在日常开发中我们经常从一个Activity跳转到另一个Activity，调用的代码无非有以下两种：
startActivity(intent);startActivityForResult(intent, requestCode);其实startActivity()最终也会调用startActivityForResult()来实现界面的跳转，下面我们就从startActivityForResult()来梳理Activity的启动流程;
进入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); } } }上面的代码只需要关注mParent ==null这部分，mParent 是一个Activity ，mMainThread.getApplicationThread()得到的是ApplicationThread对象，它是ActivityThread的一个内部类，ActivityThread和ApplicationThread在Activity的启动起着重要作用，一会儿会分析到，Instrumentation的主要作用是application的创建，生命周期管理和启动Activity，接着看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); //启动Activity的真正实现 int result = ActivityManagerNative.getDefault() .startActivity(whoThread, who.getBasePackageName(), intent, intent.resolveTypeIfNeeded(who.getContentResolver()), token, target, requestCode, 0, null, options); //检查是否在清单文件中注册这个Activity checkStartActivityResult(result, intent); } catch (RemoteException e) { throw new RuntimeException("Failure from system", e); } return null; }从上面代码可以看出启动Activity的真正实现是通过ActivityManagerNative.getDefault() 的startActivity方法完成的。 ActivityManagerService （AMS）是一个final类，它继承了ActivityManagerNative，而 ActivityManagerNative继承自Binder 并实现了 IActivityManager这个Binder 接口，所以AMS也是一个Binder，它是IActivityManager的具体实现。
public abstract class ActivityManagerNative extends Binder implements IActivityManagerpublic interface IActivityManager extends IInterfacepublic final class ActivityManagerService extends ActivityManagerNativeActivityManagerNative.getDefault()返回的是IActivityManager也就是一个Binder对象，它的具体实现是ActivityManagerService ； 在ActivityManagerNative中，AMS这个对象是采用单例模式Singleton对外提供，第一次调用它的get方法时，会通过create方法初始化AMS，在后续的调用中，直接返回之前创建的对象。
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; } };}现在我们知道了Activity的启动是ActivityManager来启动的，而ActivityManager的具体实现是AMS，所以我们只需查看AMS中的startActivity方法，先看源码：
@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的启动过程又传递给了ActivityStarter的startActivityMayWait方法，进入到startActivityMayWait类中，有进行了几次传递，由startActivityMayWait—>startActivityLocked—>startActivityUnchecked再调用ActivityStackSupervisor类的resumeFocusedStackTopActivityLocked方法再调用ActivityStack的resumeTopActivityUncheckedLocked方法，这个时候启动过程已经转移到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; }这个方法的作用是确定在栈顶的activity已经获得，这个方法又调用了 resumeTopActivityInnerLocked方法：在resumeTopActivityInnerLocked方法又调用了ActivityStackSupervisor的startSpecificActivityLocked方法，此时启动过程转到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又调用了realStartActivityLocked方法， 大概的流程图： 
其中的核心代码：
app.thread是IApplicationThread，它继承了IInterface接口，是一个Binder类型的接口，看一下IApplicationThread的一些源码：
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中声明了启动 停止activity的接口，还包含service的启动和停止，IApplicationThread的实现类是ActivityThread的内部类ApplicationThread
private class ApplicationThread extends ApplicationThreadNative {public abstract class ApplicationThreadNative extends Binder implements IApplicationThread {这个时候activity的启动过程回到了ApplicationThread 中，在ApplicationThread 中通过调用scheduleLaunchActivity来启动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方法中通过sendMessage(H.LAUNCH_ACTIVITY, r)发送一个启动activity的消息让handler处理即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中通过调用ActivityThread的handleLaunchActivity方法来启动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); ... } }由上面的代码可以看出performLaunchActivity方法完成了Activity的创建和启动，并ActivityThread通过handleResumeActivity方法来调用被启动的Activity的onResume这个生命周期方法；
看看performLaunchActivity方法的源码：
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) { // System.out.println("##### [" + System.currentTimeMillis() + "] ActivityThread.performLaunchActivity(" + r + ")"); //获取待启动的Activity的组件信息 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); } //使用类加载器创建Activity对象 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); } } //创建Application对象 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) { //创建Context对象 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; } //通过activity的attach初始化一些重要的数据 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; //调用callActivityOnCreate方法完成整个启动过程 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主要完成了以下几件事：
从ActivityClientRecord中获取待启动的Activity的组件信息通过Instrumentation的newActivity方法使用类加载器创建Activity对象；通过LoadedApk的makeApplication方法创建Application对象,在这个方法中判断了Application是否为null，为null就创建，不为null直接返回，所以Application只初始化一次；创建Context对象并通过activity的attach初始化一些重要的数据；调用Activity的OnCreate方法完成整个启动过程；此外，performLaunchActivity方法还完成了设置主题、创建window并与window建立关联，这样window接收道德外部输入事件就会传到Activity。
至此Activity的启动流程就完成了；
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