Android9.0根activity启动流程分析

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activity的启动流程

activity的启动过程分为两种，一种是根Activity的启动过程，另一种是普通Activity的启动过程。根activity的启动过程更复杂，也更具代表性，所以这里先分析根activity的启动过程。根activity的启动分为三部分：Launcher启请求AMS(ActivityManagerService)过程、AMS到ApplicationThread的调用过程以及ActivityThread启动Activity的过程。可以在这个网站在线看9.0.0_r3源码。

1，Launcher请求AMS过程

Launcher启动后会显示应用的图标于桌面上，这些应用图标就是启动根activity的入口，点击它们时就会通过Launcher类来请求AMS来启动该应用程序。Launcher的源码路径为：/packages/apps/Launcher3/src/com/android/launcher3/Launcher.java
时序图如下：

2，接下来查看ApplicationThread的scheduleLaunchActivity方法，其中ApplicationThread是ActivityThread的内部类，应用程序进程创建后会运行代表主线程的实例ActivityThread，它管理着当前应用程序进程的线程。ApplicationThread的scheduleLaunchActivity方法如下所示。
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@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;
    ...
    updatePendingConfiguration(curConfig);
    sendMessage(H.LAUNCH_ACTIVITY, r);
}

scheduleLaunchActivity方法会将启动Activity的参数封装成ActivityClientRecord ，sendMessage方法向H类发送类型为LAUNCH_ACTIVITY的消息，并将ActivityClientRecord 传递过去，sendMessage方法有多个重载方法，最终调用的sendMessage方法如下所示。

frameworks/base/core/java/android/app/ActivityThread.java:

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private void sendMessage(int what, Object obj, int arg1, int arg2, boolean async) {
        if (DEBUG_MESSAGES) Slog.v(
            TAG, "SCHEDULE " + what + " " + mH.codeToString(what)
            + ": " + arg1 + " / " + obj);
        Message msg = Message.obtain();
        msg.what = what;
        msg.obj = obj;
        msg.arg1 = arg1;
        msg.arg2 = arg2;
        if (async) {
            msg.setAsynchronous(true);
        }
        mH.sendMessage(msg);
}

这里mH指的是H，它是ActivityThread的内部类并继承Handler，是应用程序进程中主线程的消息管理类。H的代码如下所示。

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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;//1
                  r.packageInfo = getPackageInfoNoCheck(
                          r.activityInfo.applicationInfo, r.compatInfo);//2
                  handleLaunchActivity(r, null, "LAUNCH_ACTIVITY");//3
                  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;
            ...
}

查看H的handleMessage方法中对LAUNCH_ACTIVITY的处理，在注释1处将传过来的msg的成员变量obj转换为ActivityClientRecord。
在注释2处通过getPackageInfoNoCheck方法获得LoadedApk类型的对象并赋值给ActivityClientRecord 的成员变量packageInfo 。应用程序进程要启动Activity时需要将该Activity所属的APK加载进来，而LoadedApk就是用来描述已加载的APK文件。
在注释3处调用handleLaunchActivity方法，代码如下所示。

frameworks/base/core/java/android/app/ActivityThread.java

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private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) {
    ...
    WindowManagerGlobal.initialize();
    //启动Activity
    Activity a = performLaunchActivity(r, customIntent);//1
    if (a != null) {
        r.createdConfig = new Configuration(mConfiguration);
        reportSizeConfigurations(r);
        Bundle oldState = r.state;
        //将Activity的状态置为Resume
        handleResumeActivity(r.token, false, r.isForward,
                !r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason);//2
        if (!r.activity.mFinished && r.startsNotResumed) {
            performPauseActivityIfNeeded(r, reason);
            if (r.isPreHoneycomb()) {
                r.state = oldState;
            }
        }
    } else {
        try {
            //停止Activity启动
            ActivityManager.getService()
                .finishActivity(r.token, Activity.RESULT_CANCELED, null,
                        Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
        } catch (RemoteException ex) {
            throw ex.rethrowFromSystemServer();
        }
    }
}

注释1处的performLaunchActivity方法用来启动Activity ，注释2处的代码用来将Activity 的状态置为Resume。如果该Activity为null则会通知AMS停止启动Activity。来查看performLaunchActivity方法做了什么：

frameworks/base/core/java/android/app/ActivityThread.java

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private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
        //获取ActivityInfo类
        ActivityInfo aInfo = r.activityInfo;//1
        if (r.packageInfo == null) {
        //获取APK文件的描述类LoadedApk
            r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
                    Context.CONTEXT_INCLUDE_CODE);//2
        }

        ComponentName component = r.intent.getComponent();//3
        ...
        //创建要启动Activity的上下文环境
        ContextImpl appContext = createBaseContextForActivity(r);//4
        Activity activity = null;
        try {
            java.lang.ClassLoader cl = appContext.getClassLoader();
            //用类加载器来创建该Activity的实例
            activity = mInstrumentation.newActivity(
                    cl, component.getClassName(), r.intent);//5
          ...
        } catch (Exception e) {
          ...
        }

        try {
            //创建Application
            Application app = r.packageInfo.makeApplication(false, mInstrumentation);//6
            ...
            if (activity != null) {
               ...
                /**
                *7 初始化Activity
                */
                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, r.configCallback);

               ...
                if (r.isPersistable()) {
                    mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);//8
                } else {
                    mInstrumentation.callActivityOnCreate(activity, r.state);
                }
               ...
            }
            r.paused = true;
            mActivities.put(r.token, r);
        } catch (SuperNotCalledException e) {
            throw e;
        } catch (Exception e) {
          ...
        }

        return activity;
}

注释1处用来获取ActivityInfo，ActivityInfo用于存储代码和AndroidManifes设置的Activity和receiver节点信息，比如Activity的theme和launchMode。在注释2处获取APK文件的描述类LoadedApk。注释3处获取要启动的Activity的ComponentName类，ComponentName类中保存了该Activity的包名和类名。注释4处用来创建要启动Activity的上下文环境。注释5处根据ComponentName中存储的Activity类名，用类加载器来创建该Activity的实例。注释6处用来创建Application，makeApplication方法内部会调用Application的onCreate方法。注释7处调用Activity的attach方法初始化Activity，attach方法中会创建Window对象（PhoneWindow）并与Activity自身进行关联。注释8处会调用Instrumentation的callActivityOnCreate方法来启动Activity，如下所示。

frameworks/base/core/java/android/app/Instrumentation.java

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final void performCreate(Bundle icicle, PersistableBundle persistentState) {
        restoreHasCurrentPermissionRequest(icicle);
        onCreate(icicle, persistentState);
        mActivityTransitionState.readState(icicle);
        performCreateCommon();
}

performCreate方法中会调用Activity的onCreate方法，讲到这里，根Activity就启动了，即应用程序就启动了。
根Activity启动过程就讲到这里，下面我们来学习根Activity启动过程中涉及到的进程。

3,根Activity启动过程中涉及的进程
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