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学习前，建议有相关知识储备：
【Android 基础】 应用（Application）启动流程

通过本文你可以学习到Activity启动流程。

一、概览

Activity 是 android 四大组件之一，很有必要知道它的启动过程，我们在上一篇文章中介绍了 APP的启动流程，里面大概讲到了Activity的
启动流程，在本文中，我们系统的再总结一下，当成一个记录。

Activity 的启动方式我们讲有两种，一种是在应用内部启动，另一种是外部启动，比如Launcher；

• 应用内启动
  通过 startActivity、startActivityForResult等方式来启动 Activity

其流程我们总结下：

1、调用 Activity 的 startActivity 方法来启动目标 Activity
2、接着就会调用到 Instrunmentation 的 execStartActivity 方法，然后调用到 AMS 的 startActivity 中去
3、调用到 AMS 中后，会执行到ActivityStarter 的 execute 方法，接着就会进行一些校验和判断权限，包括进程检查，intent检查，权限检查、是否启用新栈等
4、所有的信息存储在ActivityRecord中，ActivityRecord是Activity在system_server进程中的镜像，Activity实例与ActivityRecord实例一一对应。ActivityRecord用来存储Activity的信息，如所在的进程名称，应用的包名，所在的任务栈的taskAffinity等
5、中间再经过一系列调用，又回调到 ActivityThread 的 handleLaunchActivity 来启动 Activity。

借用一张网络图片

• 应用外启动
  通过Launcher 进程启动，Launcher 就是我们桌面程序，当系统开机后， Launcher 也随之被启动。

1、fork并调用ActivityThread的main方法创建app进程
2、然后从 ActivityThread 调用到AMS中的attachApplicationLocked，创建Application
3、Application创建完后，调用ActivityStackSupervisor的attachApplicationLocked方法，最终调用到handleLaunchActivity，进行activity的创建

二、应用内启动源码流程 (startActivity)

我们就从源码出发，一起来看看startActivity后面的流程

2.1 startActivity()

Activity.java

    @Overridepublic void startActivity(Intent intent) {this.startActivity(intent, null);}@Overridepublic void startActivity(Intent intent, @Nullable Bundle options) {if (options != null) {startActivityForResult(intent, -1, options);} else {startActivityForResult(intent, -1);}}public void startActivityForResult(@RequiresPermission Intent intent, int requestCode) {startActivityForResult(intent, requestCode, null);}

    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) {//分析启动结果mMainThread.sendActivityResult(mToken, mEmbeddedID, requestCode, ar.getResultCode(),ar.getResultData());}} else {// 最终也是调用 execStartActivity 方法，源码如下if (options != null) {mParent.startActivityFromChild(this, intent, requestCode, options);} else {mParent.startActivityFromChild(this, intent, requestCode);}}}

    public void startActivityFromChild(@NonNull Activity child, @RequiresPermission Intent intent,int requestCode, @Nullable Bundle options) {options = transferSpringboardActivityOptions(options);Instrumentation.ActivityResult ar =mInstrumentation.execStartActivity(this, mMainThread.getApplicationThread(), mToken, child,intent, requestCode, options);if (ar != null) {mMainThread.sendActivityResult(mToken, child.mEmbeddedID, requestCode,ar.getResultCode(), ar.getResultData());}cancelInputsAndStartExitTransition(options);}

上面代码中，最终都会调用了 execStartActivity 方法，该方法会返回一个启动结果。我们一起来看看
frameworks/base/core/java/android/app/Instrumentation.java

/**** @param who              用来启动 Activity 的对象* @param contextThread Binder 对象，具有跨进程通信的能力，传入的是 ApplicationThread* @param token  Binder 对象，指向了服务端一个 ActivityRecord 对象* @param target 当前的 Activity* @param intent  Intent 对象* @param requestCode  请求码*/public ActivityResult execStartActivity(Context who, IBinder contextThread, IBinder token, String target,Intent intent, int requestCode, Bundle options) {IApplicationThread whoThread = (IApplicationThread) contextThread;try {intent.migrateExtraStreamToClipData();intent.prepareToLeaveProcess(who);// 这里的service 就是 ActivityManagerService， 具体可以跟代码看到 // 这么一句 ： ServiceManager.getService(Context.ACTIVITY_SERVICE);int result = ActivityManager.getService().startActivity(whoThread, who.getBasePackageName(), intent,intent.resolveTypeIfNeeded(who.getContentResolver()),token, target, requestCode, 0, null, options);checkStartActivityResult(result, intent);} catch (RemoteException e) {}return null;}

通过 Binder 调用 AMS 启动 Activity，我们接着往下看

2.2 startActivityAsUser()

ActivityManagerService.java

public class ActivityManagerService extends IActivityManager.Stub@Overridepublic 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());}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,boolean validateIncomingUser) {enforceNotIsolatedCaller("startActivity");// 首先 检查调用者权限userId = mActivityStartController.checkTargetUser(userId, validateIncomingUser,Binder.getCallingPid(), Binder.getCallingUid(), "startActivityAsUser");// TODO: Switch to user app stacks here.return mActivityStartController.obtainStarter(intent, "startActivityAsUser").setCaller(caller).setCallingPackage(callingPackage).setResolvedType(resolvedType).setResultTo(resultTo).setResultWho(resultWho).setRequestCode(requestCode).setStartFlags(startFlags).setProfilerInfo(profilerInfo).setActivityOptions(bOptions).setMayWait(userId).execute();}最后调用的是 ActivityStarter.execute();

上面代码最终调用到了 startActivityAsUser 方法，在内部将所有点的参数都交给了 ActivityStarter

2.3 startActivityUnchecked()

ActivityStarter 该类包含了启动的所有逻辑，比如 Intent 解析以及任务栈等。

ActivityStarter.java

   int execute() {try {if (mRequest.mayWait) {return startActivityMayWait(mRequest.caller, ...);} else {// todo return startActivity(mRequest.caller, ...);  }} finally {onExecutionComplete();}}private int startActivity(IApplicationThread caller, Intent intent, ...) {// 再次检查调用者权限，包括进程检查，intent检查，权限检查等boolean abort = !mSupervisor.checkStartAnyActivityPermission(intent, aInfo, ...);abort |= !mService.mIntentFirewall.checkStartActivity(intent, ...);// 每个 Activity 都会对应一个 ActivityRecord 对象ActivityRecord r = new ActivityRecord(mService, ...);// todo result = startActivityUnchecked(r, sourceRecord ...);}// Note: This method should only be called from {@link startActivity}.private int startActivityUnchecked(final ActivityRecord r, ActivityRecord sourceRecord ...) {//设置初始化状态setInitialState(r, options, inTask, ...);//判断启动模式，并且在 mLaunchFlags 上追加对应标记computeLaunchingTaskFlags();//设置 Activity 的栈computeSourceStack();//设置 LaunchFlags 到 intent 上mIntent.setFlags(mLaunchFlags);//决定是否用新的栈ActivityRecord reusedActivity = getReusableIntentActivity();...// Should this be considered a new task?int result = START_SUCCESS;if (mStartActivity.resultTo == null && mInTask == null && !mAddingToTask&& (mLaunchFlags & FLAG_ACTIVITY_NEW_TASK) != 0) {newTask = true;// 创建一个新的task来启动result = setTaskFromReuseOrCreateNewTask(taskToAffiliate, topStack);} else// todo mSupervisor.resumeFocusedStackTopActivityLocked();    }

上面代码中会进行一些校验和判断权限，包括进程检查，intent检查，权限检查等

2.4 resumeFocusedStackTopActivityLocked()

ActivityStackSupervisor.java

boolean resumeFocusedStackTopActivityLocked(ActivityStack targetStack...) {return targetStack.resumeTopActivityUncheckedLocked(target, targetOptions);return false;}

resumeTopActivityUncheckedLocked -> resumeTopActivityInnerLocked -> startSpecificActivityLocked()
ActivityStack.java

@GuardedBy("mService")boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {if (mStackSupervisor.inResumeTopActivity) {Activity 存在则resumetransaction.setLifecycleStateRequest( ResumeActivityItem.obtain(next.app.repProcState,。。。);mService.getLifecycleManager().scheduleTransaction(transaction);不存在则调用下面这个result = resumeTopActivityInnerLocked(prev, options);} finally {}return result;}@GuardedBy("mService")private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {mStackSupervisor.startSpecificActivityLocked(next, true, false);return true;}

2.5 startSpecificActivityLocked -> realStartActivityLocked()

ActivityStackSupervisor.java
startSpecificActivityLocked -> realStartActivityLocked(); 到这个地方，我们就可以看到真的开始启动 activity，
后面就跟Application 里面一样了

void startSpecificActivityLocked(ActivityRecord r ...) {if (app != null && app.thread != null) {try {// 真的开始启动 activity  ,看下面的方法realStartActivityLocked(r, app, andResume, checkConfig);return;} catch (RemoteException e) {}}}真的开始启动 activityfinal boolean realStartActivityLocked(ActivityRecord r, ProcessRecord app,boolean andResume, boolean checkConfig) throws RemoteException {// Create activity launch transaction.final ClientTransaction clientTransaction = ClientTransaction.obtain(app.thread,r.appToken);clientTransaction.addCallback(LaunchActivityItem.obtain(new Intent(r.intent),System.identityHashCode(r), r.info,// TODO: Have this take the merged configuration instead of separate global// and override configs.mergedConfiguration.getGlobalConfiguration(),mergedConfiguration.getOverrideConfiguration(), r.compat,r.launchedFromPackage, task.voiceInteractor, app.repProcState, r.icicle,r.persistentState, results, newIntents, mService.isNextTransitionForward(),profilerInfo));// Schedule transaction.mService.getLifecycleManager().scheduleTransaction(clientTransaction);}

ActivityManagerService

ClientLifecycleManager getLifecycleManager() {return mLifecycleManager;}

ClientLifecycleManager

    通过代码，我们可以看到，获取的client就是 ActivityThread，IApplicationThread是一个AIDL文件void scheduleTransaction(ClientTransaction transaction) throws RemoteException {final IApplicationThread client = transaction.getClient();transaction.schedule();}

ClientTransaction

        /** Target client. */private IApplicationThread mClient;public void schedule() throws RemoteException {mClient.scheduleTransaction(this);}

ClientTransactionHandler

//ActivityThread中没有复写scheduleTransaction，会执行到父类的方法//public final class ActivityThread extends ClientTransactionHandler//ClientTransactionHandler.javapublic abstract class ClientTransactionHandler {void scheduleTransaction(ClientTransaction transaction) {transaction.preExecute(this);//发送消息sendMessage(ActivityThread.H.EXECUTE_TRANSACTION, transaction);}}

ActivityThread.java

case EXECUTE_TRANSACTION:final ClientTransaction transaction = (ClientTransaction) msg.obj;mTransactionExecutor.execute(transaction);break;

这里其实就是执行LaunchActivityItem的execute方法，
其赋值的地方在realStartActivityLocked()方法，大家可以回头看看，前面有这么一句代码
clientTransaction.addCallback(LaunchActivityItem.obtain(new Intent(r.intent),

TransactionExecutor.java

public void execute(ClientTransaction transaction) {final IBinder token = transaction.getActivityToken();executeCallbacks(transaction);executeLifecycleState(transaction);mPendingActions.clear();log("End resolving transaction");}/** Cycle through all states requested by callbacks and execute them at proper times. */@VisibleForTestingpublic void executeCallbacks(ClientTransaction transaction) {final List<ClientTransactionItem> callbacks = transaction.getCallbacks();final IBinder token = transaction.getActivityToken();ActivityClientRecord r = mTransactionHandler.getActivityClient(token);final ActivityLifecycleItem finalStateRequest = transaction.getLifecycleStateRequest();final int size = callbacks.size();for (int i = 0; i < size; ++i) {final ClientTransactionItem item = callbacks.get(i);item.execute(mTransactionHandler, token, mPendingActions);item.postExecute(mTransactionHandler, token, mPendingActions);}}

到这里就调用到我们熟悉的handleLaunchActivity了
LaunchActivityItem.java

@Overridepublic void execute(ClientTransactionHandler client, IBinder token,PendingTransactionActions pendingActions) {ActivityClientRecord r = new ActivityClientRecord(token, mIntent, mIdent, mInfo,mOverrideConfig, mCompatInfo, mReferrer, mVoiceInteractor, mState, mPersistentState,mPendingResults, mPendingNewIntents, mIsForward,mProfilerInfo, client);client.handleLaunchActivity(r, pendingActions, null /* customIntent */);}

最后调用到ActivityThread

三、 Activity实例化过程

ActivityThread.java

/*** Extended implementation of activity launch. Used when server requests a launch or relaunch.*/@Overridepublic Activity handleLaunchActivity(ActivityClientRecord r,PendingTransactionActions pendingActions, Intent customIntent) {final Activity a = performLaunchActivity(r, customIntent);return a;}Activity实例化过程/**  Core implementation of activity launch. */private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {Activity activity = null;try {// 通过ClassLoader去加载需要启动的activity， 反射实例化Activity对象java.lang.ClassLoader cl = appContext.getClassLoader();activity = mInstrumentation.newActivity(cl, component.getClassName(), r.intent);} catch (Exception e) {}// 在该方法内部创建window，并设置window回调，activity.attach(appContext, this, getInstrumentation() ...);theme//当实例化Activity对象后，继续执行callActivityOnCreate， 继而调用Activity的onCreate，// 这样就完成了Activity生命周期的第一个回调onCreate方法if (r.isPersistable()) {mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);} else {mInstrumentation.callActivityOnCreate(activity, r.state);}}

下面的代码比较简单，就不贴了
frameworks/base/core/java/android/app/Instrumentation.java

四、 setContentView

这里内容太多，我们另外写一篇文章。

五、 后续生命周期

接下来就是执行 Activity 其他生命周期函数
ActivityThread.java

    @Overridepublic void handleStartActivity(ActivityClientRecord r,PendingTransactionActions pendingActions) {final Activity activity = r.activity;// Restore instance stateif (pendingActions.shouldRestoreInstanceState()) {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);}}// Call postOnCreate()if (pendingActions.shouldCallOnPostCreate()) {activity.mCalled = false;if (r.isPersistable()) {mInstrumentation.callActivityOnPostCreate(activity, r.state,r.persistentState);} else {mInstrumentation.callActivityOnPostCreate(activity, r.state);}}}@Overridepublic void handleResumeActivity(IBinder token, boolean finalStateRequest, boolean isForward,String reason) {}

写在最后：
在应用进程创建 activity 后，activity 将执行以下操作：

1. 初始化值。
2. 调用构造函数。
3. 根据 activity 的当前生命周期状态，相应地调用回调方法，如 Activity.onCreate()。
   通常，onCreate() 方法对加载时间的影响最大，因为它执行工作的开销最高：加载和渲染视图，以及初始化运行 activity 所需的对象。

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