我的 Activity 中有 5 个 fragment ，其中一个 fragment 一次保持 Activity 状态。单击 recyclerview 项目会打开另一个 fragment 并将当前 fragment 放入后台堆栈。
几天前相同的代码运行良好，但现在每当我单击主页按钮将应用程序置于后台时，应用程序都会抛出 NotSerializableException。我试过将初始化变量放在 onStart 中，然后在 onStop 中给出空值，但这没有用。
fragment 代码:

public class PaperListFragment extends Fragment implements Serializable {

private static final String TAG = "PaperListFragment";
private static final String QUESTIONS_FRAGMENT_TAG = "questions_fragment";
private static final String ADD_PAPER_FRAGMENT_TAG = "add_paper_fragment";

private OnFragmentActiveListener mOnFragmentActiveListener;
private TextView mHeadingText;
private Bundle mOutState;
private FirebaseAuth mAuth;
private DatabaseReference mDatabaseReference;
private ProgressBar mProgressBar;
private OnItemClickListener mOnItemClickListener;
private FloatingActionButton mFab;
private RecyclerView mRecyclerViewPaper;
private ArrayList<Paper> mPaperList = new ArrayList<>();
private Subject mSubject = new Subject();
private Exam mExam = new Exam();

@Nullable
@Override
public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    View rootView = inflater.inflate(R.layout.fragment_recycler_list, container, false);


    mProgressBar = (ProgressBar) rootView.findViewById(R.id.progressbar_news);
    mFab = (FloatingActionButton) rootView.findViewById(R.id.floatingActionButton);
    mProgressBar.setVisibility(View.VISIBLE);
    Log.d(TAG, "onCreateView: Fragment created");
    mAuth = FirebaseAuth.getInstance();
    mDatabaseReference = FirebaseDatabase.getInstance().getReference();
    if (mAuth.getCurrentUser() == null) {
        startActivity(new Intent(getActivity(), LoginActivity.class));
        getActivity().finish();
        return null;
    }

    if (getArguments() != null) {
        mOnFragmentActiveListener = (OnFragmentActiveListener) getArguments().getSerializable(Keys.FRAGMENT_ACTIVE_LISTENER);
        mSubject = (Subject) getArguments().getSerializable(Keys.SUBJECT_KEY);
        mExam = (Exam) getArguments().getSerializable(Keys.EXAMS_KEY);
    }

    mRecyclerViewPaper = (RecyclerView) rootView.findViewById(R.id.recycler_list);
    LinearLayoutManager layoutManager = new LinearLayoutManager(getActivity()) {
        @Override
        public boolean canScrollVertically() {
            return false;
        }
    };
    mRecyclerViewPaper.setLayoutManager(layoutManager);
    Log.d(TAG, "onCreateView: Layout Manager Set.");

    mFab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            startAddPaperFragment();
        }
    });

    mOnItemClickListener = new OnItemClickListener() {
        @Override
        public void onItemClicked(RecyclerView.ViewHolder holder, int position) {
            Log.d(TAG, "onItemClicked: Clicked item position is: "+ position);
            QuestionListFragment questionFragment = new QuestionListFragment();
            questionFragment.setRetainInstance(true);

            startFragment(position, questionFragment, QUESTIONS_FRAGMENT_TAG);

        }

        @Override
        public void OnItemLongClicked(RecyclerView.ViewHolder holder, int position) {

        }
    };

    mHeadingText = (TextView) rootView.findViewById(R.id.heading_textview);
    mHeadingText.setText(mExam.getExam_name()+" > "+ mSubject.getSubject_name());

    if (mOutState != null) {
        mPaperList = (ArrayList<Paper>) mOutState.getSerializable(Keys.PAPER_LIST_KEY);
        updateUI();
    } else {
        updateUIFromDatabase();
    }


    return rootView;
}

private void startFragment(int position, Fragment fragment, String fragmentTag) {
    Paper paper = new Paper();
    if (mPaperList.size() > 0) {
        paper = mPaperList.get(position);
    }
    Bundle args = new Bundle();
    args.putSerializable(Keys.EXAMS_KEY, mExam);
    args.putSerializable(Keys.SUBJECT_KEY, mSubject);
    args.putSerializable(Keys.PAPER, paper);
    args.putSerializable(Keys.FRAGMENT_ACTIVE_LISTENER, mOnFragmentActiveListener);
    fragment.setArguments(args);
    FragmentTransaction fragmentTransaction = getActivity().getSupportFragmentManager().beginTransaction();
    fragmentTransaction.setCustomAnimations(R.anim.slide_in_right, R.anim.slide_out_left, R.anim.slide_in_left, R.anim.slide_out_right);

    fragmentTransaction.replace(R.id.questions_fragment_container, fragment, fragmentTag);
    fragmentTransaction.addToBackStack(fragmentTag);
    fragmentTransaction.commit();
}

private void startAddPaperFragment() {
    AddPaperFragment addPaperFragment = new AddPaperFragment();
    addPaperFragment.setRetainInstance(true);
    startFragment(0, addPaperFragment, ADD_PAPER_FRAGMENT_TAG);
}

private void updateUIFromDatabase() {
    if (getArguments() != null){
        Exam exam = (Exam) getArguments().getSerializable(Keys.EXAMS_KEY);
        Subject subject = (Subject) getArguments().getSerializable(Keys.SUBJECT_KEY);
        DatabaseReference paperReference =
                mDatabaseReference
                .child(Keys.APP_DATA_KEY)
                .child(Keys.EXAM_PAPERS)
                .child(exam.getExam_name())
                .child(subject.getSubject_name());
        Query query = paperReference.orderByChild(Keys.TIME_ADDED);
        query.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                mPaperList.clear();
                for (DataSnapshot paperChild : dataSnapshot.getChildren()) {
                    mPaperList.add(paperChild.getValue(Paper.class));
                }

                updateUI();
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });
    }
}

private void updateUI() {
    PaperRecyclerAdapter adapter = new PaperRecyclerAdapter(
            getActivity(),
            mRecyclerViewPaper,
            mPaperList,
            mOnItemClickListener
    );
    mRecyclerViewPaper.setAdapter(adapter);
    mProgressBar.setVisibility(View.GONE);
}

@Override
public void onResume() {
    super.onResume();
    if (getArguments()!=null){
        mOnFragmentActiveListener.onFragmentActive(
                this,
                "Topics"
        );
    }
}

@Override
public void onPause() {
    super.onPause();
    mOutState = new Bundle();
    mOutState.putSerializable(Keys.PAPER_LIST_KEY, mPaperList);
}

异常:

2018-12-26 17:49:38.344 14834-14834/in.crazybytes.bankmaniaadmin E/AndroidRuntime: FATAL EXCEPTION: main
Process: in.crazybytes.bankmaniaadmin, PID: 14834
java.lang.RuntimeException: Parcelable encountered IOException writing serializable object (name = in.crazybytes.bankmaniaadmin.activities.QuestionsActivity)
    at android.os.Parcel.writeSerializable(Parcel.java:1526)
    at android.os.Parcel.writeValue(Parcel.java:1474)
    at android.os.Parcel.writeArrayMapInternal(Parcel.java:723)
    at android.os.BaseBundle.writeToParcelInner(BaseBundle.java:1408)
    at android.os.Bundle.writeToParcel(Bundle.java:1133)
    at android.os.Parcel.writeBundle(Parcel.java:763)
    at android.support.v4.app.FragmentState.writeToParcel(FragmentState.java:124)
    at android.os.Parcel.writeTypedArray(Parcel.java:1306)
    at android.support.v4.app.FragmentManagerState.writeToParcel(FragmentManager.java:639)
    at android.os.Parcel.writeParcelable(Parcel.java:1495)
    at android.os.Parcel.writeValue(Parcel.java:1401)
    at android.os.Parcel.writeArrayMapInternal(Parcel.java:723)
    at android.os.BaseBundle.writeToParcelInner(BaseBundle.java:1408)
    at android.os.Bundle.writeToParcel(Bundle.java:1133)
    at android.os.Parcel.writeBundle(Parcel.java:763)
    at android.app.ActivityManagerProxy.activityStopped(ActivityManagerNative.java:3697)
    at android.app.ActivityThread$StopInfo.run(ActivityThread.java:3768)
    at android.os.Handler.handleCallback(Handler.java:751)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:154)
    at android.app.ActivityThread.main(ActivityThread.java:6123)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:757)
 Caused by: java.io.NotSerializableException: com.google.firebase.auth.internal.zzj
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1224)
    at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1584)
    at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1549)
    at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1472)
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1218)
    at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1584)
    at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1549)
    at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1472)
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1218)
    at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:346)
    at android.os.Parcel.writeSerializable(Parcel.java:1521)
    at android.os.Parcel.writeValue(Parcel.java:1474) 
    at android.os.Parcel.writeArrayMapInternal(Parcel.java:723) 
    at android.os.BaseBundle.writeToParcelInner(BaseBundle.java:1408) 
    at android.os.Bundle.writeToParcel(Bundle.java:1133) 
    at android.os.Parcel.writeBundle(Parcel.java:763) 
    at android.support.v4.app.FragmentState.writeToParcel(FragmentState.java:124) 
    at android.os.Parcel.writeTypedArray(Parcel.java:1306) 
    at android.support.v4.app.FragmentManagerState.writeToParcel(FragmentManager.java:639) 
    at android.os.Parcel.writeParcelable(Parcel.java:1495) 
    at android.os.Parcel.writeValue(Parcel.java:1401) 
    at android.os.Parcel.writeArrayMapInternal(Parcel.java:723) 
    at android.os.BaseBundle.writeToParcelInner(BaseBundle.java:1408) 
    at android.os.Bundle.writeToParcel(Bundle.java:1133) 
    at android.os.Parcel.writeBundle(Parcel.java:763) 
    at android.app.ActivityManagerProxy.activityStopped(ActivityManagerNative.java:3697) 
    at android.app.ActivityThread$StopInfo.run(ActivityThread.java:3768) 
    at android.os.Handler.handleCallback(Handler.java:751) 
    at android.os.Handler.dispatchMessage(Handler.java:95) 
    at android.os.Looper.loop(Looper.java:154) 
    at android.app.ActivityThread.main(ActivityThread.java:6123) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:867) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:757)

注意:奇怪的是，其中一个 fragment 具有完全相同的代码并托管在同一个 Activity 中，但是当该 fragment 处于 Activity 状态并且应用程序进入后台时，有趣的是应用程序并没有崩溃。

**考试模型类: 包在.crazybytes.bankmaniaadmin.models;

import java.io.Serializable;

public class Exam implements Serializable {
private String mExam_name;
private String mExam_key;
private Long mTime_added;
private int mNum_subjects;
private int mNum_questions;

public Exam(String exam_name, String exam_key, Long time_added, int num_subjects, int num_questions) {
    mExam_name = exam_name;
    mExam_key = exam_key;
    mTime_added = time_added;
    mNum_subjects = num_subjects;
    mNum_questions = num_questions;
}

public Exam() {
}

public String getExam_name() {
    return mExam_name;
}

public void setExam_name(String exam_name) {
    mExam_name = exam_name;
}

public String getExam_key() {
    return mExam_key;
}

public void setExam_key(String exam_key) {
    mExam_key = exam_key;
}

public Long getTime_added() {
    return mTime_added;
}

public void setTime_added(Long time_added) {
    mTime_added = time_added;
}

public int getNum_subjects() {
    return mNum_subjects;
}

public void setNum_subjects(int num_subjects) {
    mNum_subjects = num_subjects;
}

public int getNum_questions() {
    return mNum_questions;
}

public void setNum_questions(int num_questions) {
    mNum_questions = num_questions;
}

纸模型类

package in.crazybytes.bankmaniaadmin.models;

import java.io.Serializable;

public class Paper implements Serializable {
private String mPaper_name;
private String mPaper_key;
private Long mTime_added;
private int mNum_questions;

public Paper(String paper_name, String paper_key, Long time_added, int num_questions) {
    mPaper_name = paper_name;
    mPaper_key = paper_key;
    mTime_added = time_added;
    mNum_questions = num_questions;
}

public Paper() {
}

public String getPaper_key() {
    return mPaper_key;
}

public void setPaper_key(String paper_key) {
    mPaper_key = paper_key;
}

public Long getTime_added() {
    return mTime_added;
}

public void setTime_added(Long time_added) {
    mTime_added = time_added;
}

public int getNum_questions() {
    return mNum_questions;
}

public void setNum_questions(int num_questions) {
    mNum_questions = num_questions;
}

public String getPaper_name() {
    return mPaper_name;
}

public void setPaper_name(String paper_name) {
    mPaper_name = paper_name;
}

主题模型类:

package in.crazybytes.bankmaniaadmin.models;

import java.io.Serializable;

public class Subject implements Serializable {
private String mSubject_name;
private String mSubject_key;
private Long mTime_added;
private int mNum_papers;
private int mNum_questions;

public Subject(String subject_name, String subject_key, Long time_added, int num_papers, int num_questions) {
    mSubject_name = subject_name;
    mSubject_key = subject_key;
    mTime_added = time_added;
    mNum_papers = num_papers;
    mNum_questions = num_questions;
}

public Subject() {
}

public String getSubject_name() {
    return mSubject_name;
}

public void setSubject_name(String subject_name) {
    mSubject_name = subject_name;
}

public String getSubject_key() {
    return mSubject_key;
}

public void setSubject_key(String subject_key) {
    mSubject_key = subject_key;
}

public Long getTime_added() {
    return mTime_added;
}

public void setTime_added(Long time_added) {
    mTime_added = time_added;
}

public int getNum_papers() {
    return mNum_papers;
}

public void setNum_papers(int num_papers) {
    mNum_papers = num_papers;
}

public int getNum_questions() {
    return mNum_questions;
}

public void setNum_questions(int num_questions) {
    mNum_questions = num_questions;
}

最佳答案

QuestionsActivity 以某种方式进入了 fragment 保存状态，即使您不希望这种情况发生。在序列化 QuestionsActivity 时，遇到了另一个不可序列化的对象。这就是为什么您会看到 TextView 和其他尝试序列化的原因，因为默认情况下 QuestionsActivity 的所有实例变量都会序列化。

我对为什么会发生这种情况的最佳猜测是由于这一行:

args.putSerializable(Keys.FRAGMENT_ACTIVE_LISTENER, mOnFragmentActiveListener);

但如果不查看 OnFragmentActiveListener 的定义位置，就很难确定。我假设 QuestionsActivity 实现 OnFragmentActiveListener，或者 QuestionsActivity 将 OnFragmentActiveListener 定义为内部类。无论哪种方式，如果您将 OnFragmentActiveListener 放入 fragment 参数中，您将遇到异常，因为您也间接地将整个 QuestionsActivity 存储为 fragment 参数。当 fragment 停止时，所有 fragment 参数都成为 fragment 保存状态的一部分。这就是错误的原因。

我建议不要将 OnFragmentActiveListener 作为 fragment 参数传递。如果 OnFragmentActiveListener 来自 Activity ，只需使用 getActivity() 获取对 Activity 的引用，然后获取对监听器的引用。

我还注意到 PaperListFragment 实现了 Serializable，我假设您对 QuestionsActivity 做了同样的事情。您可能这样做是为了避免编译错误。但这导致了运行时错误，因为这两个类上的实例变量并非都是可序列化的。因此，为了避免更多的运行时问题，我建议永远不要让 Activity 或 fragment 实现可序列化，因为这些类由于其成员而天生不可序列化。

关于java - 当 FragmentActivity 在 Android 中进入后台时出现 NotSerializableException，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/53932185/