Android應用程序啟動過程源代碼分析 · 老羅的Android之旅（總結）

原文出處——>[Android應用程序啟動過程源代碼分析](http://blog.csdn.net/luoshengyang/article/details/6689748) 前文簡要介紹了Android應用程序的Activity的啟動過程。在Android系統中，應用程序是由Activity組成的，因此，應用程序的啟動過程實際上就是應用程序中的默認Activity的啟動過程，本文將詳細分析應用程序框架層的源代碼，了解Android應用程序的啟動過程。在上一篇文章Android應用程序的Activity啟動過程簡要介紹和學習計劃中，我們舉例子說明了啟動Android應用程序中的Activity的兩種情景，其中，在手機屏幕中點擊應用程序圖標的情景就會引發Android應用程序中的默認Activity的啟動，從而把應用程序啟動起來。這種啟動方式的特點是會啟動一個新的進程來加載相應的Activity。這里，我們繼續以這個例子為例來說明Android應用程序的啟動過程，即MainActivity的啟動過程。 MainActivity的啟動過程如下圖所示： :-: ![](https://box.kancloud.cn/6aea0b5e66f44aa8ea18270433123611_583x768.gif) 圖1 根Activity的啟動過程上面的圖太小了，可以看我在老羅的系統源代碼書中的插圖 [點擊看大圖](https://box.kancloud.cn/51d2ff0baa74f1871a56ff5355931a9c_1501x2356.jpg) :-: ![根Activity的完整啟動過程圖](https://box.kancloud.cn/51d2ff0baa74f1871a56ff5355931a9c_1501x2356.jpg) 圖2 根Activity的完整啟動過程圖下面詳細分析每一步是如何實現的。 #### **Step 1. Launcher.startActivitySafely** 在Android系統中，應用程序是由Launcher啟動起來的，其實，Launcher本身也是一個應用程序，其它的應用程序安裝后，就會Launcher的界面上出現一個相應的圖標，點擊這個圖標時，Launcher就會對應的應用程序啟動起來。 Launcher的源代碼工程在packages/apps/Launcher2目錄下，負責啟動其它應用程序的源代碼實現在src/com/android/launcher2/Launcher.java文件中： ~~~ /** * Default launcher application. */ public final class Launcher extends Activity implements 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 shortcut final 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) { ...... } } ...... } ~~~ 回憶一下前面一篇文章Android應用程序的Activity啟動過程簡要介紹和學習計劃說到的應用程序Activity，它的默認Activity是MainActivity，這里是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，這里的intent包含的信息為：action = "android.intent.action.Main"，category="android.intent.category.LAUNCHER", cmp="shy.luo.activity/.MainActivity"，表示它要啟動的Activity為shy.luo.activity.MainActivity。Intent.FLAG_ACTIVITY_NEW_TASK表示要在一個新的Task中啟動這個Activity，注意，Task是Android系統中的概念，它不同于進程Process的概念。簡單地說，一個Task是一系列Activity的集合，這個集合是以堆棧的形式來組織的，遵循后進先出的原則。事實上，Task是一個非常復雜的概念，有興趣的讀者可以到官網http://developer.android.com/guide/topics/manifest/activity-element.html 查看相關的資料。這里，我們只要知道，這個MainActivity要在一個新的Task中啟動就可以了。 **Step 2. Activity.startActivity** 在Step 1中，我們看到，Launcher繼承于Activity類，而Activity類實現了startActivity函數，因此，這里就調用了Activity.startActivity函數，它實現在frameworks/base/core/java/android/app/Activity.java文件中： ~~~ public class Activity extends ContextThemeWrapper implements LayoutInflater.Factory, Window.Callback, KeyEvent.Callback, OnCreateContextMenuListener, ComponentCallbacks { ...... @Override public void startActivity(Intent intent) { startActivityForResult(intent, -1); } ...... } ~~~ 這個函數實現很簡單，它調用startActivityForResult來進一步處理，第二個參數傳入-1表示不需要這個Actvity結束后的返回結果。 **Step 3. Activity.startActivityForResult** 這個函數也是實現在frameworks/base/core/java/android/app/Activity.java文件中： ~~~ public class Activity extends ContextThemeWrapper implements 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 { ...... } ...... } ~~~ 這里的mInstrumentation是Activity類的成員變量，它的類型是Intrumentation，定義在frameworks/base/core/java/android/app/Instrumentation.java文件中，它用來監控應用程序和系統的交互。這里的mMainThread也是Activity類的成員變量，它的類型是ActivityThread，它代表的是應用程序的主線程，我們在Android系統在新進程中啟動自定義服務過程（startService）的原理分析一文中已經介紹過了。這里通過mMainThread.getApplicationThread獲得它里面的ApplicationThread成員變量，它是一個Binder對象，后面我們會看到，ActivityManagerService會使用它來和ActivityThread來進行進程間通信。這里我們需注意的是，這里的mMainThread代表的是Launcher應用程序運行的進程。這里的mToken也是Activity類的成員變量，它是一個Binder對象的遠程接口。 **Step 4. Instrumentation.execStartActivity** 這個函數定義在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; } ...... } ~~~ 這里的ActivityManagerNative.getDefault返回ActivityManagerService的遠程接口，即ActivityManagerProxy接口，具體可以參考Android系統在新進程中啟動自定義服務過程（startService）的原理分析一文。這里的intent.resolveTypeIfNeeded返回這個intent的MIME類型，在這個例子中，沒有AndroidManifest.xml設置MainActivity的MIME類型，因此，這里返回null。這里的target不為null，但是target.mEmbddedID為null，我們不用關注。 **Step 5. ActivityManagerProxy.startActivity** 這個函數定義在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; } ...... } ~~~ 這里的參數比較多，我們先整理一下。從上面的調用可以知道，這里的參數resolvedType、grantedUriPermissions和resultWho均為null；參數caller為ApplicationThread類型的Binder實體；參數resultTo為一個Binder實體的遠程接口，我們先不關注它；參數grantedMode為0，我們也先不關注它；參數requestCode為-1；參數onlyIfNeeded和debug均空false。 **Step 6. ActivityManagerService.startActivity** 上一步Step 5通過Binder驅動程序就進入到ActivityManagerService的startActivity函數來了，它定義在frameworks/base/services/java/com/android/server/am/ActivityManagerService.java文件中： ~~~ public final class ActivityManagerService extends ActivityManagerNative implements 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); } ...... } ~~~ 這里只是簡單地將操作轉發給成員變量mMainStack的startActivityMayWait函數，這里的mMainStack的類型為ActivityStack。 **Step 7. ActivityStack.startActivityMayWait** 這個函數定義在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，從Step 6傳下來的參數outResult和config均為null，此外，表達式(aInfo.applicationInfo.flags&ApplicationInfo.FLAG_CANT_SAVE_STATE) != 0為false，因此，這里忽略了無關代碼。下面語句對參數intent的內容進行解析，得到MainActivity的相關信息，保存在aInfo變量中： ~~~ 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的值為"shy.luo.activity"，aInfo.name的值為"shy.luo.activity.MainActivity"，這是在這個實例的配置文件AndroidManifest.xml里面配置的。此外，函數開始的地方調用intent.getComponent()函數的返回值不為null，因此，這里的componentSpecified變量為true。接下去就調用startActivityLocked進一步處理了。 **Step 8. ActivityStack.startActivityLocked** 這個函數定義在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); } ...... } ~~~ 從傳進來的參數caller得到調用者的進程信息，并保存在callerApp變量中，這里就是Launcher應用程序的進程信息了。前面說過，參數resultTo是Launcher這個Activity里面的一個Binder對象，通過它可以獲得Launcher這個Activity的相關信息，保存在sourceRecord變量中。再接下來，創建即將要啟動的Activity的相關信息，并保存在r變量中： ~~~ ActivityRecord r = new ActivityRecord(mService, this, callerApp, callingUid, intent, resolvedType, aInfo, mService.mConfiguration, resultRecord, resultWho, requestCode, componentSpecified); ~~~ 接著調用startActivityUncheckedLocked函數進行下一步操作。 **Step 9. ActivityStack.startActivityUncheckedLocked** 這個函數定義在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; } ...... } ~~~ 函數首先獲得intent的標志值，保存在launchFlags變量中。這個intent的標志值的位Intent.FLAG_ACTIVITY_NO_USER_ACTION沒有置位，因此，成員變量mUserLeaving的值為true。這個intent的標志值的位Intent.FLAG_ACTIVITY_PREVIOUS_IS_TOP也沒有置位，因此，變量notTop的值為null。由于在這個例子的AndroidManifest.xml文件中，MainActivity沒有配置launchMode屬值，因此，這里的r.launchMode為默認值0，表示以標準（Standard，或者稱為ActivityInfo.LAUNCH_MULTIPLE）的方式來啟動這個Activity。Activity的啟動方式有四種，其余三種分別是ActivityInfo.LAUNCH_SINGLE_INSTANCE、ActivityInfo.LAUNCH_SINGLE_TASK和ActivityInfo.LAUNCH_SINGLE_TOP，具體可以參考官方網站http://developer.android.com/reference/android/content/pm/ActivityInfo.html。傳進來的參數r.resultTo為null，表示Launcher不需要等這個即將要啟動的MainActivity的執行結果。由于這個intent的標志值的位Intent.FLAG_ACTIVITY_NEW_TASK被置位，而且Intent.FLAG_ACTIVITY_MULTIPLE_TASK沒有置位，因此，下面的if語句會被執行： ~~~ 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) { ...... } } } ~~~ 這段代碼的邏輯是查看一下，當前有沒有Task可以用來執行這個Activity。由于r.launchMode的值不為ActivityInfo.LAUNCH_SINGLE_INSTANCE，因此，它通過findTaskLocked函數來查找存不存這樣的Task，這里返回的結果是null，即taskTop為null，因此，需要創建一個新的Task來啟動這個Activity。接著往下看： ~~~ 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)) { ...... } } } ~~~ 這段代碼的邏輯是看一下，當前在堆棧頂端的Activity是否就是即將要啟動的Activity，有些情況下，如果即將要啟動的Activity就在堆棧的頂端，那么，就不會重新啟動這個Activity的別一個實例了，具體可以參考官方網站http://developer.android.com/reference/android/content/pm/ActivityInfo.html。現在處理堆棧頂端的Activity是Launcher，與我們即將要啟動的MainActivity不是同一個Activity，因此，這里不用進一步處理上述介紹的情況。執行到這里，我們知道，要在一個新的Task里面來啟動這個Activity了，于是新創建一個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，同時，添加到mService中去，這里的mService就是ActivityManagerService了。最后就進入startActivityLocked(r, newTask, doResume)進一步處理了。這個函數定義在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 // activity if (addPos < NH) { ...... } // Slot the activity into the history stack and proceed mHistory.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); } } ...... } ~~~ 這里的NH表示當前系統中歷史任務的個數，這里肯定是大于0，因為Launcher已經跑起來了。當NH>0時，并且現在要切換新任務時，要做一些任務切的界面操作，這段代碼我們就不看了，這里不會影響到下面啟Activity的過程，有興趣的讀取可以自己研究一下。這里傳進來的參數doResume為true，于是調用resumeTopActivityLocked進一步操作。 **Step 10. Activity.resumeTopActivityLocked** 這個函數定義在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; } ...... } ...... } ~~~ 函數先通過調用topRunningActivityLocked函數獲得堆棧頂端的Activity，這里就是MainActivity了，這是在上面的Step 9設置好的，保存在next變量中。接下來把mUserLeaving的保存在本地變量userLeaving中，然后重新設置為false，在上面的Step 9中，mUserLeaving的值為true，因此，這里的userLeaving為true。這里的mResumedActivity為Launcher，因為Launcher是當前正被執行的Activity。當我們處理休眠狀態時，mLastPausedActivity保存堆棧頂端的Activity，因為當前不是休眠狀態，所以mLastPausedActivity為null。有了這些信息之后，下面的語句就容易理解了： ~~~ // 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) { ...... } ~~~ 它首先看要啟動的Activity是否就是當前處理Resumed狀態的Activity，如果是的話，那就什么都不用做，直接返回就可以了；否則再看一下系統當前是否休眠狀態，如果是的話，再看看要啟動的Activity是否就是當前處于堆棧頂端的Activity，如果是的話，也是什么都不用做。上面兩個條件都不滿足，因此，在繼續往下執行之前，首先要把當處于Resumed狀態的Activity推入Paused狀態，然后才可以啟動新的Activity。但是在將當前這個Resumed狀態的Activity推入Paused狀態之前，首先要看一下當前是否有Activity正在進入Pausing狀態，如果有的話，當前這個Resumed狀態的Activity就要稍后才能進入Paused狀態了，這樣就保證了所有需要進入Paused狀態的Activity串行處理。這里沒有處于Pausing狀態的Activity，即mPausingActivity為null，而且mResumedActivity也不為null，于是就調用startPausingLocked函數把Launcher推入Paused狀態去了。 **Step 11. ActivityStack.startPausingLocked** 這個函數定義在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 { ...... } ...... } ...... } ~~~ 函數首先把mResumedActivity保存在本地變量prev中。在上一步Step 10中，說到mResumedActivity就是Launcher，因此，這里把Launcher進程中的ApplicationThread對象取出來，通過它來通知Launcher這個Activity它要進入Paused狀態了。當然，這里的prev.app.thread是一個ApplicationThread對象的遠程接口，通過調用這個遠程接口的schedulePauseActivity來通知Launcher進入Paused狀態。參數prev.finishing表示prev所代表的Activity是否正在等待結束的Activity列表中，由于Laucher這個Activity還沒結束，所以這里為false；參數prev.configChangeFlags表示哪些config發生了變化，這里我們不關心它的值。 **Step 12. ApplicationThreadProxy.schedulePauseActivity** 這個函數定義在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(); } ...... } ~~~ 這個函數通過Binder進程間通信機制進入到ApplicationThread.schedulePauseActivity函數中。 **Step 13. ApplicationThread.schedulePauseActivity** 這個函數定義在frameworks/base/core/java/android/app/ActivityThread.java文件中，它是ActivityThread的內部類： ~~~ 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); } ...... } ...... } ~~~ 這里調用的函數queueOrSendMessage是ActivityThread類的成員函數。上面說到，這里的finished值為false，因此，queueOrSendMessage的第一個參數值為H.PAUSE_ACTIVITY，表示要暫停token所代表的Activity，即Launcher。 **Step 14. ActivityThread.queueOrSendMessage** 這個函數定義在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); } } ...... } ~~~ 這里首先將相關信息組裝成一個msg，然后通過mH成員變量發送出去，mH的類型是H，繼承于Handler類，是ActivityThread的內部類，因此，這個消息最后由H.handleMessage來處理。 **Step 15. H.handleMessage** 這個函數定義在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; ...... } ...... } ...... } ~~~ 這里調用ActivityThread.handlePauseActivity進一步操作，msg.obj是一個ActivityRecord對象的引用，它代表的是Launcher這個Activity。 **Step 16. ActivityThread.handlePauseActivity** 這個函數定義在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) { } } } ...... } ~~~ 函數首先將Binder引用token轉換成ActivityRecord的遠程接口ActivityClientRecord，然后做了三個事情：1. 如果userLeaving為true，則通過調用performUserLeavingActivity函數來調用Activity.onUserLeaveHint通知Activity，用戶要離開它了；2. 調用performPauseActivity函數來調用Activity.onPause函數，我們知道，在Activity的生命周期中，當它要讓位于其它的Activity時，系統就會調用它的onPause函數；3. 它通知ActivityManagerService，這個Activity已經進入Paused狀態了，ActivityManagerService現在可以完成未竟的事情，即啟動MainActivity了。 **Step 17. ActivityManagerProxy.activityPaused** 這個函數定義在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(); } ...... } ~~~ 這里通過Binder進程間通信機制就進入到ActivityManagerService.activityPaused函數中去了。 **Step 18. ActivityManagerService.activityPaused** 這個函數定義在frameworks/base/services/java/com/android/server/am/ActivityManagerService.java文件中： ~~~ public final class ActivityManagerService extends ActivityManagerNative implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback { ...... public final void activityPaused(IBinder token, Bundle icicle) { ...... final long origId = Binder.clearCallingIdentity(); mMainStack.activityPaused(token, icicle, false); ...... } ...... } ~~~ 這里，又再次進入到ActivityStack類中，執行activityPaused函數。 **Step 19. ActivityStack.activityPaused** 這個函數定義在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 { ...... } } } } ...... } ~~~ 這里通過參數token在mHistory列表中得到ActivityRecord，從上面我們知道，這個ActivityRecord代表的是Launcher這個Activity，而我們在Step 11中，把Launcher這個Activity的信息保存在mPausingActivity中，因此，這里mPausingActivity等于r，于是，執行completePauseLocked操作。 **Step 20. ActivityStack.completePauseLocked** 這個函數定義在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 { ...... } ...... } ...... } ~~~ 函數首先把mPausingActivity變量清空，因為現在不需要它了，然后調用resumeTopActivityLokced進一步操作，它傳入的參數即為代表Launcher這個Activity的ActivityRecord。 **Step 21. ActivityStack.resumeTopActivityLokced** 這個函數定義在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; } ...... } ~~~ 通過上面的Step 9，我們知道，當前在堆棧頂端的Activity為我們即將要啟動的MainActivity，這里通過調用topRunningActivityLocked將它取回來，保存在next變量中。之前最后一個Resumed狀態的Activity，即Launcher，到了這里已經處于Paused狀態了，因此，mResumedActivity為null。最后一個處于Paused狀態的Activity為Launcher，因此，這里的mLastPausedActivity就為Launcher。前面我們為MainActivity創建了ActivityRecord后，它的app域一直保持為null。有了這些信息后，上面這段代碼就容易理解了，它最終調用startSpecificActivityLocked進行下一步操作。 **Step 22. ActivityStack.startSpecificActivityLocked** 這個函數定義在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，這里由于是第一次啟動應用程序的Activity，所以下面語句： ~~~ ProcessRecord app = mService.getProcessRecordLocked(r.processName, r.info.applicationInfo.uid); ~~~ 取回來的app為null。在Activity應用程序中的AndroidManifest.xml配置文件中，我們沒有指定Application標簽的process屬性，系統就會默認使用package的名稱，這里就是"shy.luo.activity"了。每一個應用程序都有自己的uid，因此，這里uid + process的組合就可以為每一個應用程序創建一個ProcessRecord。當然，我們可以配置兩個應用程序具有相同的uid和package，或者在AndroidManifest.xml配置文件的application標簽或者activity標簽中顯式指定相同的process屬性值，這樣，不同的應用程序也可以在同一個進程中啟動。函數最終執行ActivityManagerService.startProcessLocked函數進行下一步操作。 **Step 23. ActivityManagerService.startProcessLocked** 這個函數定義在frameworks/base/services/java/com/android/server/am/ActivityManagerService.java文件中： ~~~ public final class ActivityManagerService extends ActivityManagerNative implements 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 list app.addPackage(info.packageName); } ...... startProcessLocked(app, hostingType, hostingNameStr); return (app.pid != 0) ? app : null; } ...... } ~~~ 這里再次檢查是否已經有以process + uid命名的進程存在，在我們這個情景中，返回值app為null，因此，后面會創建一個ProcessRecord，并存保存在成員變量mProcessNames中，最后，調用另一個startProcessLocked函數進一步操作： ~~~ public final class ActivityManagerService extends ActivityManagerNative implements 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) { ...... } } ...... } ~~~ 這里主要是調用Process.start接口來創建一個新的進程，新的進程會導入android.app.ActivityThread類，并且執行它的main函數，這就是為什么我們前面說每一個應用程序都有一個ActivityThread實例來對應的原因。 **Step 24. ActivityThread.main** 這個函數定義在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(); ...... } } ~~~ 這個函數在進程中創建一個ActivityThread實例，然后調用它的attach函數，接著就進入消息循環了，直到最后進程退出。函數attach最終調用了ActivityManagerService的遠程接口ActivityManagerProxy的attachApplication函數，傳入的參數是mAppThread，這是一個ApplicationThread類型的Binder對象，它的作用是用來進行進程間通信的。 **Step 25. ActivityManagerProxy.attachApplication** 這個函數定義在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(); } ...... } ~~~ 這里通過Binder驅動程序，最后進入ActivityManagerService的attachApplication函數中。 **Step 26. ActivityManagerService.attachApplication** 這個函數定義在frameworks/base/services/java/com/android/server/am/ActivityManagerService.java文件中： ~~~ public final class ActivityManagerService extends ActivityManagerNative implements 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); } } ...... } ~~~ 這里將操作轉發給attachApplicationLocked函數。 **Step 27. ActivityManagerService.attachApplicationLocked** 這個函數定義在frameworks/base/services/java/com/android/server/am/ActivityManagerService.java文件中： ~~~ public final class ActivityManagerService extends ActivityManagerNative implements 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，已經創建了一個ProcessRecord，這里首先通過pid將它取回來，放在app變量中，然后對app的其它成員進行初始化，最后調用mMainStack.realStartActivityLocked執行真正的Activity啟動操作。這里要啟動的Activity通過調用mMainStack.topRunningActivityLocked(null)從堆棧頂端取回來，這時候在堆棧頂端的Activity就是MainActivity了。 **Step 28. ActivityStack.realStartActivityLocked** 這個函數定義在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; } ...... } ~~~ 這里最終通過app.thread進入到ApplicationThreadProxy的scheduleLaunchActivity函數中，注意，這里的第二個參數r，是一個ActivityRecord類型的Binder對象，用來作來這個Activity的token值。 **Step 29. ApplicationThreadProxy.scheduleLaunchActivity** 這個函數定義在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(); } ...... } ~~~ 這個函數最終通過Binder驅動程序進入到ApplicationThread的scheduleLaunchActivity函數中。 **Step 30. ApplicationThread.scheduleLaunchActivity** 這個函數定義在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); } ...... } ...... } ~~~ 函數首先創建一個ActivityClientRecord實例，并且初始化它的成員變量，然后調用ActivityThread類的queueOrSendMessage函數進一步處理。 **Step 31. ActivityThread.queueOrSendMessage** 這個函數定義在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); } } ...... } ...... } ~~~ 函數把消息內容放在msg中，然后通過mH把消息分發出去，這里的成員變量mH我們在前面已經見過，消息分發出去后，最后會調用H類的handleMessage函數。 **Step 32. H.handleMessage** 這個函數定義在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; ...... } ...... } ...... } ~~~ 這里最后調用ActivityThread類的handleLaunchActivity函數進一步處理。 **Step 33. ActivityThread.handleLaunchActivity** 這個函數定義在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 { ...... } } ...... } ~~~ 這里首先調用performLaunchActivity函數來加載這個Activity類，即shy.luo.activity.MainActivity，然后調用它的onCreate函數，最后回到handleLaunchActivity函數時，再調用handleResumeActivity函數來使這個Activity進入Resumed狀態，即會調用這個Activity的onResume函數，這是遵循Activity的生命周期的。 **Step 34. ActivityThread.performLaunchActivity** 這個函數定義在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; } ...... } ~~~ 函數前面是收集要啟動的Activity的相關信息，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); } ~~~ 然后通過ClassLoader將shy.luo.activity.MainActivity類加載進來： ~~~ 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) { ...... } ~~~ 接下來是創建Application對象，這是根據AndroidManifest.xml配置文件中的Application標簽的信息來創建的： ~~~ Application app = r.packageInfo.makeApplication(false, mInstrumentation); ~~~ 后面的代碼主要創建Activity的上下文信息，并通過attach方法將這些上下文信息設置到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); ~~~ 最后還要調用MainActivity的onCreate函數： ~~~ mInstrumentation.callActivityOnCreate(activity, r.state); ~~~ 這里不是直接調用MainActivity的onCreate函數，而是通過mInstrumentation的callActivityOnCreate函數來間接調用，前面我們說過，mInstrumentation在這里的作用是監控Activity與系統的交互操作，相當于是系統運行日志。 **Step 35. MainActivity.onCreate** 這個函數定義在packages/experimental/Activity/src/shy/luo/activity/MainActivity.java文件中，這是我們自定義的app工程文件： ~~~ public class MainActivity extends Activity implements OnClickListener { ...... @Override public void onCreate(Bundle savedInstanceState) { ...... Log.i(LOG_TAG, "Main Activity Created."); } ...... } ~~~ 這樣，MainActivity就啟動起來了，整個應用程序也啟動起來了。整個應用程序的啟動過程要執行很多步驟，但是整體來看，主要分為以下五個階段： * 一. Step1 - Step 11：Launcher通過Binder進程間通信機制通知ActivityManagerService，它要啟動一個Activity； * 二. Step 12 - Step 16：ActivityManagerService通過Binder進程間通信機制通知Launcher進入Paused狀態； * 三. Step 17 - Step 24：Launcher通過Binder進程間通信機制通知ActivityManagerService，它已經準備就緒進入Paused狀態，于是ActivityManagerService就創建一個新的進程，用來啟動一個ActivityThread實例，即將要啟動的Activity就是在這個ActivityThread實例中運行； * 四. Step 25 - Step 27：ActivityThread通過Binder進程間通信機制將一個ApplicationThread類型的Binder對象傳遞給ActivityManagerService，以便以后ActivityManagerService能夠通過這個Binder對象和它進行通信； * 五. Step 28 - Step 35：ActivityManagerService通過Binder進程間通信機制通知ActivityThread，現在一切準備就緒，它可以真正執行Activity的啟動操作了。這里不少地方涉及到了Binder進程間通信機制，相關資料請參考Android進程間通信（IPC）機制Binder簡要介紹和學習計劃一文。這樣，應用程序的啟動過程就介紹完了，它實質上是啟動應用程序的默認Activity，在下一篇文章中，我們將介紹在應用程序內部啟動另一個Activity的過程，即新的Activity與啟動它的Activity將會在同一個進程（Process）和任務（Task）運行，敬請關注。