谈谈app的启动流程

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 pendingResults, List 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。