Android Activity的启动过程源码解析

前言

Activity是Android中一个很重要的概念，堪称四大组件之首，关于Activity有很多内容，比如生命周期和启动Flags，这二者想要说清楚，恐怕又要写两篇长文，更何况分析它们的源码呢。不过本文的侧重点不是它们，我要介绍的是一个Activity典型的启动过程，本文会从源码的角度对其进行分析。我们知道，当startActivity被调用的时候，可以启动一个Activity，但是你知道这个Activity是如何被启动的吗？每个Activity也是一个对象，你知道这个对象是啥时候被创建的吗（也就是说它的构造方法是什么时候被调用的）？为什么onCreate是Activity的执行入口？所有的这一切都被系统封装好了，对我们来说是透明的，我们使用的时候仅仅是传递一个intent然后startActivity就可以达到目的了，不过，阅读了本文以后，你将会了解它的背后到底做了哪些事情。在分析之前，我先介绍几个类：

1. Activity：这个大家都熟悉，startActivity方法的真正实现在Activity中
2. Instrumentation：用来辅助Activity完成启动Activity的过程
3. ActivityThread（包含ApplicationThread + ApplicationThreadNative + IApplicationThread）：真正启动Activity的实现都在这里

源码分析

首先看入口

code：Activity#startActivity

@Override
public void startActivity(Intent intent) {
 startActivity(intent, null);
}

@Override
public void startActivity(Intent intent, Bundle options) {
 if (options != null) {
  startActivityForResult(intent, -1, options);
 } else {
  // Note we want to go through this call for compatibility with
  // applications that may have overridden the method.
  startActivityForResult(intent, -1);
 }
}

public void startActivityForResult(Intent intent, int requestCode) {
 startActivityForResult(intent, requestCode, null);
}

说明：显然，从上往下，最终都是由startActivityForResult来实现的

接着看

code：Activity#startActivityForResult

public void startActivityForResult(Intent intent, int requestCode, Bundle options) {
 //一般的Activity其mParent为null，mParent常用在ActivityGroup中，ActivityGroup已废弃
 if (mParent == null) {
  //这里会启动新的Activity，核心功能都在mMainThread.getApplicationThread()中完成
  Instrumentation.ActivityResult ar =
   mInstrumentation.execStartActivity(
    this, mMainThread.getApplicationThread(), mToken, this,
    intent, requestCode, options);
  if (ar != null) {
   //发送结果，即onActivityResult会被调用
   mMainThread.sendActivityResult(
    mToken, mEmbeddedID, requestCode, ar.getResultCode(),
    ar.getResultData());
  }
  if (requestCode >= 0) {
   // If this start is requesting a result, we can avoid making
   // the activity visible until the result is received. Setting
   // this code during onCreate(Bundle savedInstanceState) or onResume() will keep the
   // activity hidden during this time, to avoid flickering.
   // This can only be done when a result is requested because
   // that guarantees we will get information back when the
   // activity is finished, no matter what happens to it.
   mStartedActivity = true;
  }

  final View decor = mWindow != null ? mWindow.peekDecorView() : null;
  if (decor != null) {
   decor.cancelPendingInputEvents();
  }
  // TODO Consider clearing/flushing other event sources and events for child windows.
 } else {
  //在ActivityGroup内部的Activity调用startActivity的时候会走到这里，内部处理逻辑和上面是类似的
  if (options != null) {
   mParent.startActivityFromChild(this, intent, requestCode, options);
  } else {
   // Note we want to go through this method for compatibility with
   // existing applications that may have overridden it.
   mParent.startActivityFromChild(this, intent, requestCode);
  }
 }
}

说明：上述代码关键点都有注释了，可以发现，真正打开activity的实现在Instrumentation的execStartActivity方法中，去看看

code：Instrumentation#execStartActivity

public ActivityResult execStartActivity(
  Context who, IBinder contextThread, IBinder token, Activity target,
  Intent intent, int requestCode, Bundle options) {
 //核心功能在这个whoThread中完成，其内部scheduleLaunchActivity方法用于完成activity的打开
 IApplicationThread whoThread = (IApplicationThread) contextThread;
 if (mActivityMonitors != null) {
  synchronized (mSync) {
   //先查找一遍看是否存在这个activity
   final int N = mActivityMonitors.size();
   for (int i=0; i<N; i++) {
    final ActivityMonitor am = mActivityMonitors.get(i);
    if (am.match(who, null, intent)) {
     //如果找到了就跳出循环
     am.mHits++;
     //如果目标activity无法打开，直接return
     if (am.isBlocking()) {
      return requestCode >= 0 ? am.getResult() : null;
     }
     break;
    }
   }
  }
 }
 try {
  intent.migrateExtraStreamToClipData();
  intent.prepareToLeaveProcess();
  //这里才是真正打开activity的地方，核心功能在whoThread中完成。
  int result = ActivityManagerNative.getDefault()
   .startActivity(whoThread, who.getBasePackageName(), intent,
     intent.resolveTypeIfNeeded(who.getContentResolver()),
     token, target != null ? target.mEmbeddedID : null,
     requestCode, 0, null, null, options);
  //这个方法是专门抛异常的，它会对结果进行检查，如果无法打开activity，
  //则抛出诸如ActivityNotFoundException类似的各种异常
  checkStartActivityResult(result, intent);
 } catch (RemoteException e) {
 }
 return null;
}

说明：我想再说一下这个方法checkStartActivityResult，它也专业抛异常的，看代码，相信大家对下面的异常信息不陌生吧，就是它干的，其中最熟悉的非Unable to find explicit activity class莫属了，如果你在xml中没有注册目标activity，此异常将会抛出。

/*package*/ static void checkStartActivityResult(int res, Object intent) {
 if (res >= ActivityManager.START_SUCCESS) {
  return;
 }

 switch (res) {
  case ActivityManager.START_INTENT_NOT_RESOLVED:
  case ActivityManager.START_CLASS_NOT_FOUND:
   if (intent instanceof Intent && ((Intent)intent).getComponent() != null)
    throw new ActivityNotFoundException(
      "Unable to find explicit activity class "
      + ((Intent)intent).getComponent().toShortString()
      + "; have you declared this activity in your AndroidManifest.xml?");
   throw new ActivityNotFoundException(
     "No Activity found to handle " + intent);
  case ActivityManager.START_PERMISSION_DENIED:
   throw new SecurityException("Not allowed to start activity "
     + intent);
  case ActivityManager.START_FORWARD_AND_REQUEST_CONFLICT:
   throw new AndroidRuntimeException(
     "FORWARD_RESULT_FLAG used while also requesting a result");
  case ActivityManager.START_NOT_ACTIVITY:
   throw new IllegalArgumentException(
     "PendingIntent is not an activity");
  default:
   throw new AndroidRuntimeException("Unknown error code "
     + res + " when starting " + intent);
 }
}

接下来我们要去看看IApplicationThread，因为核心功能由其内部的scheduleLaunchActivity方法来完成，由于IApplicationThread是个接口，所以，我们需要找到它的实现类，我已经帮大家找到了，它就是ActivityThread中的内部类ApplicationThread，看下它的继承关系：

private class ApplicationThread extends ApplicationThreadNative;

public abstract class ApplicationThreadNative extends Binder implements IApplicationThread;

可以发现，ApplicationThread还是间接实现了IApplicationThread接口，先看下这个类的结构

看完ApplicationThread的大致结构，我们应该能够猜测到，Activity的生命周期中的resume、newIntent、pause、stop等事件都是由它触发的，事实上，的确是这样的。这里，我们为了说明问题，仅仅看scheduleLaunchActivity方法

code：ApplicationThread#scheduleLaunchActivity

public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,
  ActivityInfo info, Configuration curConfig, CompatibilityInfo compatInfo,
  int procState, Bundle state, List<ResultInfo> pendingResults,
  List<Intent> pendingNewIntents, boolean notResumed, boolean isForward,
  String profileName, ParcelFileDescriptor profileFd, boolean autoStopProfiler) {

 updateProcessState(procState, false);

 ActivityClientRecord r = new ActivityClientRecord();

 r.token = token;
 r.ident = ident;
 r.intent = intent;
 r.activityInfo = info;
 r.compatInfo = compatInfo;
 r.state = state;

 r.pendingResults = pendingResults;
 r.pendingIntents = pendingNewIntents;

 r.startsNotResumed = notResumed;
 r.isForward = isForward;

 r.profileFile = profileName;
 r.profileFd = profileFd;
 r.autoStopProfiler = autoStopProfiler;

 updatePendingConfiguration(curConfig);

 queueOrSendMessage(H.LAUNCH_ACTIVITY, r);
}

说明：上述代码很好理解，构造一个activity记录，然后发送一个消息，所以，我们要看看Handler是如何处理这个消息的，现在转到这个Handler，它有个很短的名字叫做H

code：ActivityThread#H

//这个类太长，我只帖出了我们用到的部分
private class H extends Handler {

 public void handleMessage(Message msg) {
  if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
  switch (msg.what) {
   //这里处理LAUNCH_ACTIVITY消息类型
   case LAUNCH_ACTIVITY: {
    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
    ActivityClientRecord r = (ActivityClientRecord)msg.obj;

    r.packageInfo = getPackageInfoNoCheck(
      r.activityInfo.applicationInfo, r.compatInfo);
    //这里处理startActivity消息
    handleLaunchActivity(r, null);
    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;
   case PAUSE_ACTIVITY:
    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityPause");
    handlePauseActivity((IBinder)msg.obj, false, msg.arg1 != 0, msg.arg2);
    maybeSnapshot();
    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
    break;
   ...
  }
}

说明：看来还要看handleLaunchActivity

code：ActivityThread#handleLaunchActivity

private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent) {
 // If we are getting ready to gc after going to the background, well
 // we are back active so skip it.
 unscheduleGcIdler();

 if (r.profileFd != null) {
  mProfiler.setProfiler(r.profileFile, r.profileFd);
  mProfiler.startProfiling();
  mProfiler.autoStopProfiler = r.autoStopProfiler;
 }

 // Make sure we are running with the most recent config.
 handleConfigurationChanged(null, null);

 if (localLOGV) Slog.v(
  TAG, "Handling launch of " + r);
 //终于到底了，大家都有点不耐烦了吧，从方法名可以看出，
 //performLaunchActivity真正完成了activity的调起,
 //同时activity会被实例化，并且onCreate会被调用
 Activity a = performLaunchActivity(r, customIntent);

 if (a != null) {
  r.createdConfig = new Configuration(mConfiguration);
  Bundle oldState = r.state;
  //看到没，目标activity的onResume会被调用
  handleResumeActivity(r.token, false, r.isForward,
    !r.activity.mFinished && !r.startsNotResumed);

  if (!r.activity.mFinished && r.startsNotResumed) {
   // The activity manager actually wants this one to start out
   // paused, because it needs to be visible but isn't in the
   // foreground. We accomplish this by going through the
   // normal startup (because activities expect to go through
   // onResume() the first time they run, before their window
   // is displayed), and then pausing it. However, in this case
   // we do -not- need to do the full pause cycle (of freezing
   // and such) because the activity manager assumes it can just
   // retain the current state it has.
   try {
    r.activity.mCalled = false;
    //同时，由于新activity被调起了，原activity的onPause会被调用
    mInstrumentation.callActivityOnPause(r.activity);
    // We need to keep around the original state, in case
    // we need to be created again. But we only do this
    // for pre-Honeycomb apps, which always save their state
    // when pausing, so we can not have them save their state
    // when restarting from a paused state. For HC and later,
    // we want to (and can) let the state be saved as the normal
    // part of stopping the activity.
    if (r.isPreHoneycomb()) {
     r.state = oldState;
    }
    if (!r.activity.mCalled) {
     throw new SuperNotCalledException(
      "Activity " + r.intent.getComponent().toShortString() +
      " did not call through to super.onPause()");
    }

   } catch (SuperNotCalledException e) {
    throw e;

   } catch (Exception e) {
    if (!mInstrumentation.onException(r.activity, e)) {
     throw new RuntimeException(
       "Unable to pause activity "
       + r.intent.getComponent().toShortString()
       + ": " + e.toString(), e);
    }
   }
   r.paused = true;
  }
 } else {
  // If there was an error, for any reason, tell the activity
  // manager to stop us.
  try {
   ActivityManagerNative.getDefault()
    .finishActivity(r.token, Activity.RESULT_CANCELED, null);
  } catch (RemoteException ex) {
   // Ignore
  }
 }
}

说明：关于原activity和新activity之间的状态同步，如果大家感兴趣可以自己研究下，因为逻辑太复杂，我没法把所有问题都说清楚，否则就太深入细节而淹没了整体逻辑，研究源码要的就是清楚整体逻辑。下面看最后一个方法，这个方法是activity的启动过程的真正实现。

code：ActivityThread#performLaunchActivity

private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
 // System.out.println("##### [" + System.currentTimeMillis() + "] ActivityThread.performLaunchActivity(" + r + ")");

 ActivityInfo aInfo = r.activityInfo;
 if (r.packageInfo == null) {
  r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
    Context.CONTEXT_INCLUDE_CODE);
 }
 //首先从intent中解析出目标activity的启动参数
 ComponentName component = r.intent.getComponent();
 if (component == null) {
  component = r.intent.resolveActivity(
   mInitialApplication.getPackageManager());
  r.intent.setComponent(component);
 }

 if (r.activityInfo.targetActivity != null) {
  component = new ComponentName(r.activityInfo.packageName,
    r.activityInfo.targetActivity);
 }

 Activity activity = null;
 try {
  java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
  //用ClassLoader（类加载器）将目标activity的类通过类名加载进来并调用newInstance来实例化一个对象
  //其实就是通过Activity的无参构造方法来new一个对象，对象就是在这里new出来的。
  activity = mInstrumentation.newActivity(
    cl, component.getClassName(), r.intent);
  StrictMode.incrementExpectedActivityCount(activity.getClass());
  r.intent.setExtrasClassLoader(cl);
  if (r.state != null) {
   r.state.setClassLoader(cl);
  }
 } catch (Exception e) {
  if (!mInstrumentation.onException(activity, e)) {
   throw new RuntimeException(
    "Unable to instantiate activity " + component
    + ": " + e.toString(), e);
  }
 }

 try {
  Application app = r.packageInfo.makeApplication(false, mInstrumentation);

  if (localLOGV) Slog.v(TAG, "Performing launch of " + r);
  if (localLOGV) Slog.v(
    TAG, r + ": app=" + app
    + ", appName=" + app.getPackageName()
    + ", pkg=" + r.packageInfo.getPackageName()
    + ", comp=" + r.intent.getComponent().toShortString()
    + ", dir=" + r.packageInfo.getAppDir());

  if (activity != null) {
   Context appContext = createBaseContextForActivity(r, activity);
   CharSequence id="codetool">