data, reply, 0);reply.readException();data.recycle();reply.recycle();}......} 这里通过Binder驱动程序，最后进入ActivityManagerService的attachApplication函数中。

Step 26.ActivityManagerService.attachApplication

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityManagerService.java文件中：

public final class ActivityManagerService extends ActivityManagerNativeimplements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {......public final void attachApplication(IApplicationThread thread) {synchronized (this) {int callingPid = Binder.getCallingPid();final long origId = Binder.clearCallingIdentity();attachApplicationLocked(thread, callingPid);Binder.restoreCallingIdentity(origId);}}......} 这里将操作转发给attachApplicationLocked函数。

Step 27.ActivityManagerService.attachApplicationLocked

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityManagerService.java文件中：

public final class ActivityManagerService extends ActivityManagerNativeimplements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {......private final boolean attachApplicationLocked(IApplicationThread thread,int pid) {// Find the application record that is being attached... either via// the pid if we are running in multiple processes, or just pull the// next app record if we are emulating process with anonymous threads.ProcessRecord app;if (pid != MY_PID && pid >= 0) {synchronized (mPidsSelfLocked) {app = mPidsSelfLocked.get(pid);}} else if (mStartingProcesses.size() > 0) {......} else {......}if (app == null) {......return false;}......String processName = app.processName;try {thread.asBinder().linkToDeath(new AppDeathRecipient(app, pid, thread), 0);} catch (RemoteException e) {......return false;}......app.thread = thread;app.curAdj = app.setAdj = -100;app.curSchedGroup = Process.THREAD_GROUP_DEFAULT;app.setSchedGroup = Process.THREAD_GROUP_BG_NONINTERACTIVE;app.forcingToForeground = null;app.foregroundServices = false;app.debugging = false;......boolean normalMode = mProcessesReady || isAllowedWhileBooting(app.info);......boolean badApp = false;boolean didSomething = false;// See if the top visible activity is waiting to run in this process...ActivityRecord hr = mMainStack.topRunningActivityLocked(null);if (hr != null && normalMode) {if (hr.app == null && app.info.uid == hr.info.applicationInfo.uid&& processName.equals(hr.processName)) {try {if (mMainStack.realStartActivityLocked(hr, app, true, true)) {didSomething = true;}} catch (Exception e) {......}} else {......}}......return true;}......}

在前面的Step 23中，已经创建了一个ProcessRecord，这里首先通过pid将它取回来，放在app变量中，然后对app的其它成员进行初始化，最后调用mMainStack.realStartActivityLocked执行真正的Activity启动操作。这里要启动的Activity通过调用mMainStack.topRunningActivityLocked(null)从堆栈顶端取回来，这时候在堆栈顶端的Activity就是MainActivity了。

Step 28. ActivityStack.realStartActivityLocked

这个函数定义在frameworks/base/services/java/com/android/server/am/ActivityStack.java文件中：

public class ActivityStack {......final boolean realStartActivityLocked(ActivityRecord r,ProcessRecord app, boolean andResume, boolean checkConfig)throws RemoteException {......r.app = app;......int idx = app.activities.indexOf(r);if (idx < 0) {app.activities.add(r);}......try {......List<ResultInfo> results = null;List<Intent> newIntents = null;if (andResume) {results = r.results;newIntents = r.newIntents;}......app.thread.scheduleLaunchActivity(new Intent(r.intent), r,System.identityHashCode(r),r.info, r.icicle, results, newIntents, !andResume,mService.isNextTransitionForward());......} catch (RemoteException e) {......}......return true;}......} 这里最终通过app.thread进入到ApplicationThreadProxy的scheduleLaunchActivity函数中，注意，这里的第二个参数r，是一个ActivityRecord类型的Binder对象，用来作来这个Activity的token值。

Step 29.ApplicationThreadProxy.scheduleLaunchActivity
这个函数定义在frameworks/base/core/java/android/app/ApplicationThreadNative.java文件中：

class ApplicationThreadProxy implements IApplicationThread {......public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,ActivityInfo info, Bundle state, List<ResultInfo> pendingResults,List<Intent> pendingNewIntents, boolean notResumed, boolean isForward)throws RemoteException {Parcel data = Parcel.obtain();data.writeInterfaceToken(IApplicationThread.descriptor);intent.writeToParcel(data, 0);data.writeStrongBinder(token);data.writeInt(ident);info.writeToParcel(data, 0);data.writeBundle(state);data.writeTypedList(pendingResults);data.writeTypedList(pendingNewIntents);data.writeInt(notResumed ? 1 : 0);data.writeInt(isForward ? 1 : 0);mRemote.transact(SCHEDULE_LAUNCH_ACTIVITY_TRANSACTION, data, null,IBinder.FLAG_ONEWAY);data.recycle();}......} 这个函数最终通过Binder驱动程序进入到ApplicationThread的scheduleLaunchActivity函数中。

Step 30.ApplicationThread.scheduleLaunchActivity
这个函数定义在frameworks/base/core/java/android/app/ActivityThread.java文件中：

public final class ActivityThread {......private final class ApplicationThread extends ApplicationThreadNative {......// we use token to identify this activity without having to send the// activity itself back to the activity manager. (matters more with ipc)public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,ActivityInfo info, Bundle state, List<ResultInfo> pendingResults,List<Intent> pendingNewIntents, boolean notResumed, boolean isForward) {ActivityClientRecord r = new ActivityClientRecord();r.token = token;r.ident = ident;r.intent = intent;r.activityInfo = info;r.state = state;r.pendingResults = pendingResults;r.pendingIntents = pendingNewIntents;r.startsNotResumed = notResumed;r.isForward = isForward;queueOrSendMessage(H.LAUNCH_ACTIVITY, r);}......}......} 函数首先创建一个ActivityClientRecord实例，并且初始化它的成员变量，然后调用ActivityThread类的queueOrSendMessage函数进一步处理。

Step 31.ActivityThread.queueOrSendMessage
这个函数定义在frameworks/base/core/java/android/app/ActivityThread.java文件中：

public final class ActivityThread {......private final class ApplicationThread extends ApplicationThreadNative {......// if the thread hasn't started yet, we don't have the handler, so just// save the messages until we're ready.private final void queueOrSendMessage(int what, Object obj) {queueOrSendMessage(what, obj, 0, 0);}......private final void queueOrSendMessage(int what, Object obj, int arg1, int arg2) {synchronized (this) {......Message msg = Message.obtain();msg.what = what;msg.obj = obj;msg.arg1 = arg1;msg.arg2 = arg2;mH.sendMessage(msg);}}......}......} 函数把消息内容放在msg中，然后通过mH把消息分发出去，这里的成员变量mH我们在前面已经见过，消息分发出去后，最后会调用H类的handleMessage函数。

Step 32. H.handleMessage

这个函数定义在frameworks/base/core/java/android/app/ActivityThread.java文件中：

public final class ActivityThread {......private final class H extends Handler {......public void handleMessage(Message msg) {......switch (msg.what) {case LAUNCH_ACTIVITY: {ActivityClientRecord r = (ActivityClientRecord)msg.obj;r.packageInfo = getPackageInfoNoCheck(r.activityInfo.applicationInfo);handleLaunchActivity(r, null);} break;......}......}......} 这里最后调用ActivityThread类的handleLaunchActivity函数进一步处理。

Step 33.ActivityThread.handleLaunchActivity

这个函数定义在frameworks/base/core/java/android/app/ActivityThread.java文件中：

public final class ActivityThread {......private final void handleLaunchActivity(ActivityClientRecord r, Intent customIntent) {......Activity a = performLaunchActivity(r, customIntent);if (a != null) {r.createdConfig = new Configuration(mConfiguration);Bundle oldState = r.state;handleResumeActivity(r.token, false, r.isForward);......} else {......}}......} 这里首先调用performLaunchActivity函数来加载这个Activity类，即shy.luo.activity.MainActivity，然后调用它的onCreate函数，最后回到handleLaunchActivity函数时，再调用handleResumeActivity函数来使这个Activity进入Resumed状态，即会调用这个Activity的onResume函数，这是遵循Activity的生命周期的。

Step 34.ActivityThread.performLaunchActivity
这个函数定义在frameworks/base/core/java/android/app/ActivityThread.java文件中：

public final class ActivityThread {......private final Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {ActivityInfo aInfo = r.activityInfo;if (r.packageInfo == null) {r.packageInfo = getPackageInfo(aInfo.applicationInfo,Context.CONTEXT_INCLUDE_CODE);}ComponentName component = r.intent.getComponent();if (component == null) {component = r.intent.resolveActivity(mInitialApplication.getPackageManager());r.intent.setComponent(component);}if (r.activityInfo.targetActivity != null) {component = new ComponentName(r.activityInfo.packageName,r.activityInfo.targetActivity);}Activity activity = null;try {java.lang.ClassLoader cl = r.packageInfo.getClassLoader();activity = mInstrumentation.newActivity(cl, component.getClassName(), r.intent);r.intent.setExtrasClassLoader(cl);if (r.state != null) {r.state.setClassLoader(cl);}} catch (Exception e) {......}try {Application app = r.packageInfo.makeApplication(false, mInstrumentation);......if (activity != null) {ContextImpl appContext = new ContextImpl();appContext.init(r.packageInfo, r.token, this);appContext.setOuterContext(activity);CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());Configuration config = new Configuration(mConfiguration);......activity.attach(appContext, this, getInstrumentation(), r.token,r.ident, app, r.intent, r.activityInfo, title, r.parent,r.embeddedID, r.lastNonConfigurationInstance,r.lastNonConfigurationChildInstances, config);if (customIntent != null) {activity.mIntent = customIntent;}r.lastNonConfigurationInstance = null;r.lastNonConfigurationChildInstances = null;activity.mStartedActivity = false;int theme = r.activityInfo.getThemeResource();if (theme != 0) {activity.setTheme(theme);}activity.mCalled = false;mInstrumentation.callActivityOnCreate(activity, r.state);......r.activity = activity;r.stopped = true;if (!r.activity.mFinished) {activity.performStart();r.stopped = false;}if (!r.activity.mFinished) {if (r.state != null) {mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state);}}if (!r.activity.mFinished) {activity.mCalled = false;mInstrumentation.callActivityOnPostCreate(activity, r.state);if (!activity.mCalled) {throw new SuperNotCalledException("Activity " + r.intent.getComponent().toShortString() +" did not call through to super.onPostCreate()");}}}r.paused = true;mActivities.put(r.token, r);} catch (SuperNotCalledException e) {......} catch (Exception e) {......}return activity;}......}

函数前面是收集要启动的Activity的相关信息，主要package和component信息：

ActivityInfo aInfo = r.activityInfo; if (r.packageInfo == null) { r.packageInfo = getPackageInfo(aInfo.applicationInfo, Context.CONTEXT_INCLUDE_CODE); } ComponentName component = r.intent.getComponent(); if (component == null) { component = r.intent.resolveActivity( mInitialApplication.getPackageManager()); r.intent.setComponent(component); } if (r.activityInfo.targetActivity != null) { component = new ComponentName(r.activityInfo.packageName, r.activityInfo.targetActivity); } 然后通过ClassLoader将shy.luo.activity.MainActivity类加载进来：

Activity activity = null; try {java.lang.ClassLoader cl = r.packageInfo.getClassLoader();activity = mInstrumentation.newActivity(cl, component.getClassName(), r.intent);r.intent.setExtrasClassLoader(cl);if (r.state != null) {r.state.setClassLoader(cl);} } catch (Exception e) {...... } 接下来是创建Application对象，这是根据AndroidManifest.xml配置文件中的Application标签的信息来创建的：

Application app = r.packageInfo.makeApplication(false, mInstrumentation); 后面的代码主要创建Activity的上下文信息，并通过attach方法将这些上下文信息设置到MainActivity中去：

activity.attach(appContext, this, getInstrumentation(), r.token,r.ident, app, r.intent, r.activityInfo, title, r.parent,r.embeddedID, r.lastNonConfigurationInstance,r.lastNonConfigurationChildInstances, config); 最后还要调用MainActivity的onCreate函数：

mInstrumentation.callActivityOnCreate(activity, r.state); 这里不是直接调用MainActivity的onCreate函数，而是通过mInstrumentation的callActivityOnCreate函数来间接调用，前面我们说过，mInstrumentation在这里的作用是监控Activity与系统的交互操作，相当于是系统运行日志。

Step 35. MainActivity.onCreate

这个函数定义在packages/experimental/Activity/src/shy/luo/activity/MainActivity.java文件中，这是我们自定义的app工程文件：

public class MainActivity extends Activity implements OnClickListener {......@Overridepublic void onCreate(Bundle savedInstanceState) {......Log.i(LOG_TAG, "Main Activity Created.");}......} 这样，MainActivity就启动起来了，整个应用程序也启动起来了。

整个应用程序的启动过程要执行很多步骤，但是整体来看，主要分为以下五个阶段：

一. Step1 - Step 11：Launcher通过Binder进程间通信机制通知ActivityManagerService，它要启动一个Activity；

二. Step 12 - Step 16：ActivityManagerService通过Binder进程间通信机制通知Launcher进入Paused状态；

三. Step 17 - Step 24：Launcher通过Binder进程间通信机制通知ActivityManagerService，它已经准备就绪进入Paused状态，于是ActivityManagerService就创建一个新的进程，用来启动一个ActivityThread实例，即将要启动的Activity就是在这个ActivityThread实例中运行；

四. Step 25 - Step 27：ActivityThread通过Binder进程间通信机制将一个ApplicationThread类型的Binder对象传递给ActivityManagerService，以便以后ActivityManagerService能够通过这个Binder对象和它进行通信；

五. Step 28 - Step 35：ActivityManagerService通过Binder进程间通信机制通知ActivityThread，现在一切准备就绪，它可以真正执行Activity的启动操作了。

这里不少地方涉及到了Binder进程间通信机制，相关资料请参考Android进程间通信（IPC）机制Binder简要介绍和学习计划一文。

这样，应用程序的启动过程就介绍完了，它实质上是启动应用程序的默认Activity，在下一篇文章中，我们将介绍在应用程序内部启动另一个Activity的过程，即新的Activity与启动它的Activity将会在同一个进程（Process）和任务（Task）运行，敬请关注。

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