App的启动流程挺复杂的，涉及到很多东西，有System_server进程的Ams，Launcher进程的AMP等等。

大致流程

屏幕快照 2019-08-13 下午10.31.12.png

第一阶段：孵化进程

从图中我们可以看出，点击桌面Launcher上的图标之后,会调用startActivity,然后会走到AMS，最终经过一系列的操作初始化，会调用到Zygote进程的main方法，进而会调用到ActivityThread方法，到了这里就是我们所熟悉的东西了。

第二阶段：绑定应用程序和进程(android 版本26)

这部分的代码大部分都在ActivityManagerService.java中(一般在.../sdk/sources/android-xx/com/android/server/am目录下，主要是ActivityManagerServer和ActivityStackSupervisor这两个文件)

    1.ActivityThread.java
    public static void main(String[] args) {
        Looper.prepareMainLooper();
        ActivityThread thread = new ActivityThread();
        thread.attach(false);
        Looper.loop();
    }

    private void attach(boolean system) {
        // TODO 通过binder机制获取system_server进程的ams对象
        final IActivityManager mgr = ActivityManager.getService();
            try {
                mgr.attachApplication(mAppThread);
            } catch (RemoteException ex) {
                throw ex.rethrowFromSystemServer();
            }
    }

    2.ActivityManagerService
    public final void attachApplication(IApplicationThread thread) {
        synchronized (this) {
            int callingPid = Binder.getCallingPid();
            final long origId = Binder.clearCallingIdentity();
            attachApplicationLocked(thread, callingPid);
            Binder.restoreCallingIdentity(origId);
        }
    }
    private final boolean attachApplicationLocked(IApplicationThread thread,
            int pid) {
          // TODO 回调activityThread的bindApplication方法，进而回调到Application的onCreate方法进行初始化
         thread.bindApplication(processName, appInfo, providers, null, profilerInfo,
                        null, null, null, testMode,
                        mBinderTransactionTrackingEnabled, enableTrackAllocation,
                        isRestrictedBackupMode || !normalMode, app.persistent,
                        new Configuration(getGlobalConfiguration()), app.compat,
                        getCommonServicesLocked(app.isolated),
                        mCoreSettingsObserver.getCoreSettingsLocked(),
                        buildSerial);
        if (normalMode) {
          try {
              if (mStackSupervisor.attachApplicationLocked(app)) {
                didSomething = true;
              }
          } catch (Exception e) {
        }
      }
       return true;
}
boolean attachApplicationLocked(ProcessRecord app) throws RemoteException {
    if (realStartActivityLocked(hr, app, true, true)) {
            didSomething = true;
    }
}
final boolean realStartActivityLocked(ActivityRecord r,
                                      ProcessRecord app, boolean andResume, boolean checkConfig)
        throws RemoteException {
    app.thread.scheduleLaunchActivity(new Intent(r.intent), r.appToken,
                System.identityHashCode(r), r.info, new Configuration(mService.mConfiguration),
                r.compat, r.launchedFromPackage, r.task.voiceInteractor, app.repProcState,
                r.icicle, r.persistentState, results, newIntents, !andResume,
                mService.isNextTransitionForward(), profilerInfo);
}

从这里我们可以看到又回调到了ActivityThread的scheduleLaunchActivity方法。

第三阶段：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) {
            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);
        }
// TODO final H mH = new H();
private void sendMessage(int what, Object obj, int arg1, int arg2, boolean async) {
        Message msg = Message.obtain();
        msg.what = what;
        msg.obj = obj;
        msg.arg1 = arg1;
        msg.arg2 = arg2;
        mH.sendMessage(msg);
    }
// TODO 里面存在一个名为H的handler，这里只拷出部分相关代码
public void handleMessage(Message msg) {
            switch (msg.what) {
                case LAUNCH_ACTIVITY: {
                    final ActivityClientRecord r = (ActivityClientRecord) msg.obj;
                    r.packageInfo = getPackageInfoNoCheck(
                            r.activityInfo.applicationInfo, r.compatInfo);
                    handleLaunchActivity(r, null, "LAUNCH_ACTIVITY");
                } break;
}
private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) {
        // TODO 最终调用activity的onCreate方法
        Activity a = performLaunchActivity(r, customIntent);
        if (a != null) {
            // TODO 最终调用activity的onResume方法
            handleResumeActivity(r.token, false, r.isForward,
                    !r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason);
        } else {
        }
    }
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
        ...
        mInstrumentation.callActivityOnCreate(activity, r.state);
        ...
    }

public void callActivityOnCreate(Activity activity, Bundle icicle) {
        prePerformCreate(activity);
        activity.performCreate(icicle);
        postPerformCreate(activity);
    }
final void performCreate(Bundle icicle) {
        restoreHasCurrentPermissionRequest(icicle);
        onCreate(icicle);
        mActivityTransitionState.readState(icicle);
        performCreateCommon();
    }

部分代码已经在图中注释了，其他的生命周期和onCreate是相似的，就不多说了，可以自己查看源码。