IssueTracker

I found the root cause! The fragment savedstate class uses a null class loader. Come on, that's Android 101! The correct creator should be:
        public static final Parcelable.Creator<SavedState> CREATOR
                = new Parcelable.Creator<SavedState>() {
            @Override
            public SavedState createFromParcel(Parcel in) {
                return new SavedState(in, getClass().getClassLoader()); // <------
            }

            @Override
            public SavedState[] newArray(int size) {
                return new SavedState[size];
            }
        };

For wondering how to fix this, reflection will do the trick:
        // TODO https://issuetracker.google.com/issues/74354091
        val fragmentStateField: Field = Fragment.SavedState::class.java
                .getDeclaredField("mState").apply {
                    isAccessible = true
                }

            // ...
            (fragmentStateField.get(savedState) as Bundle).classLoader = javaClass.classLoader

Thank you for reporting this issue. For us to further investigate this issue, please provide the following additional information:

Android bug report
After reproducing the issue, press the volume up, volume down, and power button simultaneously. This will capture a bug report on your device in the “bug reports” directory. Attach the bug report  file to this issue.

Alternate method:
After reproducing the issue, navigate to developer settings, ensure ‘USB debugging’ is enabled, then enable ‘Bug report shortcut’. To take bug report, hold the power button and select the ‘Take bug report’ option.

Please provide the information requested in comment #3 to investigate this issue further.

I think this a duplicate of https://issuetracker.google.com/issues/74607887 you can find more information there.

nabrozidhs@, can you please provide the android bug report to confirm that is duplicate or not.

You can see it on the issue link I posted on the previous email. Checking the stack trace both go through a ViewPager/FragmentStatePagerAdapter before failing on the same issue.

Here, I have a simple sample which reproduces the issue: https://github.com/runningcode/classnotfound
Run `./gradlew connectedCheck` to reproduce.

The manual repro steps are as follows:
open an activity (with button)
press back
switch to 3rd tab
ppen activity again (with button)
Press back
switch back to 1st tab

This is very urgent to us. We are getting a lot of crashes because of this issue on support lib 27.1.0.

To clarify, the repro steps are only 100% reproducible on SDK 19 (Kitkat) but we are seeing it happen in production against all SDK levels.

This bug has been fixed in  Support Library 27.1.1.

this is same in 28.0.2 show again, please help to fix it.thanks

this is same in 28.0.0 show again。

=======

Fatal Exception: android.os.BadParcelableException:ClassNotFoundException when unmarshalling: android.support.v4.app.FragmentManagerState
at android.os.Parcel.readParcelableCreator(Parcel.java:2855)
at android.os.Parcel.readParcelable(Parcel.java:2781)
at android.os.Parcel.readValue(Parcel.java:2684)
at android.os.Parcel.readArrayMapInternal(Parcel.java:3053)
at android.os.BaseBundle.initializeFromParcelLocked(BaseBundle.java:288)
at android.os.BaseBundle.unparcel(BaseBundle.java:232)
at android.os.BaseBundle.size(BaseBundle.java:350)
at android.app.servertransaction.LaunchActivityItem.hashCode(LaunchActivityItem.java:193)
at java.util.AbstractList.hashCode(AbstractList.java:541)
at java.util.Objects.hashCode(Objects.java:98)
at android.app.servertransaction.ClientTransaction.hashCode(ClientTransaction.java:236)
at java.lang.Object.toString(Object.java:273)
at java.lang.String.valueOf(String.java:2896)
at java.lang.StringBuilder.append(StringBuilder.java:132)
at android.os.Message.toStringLite(Message.java:533)
at android.os.Looper.loop(Looper.java:261)
at android.app.ActivityThread.main(ActivityThread.java:7154)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:576)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:878)

Same crash info as above here in Support Library 27.1.1.    https://issuetracker.google.com/issues/147449674

The fix is to migrate the app to use AndroidX, instead of using the original support libraries.