详解Android Activity的启动流程

Launcher.java
 public boolean startActivitySafely(View v, Intent intent, ItemInfo item) {
  ...
  //标记在新的栈启动
  intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
  ...
  startActivity(intent, optsBundle);
  ...
 }

Activity.java
 @Override
 public void startActivity(Intent intent) {
  this.startActivity(intent, null);
 }

 @Override
 public void startActivity(Intent intent, @Nullable Bundle options) {
  ...
  if (options != null) {
   //-1为requestCode表明不需要知道是否启动成功
   startActivityForResult(intent, -1, options);
  } else {
   startActivityForResult(intent, -1);
  }
 }

 public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
   @Nullable Bundle options) {
   ...
   Instrumentation.ActivityResult ar =
    mInstrumentation.execStartActivity(
     this, mMainThread.getApplicationThread(), mToken,this,intent, requestCode, options);
   ...
 }

Instrumentation.java
public ActivityResult execStartActivity(
   Context who, IBinder contextThread, IBinder token, Activity target,
   Intent intent, int requestCode, Bundle options) {
 ...
	int result = ActivityTaskManager.getService().startActivity(whoThread,who.getBasePackageName(),
  who.getAttributionTag(),intent,intent.resolveTypeIfNeeded(who.getContentResolver()),
  token,target != null ? target.mEmbeddedID : null, requestCode, 0, null, options); 
 ...
}

ActivityTaskManager.java
public static IActivityTaskManager getService() {
  return IActivityTaskManagerSingleton.get();
}
 
private static final Singleton<IActivityTaskManager> IActivityTaskManagerSingleton =
   new Singleton<IActivityTaskManager>() {
    @Override
    protected IActivityTaskManager create() {
     final IBinder b = ServiceManager.getService(Context.ACTIVITY_TASK_SERVICE);
     return IActivityTaskManager.Stub.asInterface(b);
    }
    }
};

ActivityTaskManagerService.java
 @Override
 public final int startActivity(IApplicationThread caller, String callingPackage,
   String callingFeatureId, Intent intent, String resolvedType, IBinder resultTo,
   String resultWho, int requestCode, int startFlags, ProfilerInfo profilerInfo,
   Bundle bOptions) {
  return startActivityAsUser(caller, callingPackage, callingFeatureId, intent, resolvedType,
    resultTo, resultWho, requestCode, startFlags, profilerInfo, bOptions,
    UserHandle.getCallingUserId());
 }
 
 @Override
 public int startActivityAsUser(IApplicationThread caller, String callingPackage,
   String callingFeatureId, Intent intent, String resolvedType, IBinder resultTo,
   String resultWho, int requestCode, int startFlags, ProfilerInfo profilerInfo,
   Bundle bOptions, int userId) {
  return startActivityAsUser(caller, callingPackage, callingFeatureId, intent, resolvedType,
    resultTo, resultWho, requestCode, startFlags, profilerInfo, bOptions, userId,
    true /*validateIncomingUser*/);
 }
 
 private int startActivityAsUser(IApplicationThread caller, String callingPackage,
   @Nullable String callingFeatureId, Intent intent, String resolvedType,
   IBinder resultTo, String resultWho, int requestCode, int startFlags,
   ProfilerInfo profilerInfo, Bundle bOptions, int userId, boolean validateIncomingUser) {
	 ...
  userId = getActivityStartController().checkTargetUser(userId, validateIncomingUser,
    Binder.getCallingPid(), Binder.getCallingUid(), "startActivityAsUser");
    
  return getActivityStartController().obtainStarter(intent, "startActivityAsUser")
    .setCaller(caller)
    .setCallingPackage(callingPackage)
    .setCallingFeatureId(callingFeatureId)
    .setResolvedType(resolvedType)
    .setResultTo(resultTo)
    .setResultWho(resultWho)
    .setRequestCode(requestCode)
    .setStartFlags(startFlags)
    .setProfilerInfo(profilerInfo)
    .setActivityOptions(bOptions)
    .setUserId(userId)
    .execute();
 }
 
 ActivityStarter obtainStarter(Intent intent, String reason) {
  return mFactory.obtain().setIntent(intent).setReason(reason);
 }

ActivityStarter.java
 int execute() {
 ...
 res = executeRequest(mRequest);
 ...
 }
//层层调用会到下面这个方法
ActivityStack.java
 private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {
 ...
 if (mResumedActivity != null) {
  pausing |= startPausingLocked(userLeaving, false , next);
 }
 ...
 mStackSupervisor.startSpecificActivity(next, true, false);
 ...
 }

ActivityStackSupervisor.java
void startSpecificActivity(ActivityRecord r, boolean andResume, boolean checkConfig) {
  // 获取要启动的Activity进程信息
  final WindowProcessController wpc =
    mService.getProcessController(r.processName, r.info.applicationInfo.uid);
  boolean knownToBeDead = false;
  //如果进程存在且有进程中有线程存在 就是启动一个同应用的Activity（普通Activity就在此执行）
  if (wpc != null && wpc.hasThread()) {
   try {
    realStartActivityLocked(r, wpc, andResume, checkConfig);
    return;
   } catch (RemoteException e) {
    Slog.w(TAG, "Exception when starting activity "
        r.intent.getComponent().flattenToShortString(), e);
   }

   // If a dead object exception was thrown -- fall through to
   // restart the application.
   knownToBeDead = true;
  }
	//否则通过AMS向Zygote进程请求创建新的进程
  r.notifyUnknownVisibilityLaunchedForKeyguardTransition();
  final boolean isTop = andResume && r.isTopRunningActivity();
  mService.startProcessAsync(r, knownToBeDead, isTop, isTop ? "top-activity" : "activity");
}

ActivityThread.java
 public static void main(String[] args) {
  ...
  Looper.prepareMainLooper();
	...
  ActivityThread thread = new ActivityThread();
  thread.attach(false, startSeq);
	...
  Looper.loop();
	...
 }
 
 private void attach(boolean system, long startSeq) {
   final IActivityManager mgr = ActivityManager.getService();
   try {
    mgr.attachApplication(mAppThread, startSeq);
   } catch (RemoteException ex) {
    throw ex.rethrowFromSystemServer();
   }
   ...
 }
ActivityManagerService.java
 private boolean attachApplicationLocked(@NonNull IApplicationThread thread,
   int pid, int callingUid, long startSeq) {
   ...
   thread.bindApplication(processName, appInfo, providerList,
      instr2.mClass,
      profilerInfo, instr2.mArguments,
      instr2.mWatcher,
      instr2.mUiAutomationConnection, testMode,
      mBinderTransactionTrackingEnabled, enableTrackAllocation,
      isRestrictedBackupMode || !normalMode, app.isPersistent(),
      new Configuration(app.getWindowProcessController().getConfiguration()),
      app.compat, getCommonServicesLocked(app.isolated),
      mCoreSettingsObserver.getCoreSettingsLocked(),
      buildSerial, autofillOptions, contentCaptureOptions,
      app.mDisabledCompatChanges);
    ...
    didSomething = mAtmInternal.attachApplication(app.getWindowProcessController());
    ...
 }

public final void bindApplication(String processName, ApplicationInfo appInfo,
    ProviderInfoList providerList, ComponentName instrumentationName,
    ProfilerInfo profilerInfo, Bundle instrumentationArgs,
    IInstrumentationWatcher instrumentationWatcher,
    IUiAutomationConnection instrumentationUiConnection, int debugMode,
    boolean enableBinderTracking, boolean trackAllocation,
    boolean isRestrictedBackupMode, boolean persistent, Configuration config,
    CompatibilityInfo compatInfo, Map services, Bundle coreSettings,
    String buildSerial, AutofillOptions autofillOptions,
    ContentCaptureOptions contentCaptureOptions, long[] disabledCompatChanges) {
    	...
    	sendMessage(H.BIND_APPLICATION, data);
 }
    
public void handleMessage(Message msg) {
 switch (msg.what) {
  case BIND_APPLICATION:
  	AppBindData data = (AppBindData)msg.obj;
  	handleBindApplication(data);
  	Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
  	break;
  ...
  }
}

private void handleBindApplication(AppBindData data) {
	...
	mInstrumentation.callApplicationOnCreate(app);
	...
}

ActivityStackSupervisor.java
boolean realStartActivityLocked(ActivityRecord r, WindowProcessController proc,
  boolean andResume, boolean checkConfig) throws RemoteException {
  ...
  final ClientTransaction clientTransaction = ClientTransaction.obtain(
    proc.getThread(), r.appToken);
   final DisplayContent dc = r.getDisplay().mDisplayContent;
   clientTransaction.addCallback(LaunchActivityItem.obtain(new Intent(r.intent),
     System.identityHashCode(r), r.info,
     mergedConfiguration.getGlobalConfiguration(),
     mergedConfiguration.getOverrideConfiguration(), r.compat,
     r.launchedFromPackage, task.voiceInteractor, proc.getReportedProcState(),
     r.getSavedState(), r.getPersistentSavedState(), results, newIntents,
     dc.isNextTransitionForward(), proc.createProfilerInfoIfNeeded(),
     r.assistToken, r.createFixedRotationAdjustmentsIfNeeded()));

   final ActivityLifecycleItem lifecycleItem;
   if (andResume) {
    lifecycleItem = ResumeActivityItem.obtain(dc.isNextTransitionForward());
   } else {
    lifecycleItem = PauseActivityItem.obtain();
   }
   clientTransaction.setLifecycleStateRequest(lifecycleItem);
   //执行clientTransaction
   mService.getLifecycleManager().scheduleTransaction(clientTransaction);
   ...
}

 void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
  final IApplicationThread client = transaction.getClient();
  transaction.schedule();
  ...
 }

 public void schedule() throws RemoteException {
  mClient.scheduleTransaction(this);
 }

 void scheduleTransaction(ClientTransaction transaction) {
  transaction.preExecute(this);
  sendMessage(ActivityThread.H.EXECUTE_TRANSACTION, transaction);
 }

 class H extends Handler {
   ...
   public void handleMessage(Message msg) {
   ...
   case EXECUTE_TRANSACTION:
    final ClientTransaction transaction = (ClientTransaction) msg.obj;
    mTransactionExecutor.execute(transaction);
    if (isSystem()) {
     transaction.recycle();
    }
    break;
   ...
   } 
   ...
 }

 public void execute(ClientTransactionHandler client, IBinder token,
   PendingTransactionActions pendingActions) {
  ...
  client.handleLaunchActivity(r, pendingActions, null /* customIntent */);
 }

public Activity handleLaunchActivity(ActivityClientRecord r,
   PendingTransactionActions pendingActions, Intent customIntent) {
  ...
  final Activity a = performLaunchActivity(r, customIntent);
  ...
  return a;
}

private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
  ContextImpl appContext = createBaseContextForActivity(r);
  ...
  java.lang.ClassLoader cl = appContext.getClassLoader();
  activity = mInstrumentation.newActivity(cl, component.getClassName(), r.intent);
  ...
  Application app = r.packageInfo.makeApplication(false, mInstrumentation);
  ...
  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,
      r.assistToken);
  ...
  activity.setTheme(theme);
  ...
  mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
}