Android深入四大组件（一）应用程序启动过程（后篇）

本文首发于微信公众号「后厂技术官」

1.ActivityManageService到ApplicationThread的调用流程

AMS的startActivity方法中return了startActivityAsUser方法：

frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java

@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);
     return mActivityStarter.startActivityMayWait(caller, -1, callingPackage, intent,
             resolvedType, null, null, resultTo, resultWho, requestCode, startFlags,
             profilerInfo, null, null, bOptions, false, userId, null, null);

startActivityAsUser方法中又return了mActivityStarter的startActivityMayWait方法，代码如下所示。
frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java

final int startActivityMayWait(IApplicationThread caller, int callingUid,
         String callingPackage, Intent intent, String resolvedType,
         IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
         IBinder resultTo, String resultWho, int requestCode, int startFlags,
         ProfilerInfo profilerInfo, IActivityManager.WaitResult outResult, Configuration config,
         Bundle bOptions, boolean ignoreTargetSecurity, int userId,
         IActivityContainer iContainer, TaskRecord inTask) {
   ...
         int res = startActivityLocked(caller, intent, ephemeralIntent, resolvedType,
                 aInfo, rInfo, voiceSession, voiceInteractor,
                 resultTo, resultWho, requestCode, callingPid,
                 callingUid, callingPackage, realCallingPid, realCallingUid, startFlags,
                 options, ignoreTargetSecurity, componentSpecified, outRecord, container,
                 inTask);

     ...
         return res;
     }
 }

又调用了startActivityLocked方法，代码如下所示。
frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java

final int startActivityLocked(IApplicationThread caller, Intent intent, Intent ephemeralIntent,
           String resolvedType, ActivityInfo aInfo, ResolveInfo rInfo,
           IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
           IBinder resultTo, String resultWho, int requestCode, int callingPid, int callingUid,
           String callingPackage, int realCallingPid, int realCallingUid, int startFlags,
           ActivityOptions options, boolean ignoreTargetSecurity, boolean componentSpecified,
           ActivityRecord[] outActivity, ActivityStackSupervisor.ActivityContainer container,
           TaskRecord inTask) {
      ...

       doPendingActivityLaunchesLocked(false);

      ...
       return err;
   }

startActivityLocked函数代码非常多，我们只需要关注doPendingActivityLaunchesLocked方法，代码如下所示。
frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java

final void doPendingActivityLaunchesLocked(boolean doResume) {
       while (!mPendingActivityLaunches.isEmpty()) {
           final PendingActivityLaunch pal = mPendingActivityLaunches.remove(0);
           final boolean resume = doResume && mPendingActivityLaunches.isEmpty();
           try {
               final int result = startActivityUnchecked(
                       pal.r, pal.sourceRecord, null, null, pal.startFlags, resume, null, null);
               postStartActivityUncheckedProcessing(
                       pal.r, result, mSupervisor.mFocusedStack.mStackId, mSourceRecord,
                       mTargetStack);
           } catch (Exception e) {
               Slog.e(TAG, "Exception during pending activity launch pal=" + pal, e);
               pal.sendErrorResult(e.getMessage());
           }
       }
   }

接着又调用startActivityUnchecked方法：
frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java

private int startActivityUnchecked(final ActivityRecord r, ActivityRecord sourceRecord,
           IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
           int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask) {
     ...  
        mSupervisor.resumeFocusedStackTopActivityLocked();  
     ... 
       return START_SUCCESS;
   }

startActivityUnchecked方法中调用了ActivityStackSupervisor类型的mSupervisor的resumeFocusedStackTopActivityLocked方法，如下所示。
frameworks/base/services/core/java/com/android/server/am/ActivityStackSupervisor.java

boolean resumeFocusedStackTopActivityLocked(
           ActivityStack targetStack, ActivityRecord target, ActivityOptions targetOptions) {
       if (targetStack != null && isFocusedStack(targetStack)) {
           return targetStack.resumeTopActivityUncheckedLocked(target, targetOptions);
       }
       final ActivityRecord r = mFocusedStack.topRunningActivityLocked();
       if (r == null || r.state != RESUMED) {
           mFocusedStack.resumeTopActivityUncheckedLocked(null, null);//1
       }
       return false;
   }

在注释1处又调用了ActivityStack类型mFocusedStack的resumeTopActivityUncheckedLocked方法：
frameworks/base/services/core/java/com/android/server/am/ActivityStack.java

 boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {
...
        try {
           ...
            result = resumeTopActivityInnerLocked(prev, options);
        } finally {
            mStackSupervisor.inResumeTopActivity = false;
        }
        return result;
    }

紧接着查看ActivityStack的resumeTopActivityInnerLocked方法：
frameworks/base/services/core/java/com/android/server/am/ActivityStack.java

private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {
      ...
           mStackSupervisor.startSpecificActivityLocked(next, true, true);
       }
        if (DEBUG_STACK) mStackSupervisor.validateTopActivitiesLocked();
       return true; 

resumeTopActivityInnerLocked方法代码非常多，我们只需要关注调用了ActivityStackSupervisor类型mStackSupervisor的startSpecificActivityLocked方法，代码如下所示。
frameworks/base/services/core/java/com/android/server/am/ActivityStackSupervisor.java

void startSpecificActivityLocked(ActivityRecord r,
          boolean andResume, boolean checkConfig) {
      ProcessRecord app = mService.getProcessRecordLocked(r.processName,
              r.info.applicationInfo.uid, true);
      r.task.stack.setLaunchTime(r);
      if (app != null && app.thread != null) {//1
          try {
              if ((r.info.flags&ActivityInfo.FLAG_MULTIPROCESS) == 0
                      || !"android".equals(r.info.packageName)) {
                  app.addPackage(r.info.packageName, r.info.applicationInfo.versionCode,
                          mService.mProcessStats);
              }
              realStartActivityLocked(r, app, andResume, checkConfig);//2
              return;
          } catch (RemoteException e) {
              Slog.w(TAG, "Exception when starting activity "
                      + r.intent.getComponent().flattenToShortString(), e);
          }
      }
      mService.startProcessLocked(r.processName, r.info.applicationInfo, true, 0,
              "activity", r.intent.getComponent(), false, false, true);
  }

在注释1处如果当前Activity所在的Application运行的话，会执行注释2处的代码。realStartActivityLocked方法的代码如下所示。
frameworks/base/services/core/java/com/android/server/am/ActivityStackSupervisor.java

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),
                  new Configuration(task.mOverrideConfig), r.compat, r.launchedFromPackage,
                  task.voiceInteractor, app.repProcState, r.icicle, r.persistentState, results,
                  newIntents, !andResume, mService.isNextTransitionForward(), profilerInfo);

  ...      

      return true;
  }

这里的 app.thread指的是IApplicationThread，它的实现是ActivityThread的内部类ApplicationThread，其中ApplicationThread继承了ApplicationThreadNative，而ApplicationThreadNative继承了Binder并实现了IApplicationThread接口。
讲到这里再给出ActivityManageService到ApplicationThread调用过程的时序图，如下图所示。

2.ActivityThread启动Activity

Android应用程序进程启动过程（前篇）这篇文章中提到过ActivityThread，在应用程序进程启动时会创建ActivityThread实例。ActivityThread作为应用程序进程的核心类，它是如何启动应用程序（Activity）的呢？我们接着往下看。
根据上文接着查看ApplicationThread的scheduleLaunchActivity方法，代码如下所示。
frameworks/base/core/java/android/app/ActivityThread.java

@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方法的代码如下所示。
frameworks/base/core/java/android/app/ActivityThread.java

private void sendMessage(int what, Object obj, int arg1, int arg2, boolean async) {
   ...
      mH.sendMessage(msg);
  }

这里mH指的是H，它是ActivityThread的内部类并继承Handler，H的代码如下所示。
frameworks/base/core/java/android/app/ActivityThread.java

private class H extends Handler {
      public static final int LAUNCH_ACTIVITY         = 100;
      public static final int PAUSE_ACTIVITY          = 101;
...
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

private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) {
    ...
      Activity a = performLaunchActivity(r, customIntent);//1
      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);//2

          if (!r.activity.mFinished && r.startsNotResumed) {      
              performPauseActivityIfNeeded(r, reason);
              if (r.isPreHoneycomb()) {
                  r.state = oldState;
              }
          }
      } else {
          try {
              ActivityManagerNative.getDefault()
                  .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ActivityManager停止启动Activity。来查看performLaunchActivity方法做了什么：
frameworks/base/core/java/android/app/ActivityThread.java

  private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
  ...
        ActivityInfo aInfo = r.activityInfo;//1
        if (r.packageInfo == null) {
            r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
                    Context.CONTEXT_INCLUDE_CODE);//2
        }
        ComponentName component = r.intent.getComponent();//3
      ...
        Activity activity = null;
        try {
            java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
            activity = mInstrumentation.newActivity(
                    cl, component.getClassName(), r.intent);//4
           ...
            }
        } catch (Exception e) {
         ...
        }
        try {
            Application app = r.packageInfo.makeApplication(false, mInstrumentation);//5

        ...
            if (activity != null) {
                Context appContext = createBaseContextForActivity(r, activity);//6
         ...
                }
                /**
                *7
                */
                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 (r.isPersistable()) {
                    mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);//8
                } else {
                    mInstrumentation.callActivityOnCreate(activity, r.state);
                }
                ...
        }
        return activity;
}        

注释1处用来获取ActivityInfo，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5处用来创建Application，makeApplication方法内部会调用Application的onCreate方法。注释6处用来创建要启动Activity的上下文环境。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：
frameworks/base/core/java/android/app/Instrumentation.java

public void callActivityOnCreate(Activity activity, Bundle icicle,
         PersistableBundle persistentState) {
     prePerformCreate(activity);
     activity.performCreate(icicle, persistentState);//1
     postPerformCreate(activity);
 }

注释1处调用了Activity的performCreate方法，代码如下所示。
frameworks/base/core/java/android/app/Activity.java

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

performCreate方法中会调用Activity的onCreate方法，这样Activity就启动了，即应用程序就启动了。
应用程序启动过程就讲到这里，最后附上ActivityThread启动Activity的时序图。