Why does "dumpsys activity top" list "fragment" in its output while I did not use fragment?

The SecondActivity only has simple layout which is set as ContentView in its onCreate() method:

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_second);
}

And the activity_second.xml is:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".SecondActivity" />

But when I use dumpsys to dump the top activity, it lists an unknown Fragment ("ReportFragment") in output:

$ adb shell dumpsys activity top
TASK xxx.lifecycleapplication id=4
  ACTIVITY xxx.lifecycleapplication/.SecondActivity 538274ac pid=1519
    Local Activity 5368915c State:
      mResumed=true mStopped=false mFinished=false
      mLoadersStarted=true
      mChangingConfigurations=false
      mCurrentConfig={1.0 310mcc270mnc en_US sw384dp w384dp h567dp nrml port finger qwerty/v/v dpad/v s.5}
    Active Fragments in 536891f0:
      #0: ReportFragment{5369f99c #0 androidx.lifecycle.LifecycleDispatcher.report_fragment_tag}
        mFragmentId=#0 mContainerId=#0 mTag=androidx.lifecycle.LifecycleDispatcher.report_fragment_tag
        mState=5 mIndex=0 mWho=android:fragment:0 mBackStackNesting=0
        mAdded=true mRemoving=false mResumed=true mFromLayout=false mInLayout=false
        mHidden=false mDetached=false mMenuVisible=true mHasMenu=false
        mRetainInstance=false mRetaining=false mUserVisibleHint=true
        mFragmentManager=FragmentManager{536891f0 in SecondActivity{5368915c}}
        mActivity=xxx.lifecycleapplication.SecondActivity@5368915c
    Added Fragments:
      #0: ReportFragment{5369f99c #0 androidx.lifecycle.LifecycleDispatcher.report_fragment_tag}
    FragmentManager misc state:
      mCurState=5 mStateSaved=false mDestroyed=false
    Local FragmentActivity 5368915c State:
      mCreated=true mResumed=true mStopped=false    FragmentManager misc state:
      mHost=androidx.fragment.app.FragmentActivity$HostCallbacks@5368b814
      mContainer=androidx.fragment.app.FragmentActivity$HostCallbacks@5368b814
      mCurState=4 mStateSaved=false mStopped=false mDestroyed=false

Why output shows ReportFragment in "Active Fragments" and "Added Fragments" while I has not use any Fragment?

Solution

This line is the clue:

mTag=androidx.lifecycle.LifecycleDispatcher.report_fragment_tag

The ReportFragment is an implementation detail used by the lifecycle-process and the ProcessLifecycleOwner for tracking whether your app has any started activity. Prior to API 29, it uses a framework Fragment for tracking this across all activities (whether they are AppCompatActivity or regular Activity subclasses).