在日常開發中我們經常從一個Activity跳轉到另一個Activity，調用的代碼無非有以下兩種：
startActivity(intent);startActivityForResult(intent, requestCode);其實startActivity()最終也會調用startActivityForResult()來實現界面的跳轉，下面我們就從startActivityForResult()來梳理Activity的啟動流程;
進入startActivityForResult()方法瞧瞧：
PRivate Instrumentation mInstrumentation;private IBinder mToken;ActivityThread mMainThread;Activity mParent;public void startActivityForResult(@RequiresPermission Intent intent, int requestCode, @Nullable Bundle options) { if (mParent == null) { options = transferSpringboardActivityOptions(options); Instrumentation.ActivityResult ar = mInstrumentation.execStartActivity( this, mMainThread.getapplicationThread(), mToken, this, intent, requestCode, options); if (ar != null) { mMainThread.sendActivityResult( mToken, mEmbeddedID, requestCode, ar.getResultCode(), ar.getResultData()); } if (requestCode >= 0) { mStartedActivity = true; } cancelInputsAndStartExitTransition(options); // TODO Consider clearing/flushing other event sources and events for child windows. } else { if (options != null) { mParent.startActivityFromChild(this, intent, requestCode, options); } else { // Note we want to go through this method for compatibility with // existing applications that may have overridden it. mParent.startActivityFromChild(this, intent, requestCode); } } }上面的代碼只需要關注mParent ==null這部分，mParent 是一個Activity ，mMainThread.getApplicationThread()得到的是ApplicationThread對象，它是ActivityThread的一個內部類，ActivityThread和ApplicationThread在Activity的啟動起著重要作用，一會兒會分析到，Instrumentation的主要作用是application的創建，生命周期管理和啟動Activity，接著看mInstrumentation.execStartActivity()方法：
public ActivityResult execStartActivity( Context who, IBinder contextThread, IBinder token, String target, Intent intent, int requestCode, Bundle options) { IApplicationThread whoThread = (IApplicationThread) contextThread; if (mActivityMonitors != null) { synchronized (mSync) { final int N = mActivityMonitors.size(); for (int i=0; i<N; i++) { final ActivityMonitor am = mActivityMonitors.get(i); if (am.match(who, null, intent)) { am.mHits++; if (am.isBlocking()) { return requestCode >= 0 ? am.getResult() : null; } break; } } } } try { intent.migrateExtraStreamToClipData(); intent.prepareToLeaveProcess(who); //啟動Activity的真正實現 int result = ActivityManagerNative.getDefault() .startActivity(whoThread, who.getBasePackageName(), intent, intent.resolveTypeIfNeeded(who.getContentResolver()), token, target, requestCode, 0, null, options); //檢查是否在清單文件中注冊這個Activity checkStartActivityResult(result, intent); } catch (RemoteException e) { throw new RuntimeException("Failure from system", e); } return null; }從上面代碼可以看出啟動Activity的真正實現是通過ActivityManagerNative.getDefault() 的startActivity方法完成的。 ActivityManagerService （AMS）是一個final類，它繼承了ActivityManagerNative，而 ActivityManagerNative繼承自Binder 并實現了 IActivityManager這個Binder 接口，所以AMS也是一個Binder，它是IActivityManager的具體實現。
public abstract class ActivityManagerNative extends Binder implements IActivityManagerpublic interface IActivityManager extends IInterfacepublic final class ActivityManagerService extends ActivityManagerNativeActivityManagerNative.getDefault()返回的是IActivityManager也就是一個Binder對象，它的具體實現是ActivityManagerService ； 在ActivityManagerNative中，AMS這個對象是采用單例模式Singleton對外提供，第一次調用它的get方法時，會通過create方法初始化AMS，在后續的調用中，直接返回之前創建的對象。
static public IActivityManager getDefault() { return gDefault.get(); }private static final Singleton<IActivityManager> gDefault = new Singleton<IActivityManager>() { protected IActivityManager create() { IBinder b = ServiceManager.getService("activity"); if (false) { Log.v("ActivityManager", "default service binder = " + b); } IActivityManager am = asInterface(b); if (false) { Log.v("ActivityManager", "default service = " + am); } return am; } };}現在我們知道了Activity的啟動是ActivityManager來啟動的，而ActivityManager的具體實現是AMS，所以我們只需查看AMS中的startActivity方法，先看源碼：
@Override public final int startActivity(IApplicationThread caller, String callingPackage, Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode, int startFlags, ProfilerInfo profilerInfo, Bundle bOptions) { return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo, resultWho, requestCode, startFlags, profilerInfo, bOptions, UserHandle.getCallingUserId()); }@Override public final int startActivityAsUser(IApplicationThread caller, String callingPackage, Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode, int startFlags, ProfilerInfo profilerInfo, Bundle bOptions, int userId) { enforceNotIsolatedCaller("startActivity"); userId = mUserController.handleIncomingUser(Binder.getCallingPid(), Binder.getCallingUid(), userId, false, ALLOW_FULL_ONLY, "startActivity", null); // TODO: Switch to user app stacks here. return mActivityStarter.startActivityMayWait(caller, -1, callingPackage, intent, resolvedType, null, null, resultTo, resultWho, requestCode, startFlags, profilerInfo, null, null, bOptions, false, userId, null, null); }可以看出Activity的啟動過程又傳遞給了ActivityStarter的startActivityMayWait方法，進入到startActivityMayWait類中，有進行了幾次傳遞，由startActivityMayWait—>startActivityLocked—>startActivityUnchecked再調用ActivityStackSupervisor類的resumeFocusedStackTopActivityLocked方法再調用ActivityStack的resumeTopActivityUncheckedLocked方法，這個時候啟動過程已經轉移到ActivityStack中。 看看ActivityStack的resumeTopActivityUncheckedLocked方法：
/** * Ensure that the top activity in the stack is resumed. */boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) { if (mStackSupervisor.inResumeTopActivity) { // Don't even start recursing. return false; } boolean result = false; try { // Protect against recursion. mStackSupervisor.inResumeTopActivity = true; if (mService.mLockScreenShown == ActivityManagerService.LOCK_SCREEN_LEAVING) { mService.mLockScreenShown = ActivityManagerService.LOCK_SCREEN_HIDDEN; mService.updateSleepIfNeededLocked(); } result = resumeTopActivityInnerLocked(prev, options); } finally { mStackSupervisor.inResumeTopActivity = false; } return result; }這個方法的作用是確定在棧頂的activity已經獲得，這個方法又調用了 resumeTopActivityInnerLocked方法：在resumeTopActivityInnerLocked方法又調用了ActivityStackSupervisor的startSpecificActivityLocked方法，此時啟動過程轉到ActivityStackSupervisor中，看看startSpecificActivityLocked方法：
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, true); r.task.stack.setLaunchTime(r); if (app != null && app.thread != null) { try { if ((r.info.flags&ActivityInfo.FLAG_MULTIPROCESS) == 0 || !"android".equals(r.info.packageName)) { // Don't add this if it is a platform component that is marked // to run in multiple processes, because this is actually // part of the framework so doesn't make sense to track as a // separate apk in the process. app.addPackage(r.info.packageName, r.info.applicationInfo.versionCode, mService.mProcessStats); } realStartActivityLocked(r, app, andResume, checkConfig); return; } catch (RemoteException e) { Slog.w(TAG, "Exception when starting activity " + r.intent.getComponent().flattenToShortString(), e); } // If a dead object exception was thrown -- fall through to // restart the application. } mService.startProcessLocked(r.processName, r.info.applicationInfo, true, 0, "activity", r.intent.getComponent(), false, false, true); }startSpecificActivityLocked又調用了realStartActivityLocked方法， 大概的流程圖： 
其中的核心代碼：
app.thread是IApplicationThread，它繼承了IInterface接口，是一個Binder類型的接口，看一下IApplicationThread的一些源碼：
public interface IApplicationThread extends IInterface { void schedulePauseActivity(IBinder token, boolean finished, boolean userLeaving, int configChanges, boolean dontReport) throws RemoteException; void scheduleStopActivity(IBinder token, boolean showWindow, int configChanges) throws RemoteException; void scheduleWindowVisibility(IBinder token, boolean showWindow) throws RemoteException; void scheduleSleeping(IBinder token, boolean sleeping) throws RemoteException; void scheduleResumeActivity(IBinder token, int procState, boolean isForward, Bundle resumeArgs) throws RemoteException; void scheduleSendResult(IBinder token, List<ResultInfo> results) throws RemoteException; void scheduleLaunchActivity(Intent intent, IBinder token, int ident, ActivityInfo info, Configuration curConfig, Configuration overrideConfig, CompatibilityInfo compatInfo, String referrer, IVoiceInteractor voiceInteractor, int procState, Bundle state, PersistableBundle persistentState, List<ResultInfo> pendingResults, List<ReferrerIntent> pendingNewIntents, boolean notResumed, boolean isForward, ProfilerInfo profilerInfo) throws RemoteException; void scheduleRelaunchActivity(IBinder token, List<ResultInfo> pendingResults, List<ReferrerIntent> pendingNewIntents, int configChanges, boolean notResumed, Configuration config, Configuration overrideConfig, boolean preserveWindow) throws RemoteException; void scheduleNewIntent( List<ReferrerIntent> intent, IBinder token, boolean andPause) throws RemoteException; void scheduleDestroyActivity(IBinder token, boolean finished, int configChanges) throws RemoteException; void scheduleReceiver(Intent intent, ActivityInfo info, CompatibilityInfo compatInfo, int resultCode, String data, Bundle extras, boolean sync, int sendingUser, int processState) throws RemoteException; static final int BACKUP_MODE_INCREMENTAL = 0; static final int BACKUP_MODE_FULL = 1; static final int BACKUP_MODE_RESTORE = 2; static final int BACKUP_MODE_RESTORE_FULL = 3; void scheduleCreateBackupAgent(ApplicationInfo app, CompatibilityInfo compatInfo, int backupMode) throws RemoteException; void scheduleDestroyBackupAgent(ApplicationInfo app, CompatibilityInfo compatInfo) throws RemoteException; void scheduleCreateService(IBinder token, ServiceInfo info, CompatibilityInfo compatInfo, int processState) throws RemoteException; void scheduleBindService(IBinder token, Intent intent, boolean rebind, int processState) throws RemoteException; void scheduleUnbindService(IBinder token, Intent intent) throws RemoteException; void scheduleServiceArgs(IBinder token, boolean taskRemoved, int startId, int flags, Intent args) throws RemoteException; void scheduleStopService(IBinder token) throws RemoteException; ...... }可以看出IApplicationThread中聲明了啟動 停止activity的接口，還包含service的啟動和停止，IApplicationThread的實現類是ActivityThread的內部類ApplicationThread
private class ApplicationThread extends ApplicationThreadNative {public abstract class ApplicationThreadNative extends Binder implements IApplicationThread {這個時候activity的啟動過程回到了ApplicationThread 中，在ApplicationThread 中通過調用scheduleLaunchActivity來啟動activity：
@Override public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident, ActivityInfo info, Configuration curConfig, Configuration overrideConfig, CompatibilityInfo compatInfo, String referrer, IVoiceInteractor voiceInteractor, int procState, Bundle state, PersistableBundle persistentState, List<ResultInfo> pendingResults, List<ReferrerIntent> pendingNewIntents, boolean notResumed, boolean isForward, ProfilerInfo profilerInfo) { updateProcessState(procState, false); ActivityClientRecord r = new ActivityClientRecord(); r.token = token; r.ident = ident; r.intent = intent; r.referrer = referrer; r.voiceInteractor = voiceInteractor; r.activityInfo = info; r.compatInfo = compatInfo; r.state = state; r.persistentState = persistentState; r.pendingResults = pendingResults; r.pendingIntents = pendingNewIntents; r.startsNotResumed = notResumed; r.isForward = isForward; r.profilerInfo = profilerInfo; r.overrideConfig = overrideConfig; updatePendingConfiguration(curConfig); sendMessage(H.LAUNCH_ACTIVITY, r); }scheduleLaunchActivity方法中通過sendMessage(H.LAUNCH_ACTIVITY, r)發送一個啟動activity的消息讓handler處理即H;
private class H extends Handler { public static final int LAUNCH_ACTIVITY = 100; public static final int PAUSE_ACTIVITY = 101; public static final int PAUSE_ACTIVITY_FINISHING= 102; public static final int STOP_ACTIVITY_SHOW = 103; public static final int STOP_ACTIVITY_HIDE = 104; public static final int SHOW_WINDOW = 105; public static final int HIDE_WINDOW = 106; public static final int RESUME_ACTIVITY = 107; public static final int SEND_RESULT = 108; public static final int DESTROY_ACTIVITY = 109; public static final int BIND_APPLICATION = 110; public static final int EXIT_APPLICATION = 111; public static final int NEW_INTENT = 112; public static final int RECEIVER = 113; public static final int CREATE_SERVICE = 114; public static final int SERVICE_ARGS = 115; public static final int STOP_SERVICE = 116;....... public void handleMessage(Message msg) { if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what)); switch (msg.what) { case LAUNCH_ACTIVITY: { Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart"); final ActivityClientRecord r = (ActivityClientRecord) msg.obj; r.packageInfo = getPackageInfoNoCheck( r.activityInfo.applicationInfo, r.compatInfo); handleLaunchActivity(r, null, "LAUNCH_ACTIVITY"); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); } break; case RELAUNCH_ACTIVITY: { Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityRestart"); ActivityClientRecord r = (ActivityClientRecord)msg.obj; handleRelaunchActivity(r); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); } break; case PAUSE_ACTIVITY: { Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityPause"); SomeArgs args = (SomeArgs) msg.obj; handlePauseActivity((IBinder) args.arg1, false, (args.argi1 & USER_LEAVING) != 0, args.argi2, (args.argi1 & DONT_REPORT) != 0, args.argi3); maybeSnapshot(); Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER); } break; ...... } Object obj = msg.obj; if (obj instanceof SomeArgs) { ((SomeArgs) obj).recycle(); } if (DEBUG_MESSAGES) Slog.v(TAG, "<<< done: " + codeToString(msg.what)); }在H中通過調用ActivityThread的handleLaunchActivity方法來啟動Activity：
private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) { ... if (localLOGV) Slog.v( TAG, "Handling launch of " + r); // Initialize before creating the activity WindowManagerGlobal.initialize(); Activity a = performLaunchActivity(r, customIntent); if (a != null) { r.createdConfig = new Configuration(mConfiguration); reportSizeConfigurations(r); Bundle oldState = r.state; handleResumeActivity(r.token, false, r.isForward, !r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason); ... } }由上面的代碼可以看出performLaunchActivity方法完成了Activity的創建和啟動，并ActivityThread通過handleResumeActivity方法來調用被啟動的Activity的onResume這個生命周期方法；
看看performLaunchActivity方法的源碼：
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) { // System.out.println("##### [" + System.currentTimeMillis() + "] ActivityThread.performLaunchActivity(" + r + ")"); //獲取待啟動的Activity的組件信息 ActivityInfo aInfo = r.activityInfo; if (r.packageInfo == null) { r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo, 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); } //使用類加載器創建Activity對象 Activity activity = null; try { java.lang.ClassLoader cl = r.packageInfo.getClassLoader(); activity = mInstrumentation.newActivity( cl, component.getClassName(), r.intent); StrictMode.incrementExpectedActivityCount(activity.getClass()); r.intent.setExtrasClassLoader(cl); r.intent.prepareToEnterProcess(); if (r.state != null) { r.state.setClassLoader(cl); } } catch (Exception e) { if (!mInstrumentation.onException(activity, e)) { throw new RuntimeException( "Unable to instantiate activity " + component + ": " + e.toString(), e); } } //創建Application對象 try { Application app = r.packageInfo.makeApplication(false, mInstrumentation); if (localLOGV) Slog.v(TAG, "Performing launch of " + r); if (localLOGV) Slog.v( TAG, r + ": app=" + app + ", appName=" + app.getPackageName() + ", pkg=" + r.packageInfo.getPackageName() + ", comp=" + r.intent.getComponent().toShortString() + ", dir=" + r.packageInfo.getAppDir()); if (activity != null) { //創建Context對象 Context appContext = createBaseContextForActivity(r, activity); CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager()); Configuration config = new Configuration(mCompatConfiguration); if (r.overrideConfig != null) { config.updateFrom(r.overrideConfig); } if (DEBUG_CONFIGURATION) Slog.v(TAG, "Launching activity " + r.activityInfo.name + " with config " + config); Window window = null; if (r.mPendingRemoveWindow != null && r.mPreserveWindow) { window = r.mPendingRemoveWindow; r.mPendingRemoveWindow = null; r.mPendingRemoveWindowManager = null; } //通過activity的attach初始化一些重要的數據 activity.attach(appContext, this, getInstrumentation(), r.token, r.ident, app, r.intent, r.activityInfo, title, r.parent, r.embeddedID, r.lastNonConfigurationInstances, config, r.referrer, r.voiceInteractor, window); if (customIntent != null) { activity.mIntent = customIntent; } r.lastNonConfigurationInstances = null; activity.mStartedActivity = false; int theme = r.activityInfo.getThemeResource(); if (theme != 0) { activity.setTheme(theme); } activity.mCalled = false; //調用callActivityOnCreate方法完成整個啟動過程 if (r.isPersistable()) { mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState); } else { mInstrumentation.callActivityOnCreate(activity, r.state); } if (!activity.mCalled) { throw new SuperNotCalledException( "Activity " + r.intent.getComponent().toShortString() + " did not call through to super.onCreate()"); } r.activity = activity; r.stopped = true; if (!r.activity.mFinished) { activity.performStart(); r.stopped = false; } if (!r.activity.mFinished) { if (r.isPersistable()) { if (r.state != null || r.persistentState != null) { mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state, r.persistentState); } } else if (r.state != null) { mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state); } } if (!r.activity.mFinished) { activity.mCalled = false; if (r.isPersistable()) { mInstrumentation.callActivityOnPostCreate(activity, r.state, r.persistentState); } else { 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) { throw e; } catch (Exception e) { if (!mInstrumentation.onException(activity, e)) { throw new RuntimeException( "Unable to start activity " + component + ": " + e.toString(), e); } } return activity; }performLaunchActivity主要完成了以下幾件事：
從ActivityClientRecord中獲取待啟動的Activity的組件信息通過Instrumentation的newActivity方法使用類加載器創建Activity對象；通過LoadedApk的makeApplication方法創建Application對象,在這個方法中判斷了Application是否為null，為null就創建，不為null直接返回，所以Application只初始化一次；創建Context對象并通過activity的attach初始化一些重要的數據；調用Activity的OnCreate方法完成整個啟動過程；此外，performLaunchActivity方法還完成了設置主題、創建window并與window建立關聯，這樣window接收道德外部輸入事件就會傳到Activity。
至此Activity的啟動流程就完成了；
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