Java 访问片段时偶尔出现NPE'；s观点

当输入片段时，我偶尔会得到

NullPointerException

。它发生在应用程序在后台很长一段时间，然后我打开它并滑动到这个片段

public class SummaryFragment extends Fragment implements FragmentLifecycle {

    private static final String TAG = "DTAG";
    private DateFormat dateFormatName;
    private Preference prefs;
    private List<String> monthList;
    private TextView totalTimeFullTv;
    private TextView totalTimeNetTv;
    private TextView averageTimeTv;
    private TextView overUnderTv;
    private TextView minTimeTv;
    private TextView maxTimeTv;
    private TextView vacationsTv;
    private TextView sickTv;
    private TextView headlineTv;
    private TextView overUnderTvH;
    private OnFragmentInteractionListener mListener;

    public SummaryFragment() {
        // Required empty public constructor
    }


    public static SummaryFragment newInstance(String param1, String param2) {
        SummaryFragment fragment = new SummaryFragment();
        return fragment;
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {

        View RootView = inflater.inflate(R.layout.fragment_summary, container, false);

        dateFormatName = new SimpleDateFormat(getResources().getString(R.string.month_text));
        monthList = Arrays.asList(new DateFormatSymbols().getMonths());
        prefs = new Preference(GeneralAdapter.getContext());


        totalTimeFullTv = RootView.findViewById(R.id.textView_sum_ttf);
        totalTimeNetTv = RootView.findViewById(R.id.textView_sum_ttn);
        averageTimeTv = RootView.findViewById(R.id.textView_sum_av);
        overUnderTv = RootView.findViewById(R.id.textView_sum_ou);
        overUnderTvH = RootView.findViewById(R.id.textView_sum_ou_h);


        minTimeTv = RootView.findViewById(R.id.textView_sum_min);
        maxTimeTv = RootView.findViewById(R.id.textView_sum_max);
        vacationsTv = RootView.findViewById(R.id.textView_sum_vac);
        sickTv = RootView.findViewById(R.id.textView_sum_sick);
        headlineTv= RootView.findViewById(R.id.textView_sum_headline);

        return RootView;
    }

    private void refreshData() {

        if (prefs == null)
        {
            prefs = new Preference(GeneralAdapter.getContext());
        }

        String month = prefs.getString(Preference.CURRENT_MONTH);

        MonthData monthData = Calculators.CalculateLocalData(MainActivity.db.getAllDays(month));

        totalTimeFullTv.setText(monthData.getTotalTimeFull()); //Crash here
        totalTimeNetTv.setText(monthData.getTotalTimeNet());
        averageTimeTv.setText(monthData.getAverageTime());
        overUnderTv.setText(monthData.getOverUnder());
        if (monthData.getOverUnderFloat()<0)
        {
            overUnderTvH.setText(R.string.sum_over_time_neg);
            overUnderTv.setTextColor(ContextCompat.getColor(GeneralAdapter.getContext(),R.color.negative_color));
        }
        else
        {
            overUnderTvH.setText(R.string.sum_over_time_pos);
            overUnderTv.setTextColor(ContextCompat.getColor(GeneralAdapter.getContext(),R.color.positive_color));
        }

        minTimeTv.setText(monthData.getMinTime());
        maxTimeTv.setText(monthData.getMaxTime());
        vacationsTv.setText(""+monthData.getVacations());
        sickTv.setText(""+monthData.getSick());
        headlineTv.setText(month);
    }

    public void onButtonPressed(Uri uri) {
        if (mListener != null) {
            mListener.onFragmentInteraction(uri);
        }
    }

    @Override
    public void onAttachFragment(Fragment childFragment) {
        super.onAttachFragment(childFragment);

    }

    @Override
    public void onDetach() {
        super.onDetach();
        mListener = null;
    }

    @Override
    public void onPauseFragment() {

    }

    @Override
    public void onResumeFragment()
    {
        refreshData();
    }


    public interface OnFragmentInteractionListener {
        // TODO: Update argument type and name
        void onFragmentInteraction(Uri uri);
    }
}

日志：

viewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
            int currentPosition = 0;

            @Override
            public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
            }

            @Override
            public void onPageSelected(int position) {

                FragmentLifecycle fragmentToHide = (FragmentLifecycle) adapter.getItem(currentPosition);
                fragmentToHide.onPauseFragment();

                FragmentLifecycle fragmentToShow = (FragmentLifecycle) adapter.getItem(position);
                fragmentToShow.onResumeFragment(); //Crash start

                currentPosition = position;
            }

            @Override
            public void onPageScrollStateChanged(int state) {

            }
        });

                                            E/AndroidRuntime: FATAL EXCEPTION: main
                                               Process: michlind.com.workcalendar, PID: 25038
                                               java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
                                                   at michlind.com.workcalendar.mainfragments.SummaryFragment.refreshData(SummaryFragment.java:99)
                                                   at michlind.com.workcalendar.mainfragments.SummaryFragment.onResumeFragment(SummaryFragment.java:147)
                                                   at michlind.com.workcalendar.activities.MainActivity.onPageSelected(MainActivity.java:84)
                                                   at android.support.v4.view.ViewPager.dispatchOnPageSelected(ViewPager.java:1941)
                                                   at android.support.v4.view.ViewPager.scrollToItem(ViewPager.java:680)
                                                   at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:664)
                                                   at android.support.v4.view.ViewPager.onTouchEvent(ViewPager.java:2257)
                                                   at android.view.View.dispatchTouchEvent(View.java:11776)
                                                   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2962)
                                                   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2643)
                                                   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2968)
                                                   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2657)
                                                   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2968)
                                                   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2657)
                                                   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2968)
                                                   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2657)
                                                   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2968)
                                                   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2657)
                                                   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2968)
                                                   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2657)
                                                   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2968)
                                                   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2657)
                                                   at com.android.internal.policy.DecorView.superDispatchTouchEvent(DecorView.java:448)
                                                   at com.android.internal.policy.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1829)
                                                   at android.app.Activity.dispatchTouchEvent(Activity.java:3307)
                                                   at android.support.v7.view.WindowCallbackWrapper.dispatchTouchEvent(WindowCallbackWrapper.java:68)
                                                   at com.android.internal.policy.DecorView.dispatchTouchEvent(DecorView.java:410)
                                                   at android.view.View.dispatchPointerEvent(View.java:12015)
                                                   at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent(ViewRootImpl.java:4795)
                                                   at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:4609)
                                                   at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4147)
                                                   at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4200)
                                                   at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4166)
                                                   at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:4293)
                                                   at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4174)
                                                   at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:4350)
                                                   at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4147)
                                                   at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4200)
                                                   at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4166)
                                                   at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4174)
                                                   at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4147)
                                                   at android.view.ViewRootImpl.deliverInputEvent(ViewRootImpl.java:6661)
                                                   at android.view.ViewRootImpl.doProcessInputEvents(ViewRootImpl.java:6635)
                                                   at android.view.ViewRootImpl.enqueueInputEvent(ViewRootImpl.java:6596)
                                                   at android.view.ViewRootImpl$WindowInputEventReceiver.onInputEvent(ViewRootImpl.java:6764)
                                                   at android.view.InputEventReceiver.dispatchInputEvent(InputEventReceiver.java:186)
                                                   at android.os.MessageQueue.nativePollOnce(Native Method)
                                                   at android.os.MessageQueue.next(MessageQueue.java:325)
                                                   at android.os.Looper.loop(Looper.java:142)
                                                   at android.app.ActivityThread.main(ActivityThread.java:6494)

更新：

viewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
            int currentPosition = 0;

            @Override
            public void onPageScrolled(int position, float positionOffset, int positionOffsetPixels) {
            }

            @Override
            public void onPageSelected(int position) {

                FragmentLifecycle fragmentToHide = (FragmentLifecycle) adapter.getItem(currentPosition);
                fragmentToHide.onPauseFragment();

                FragmentLifecycle fragmentToShow = (FragmentLifecycle) adapter.getItem(position);
                fragmentToShow.onResumeFragment(); //Crash start

                currentPosition = position;
            }

            @Override
            public void onPageScrollStateChanged(int state) {

            }
        });

                                            E/AndroidRuntime: FATAL EXCEPTION: main
                                               Process: michlind.com.workcalendar, PID: 25038
                                               java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
                                                   at michlind.com.workcalendar.mainfragments.SummaryFragment.refreshData(SummaryFragment.java:99)
                                                   at michlind.com.workcalendar.mainfragments.SummaryFragment.onResumeFragment(SummaryFragment.java:147)
                                                   at michlind.com.workcalendar.activities.MainActivity.onPageSelected(MainActivity.java:84)
                                                   at android.support.v4.view.ViewPager.dispatchOnPageSelected(ViewPager.java:1941)
                                                   at android.support.v4.view.ViewPager.scrollToItem(ViewPager.java:680)
                                                   at android.support.v4.view.ViewPager.setCurrentItemInternal(ViewPager.java:664)
                                                   at android.support.v4.view.ViewPager.onTouchEvent(ViewPager.java:2257)
                                                   at android.view.View.dispatchTouchEvent(View.java:11776)
                                                   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2962)
                                                   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2643)
                                                   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2968)
                                                   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2657)
                                                   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2968)
                                                   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2657)
                                                   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2968)
                                                   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2657)
                                                   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2968)
                                                   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2657)
                                                   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2968)
                                                   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2657)
                                                   at android.view.ViewGroup.dispatchTransformedTouchEvent(ViewGroup.java:2968)
                                                   at android.view.ViewGroup.dispatchTouchEvent(ViewGroup.java:2657)
                                                   at com.android.internal.policy.DecorView.superDispatchTouchEvent(DecorView.java:448)
                                                   at com.android.internal.policy.PhoneWindow.superDispatchTouchEvent(PhoneWindow.java:1829)
                                                   at android.app.Activity.dispatchTouchEvent(Activity.java:3307)
                                                   at android.support.v7.view.WindowCallbackWrapper.dispatchTouchEvent(WindowCallbackWrapper.java:68)
                                                   at com.android.internal.policy.DecorView.dispatchTouchEvent(DecorView.java:410)
                                                   at android.view.View.dispatchPointerEvent(View.java:12015)
                                                   at android.view.ViewRootImpl$ViewPostImeInputStage.processPointerEvent(ViewRootImpl.java:4795)
                                                   at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:4609)
                                                   at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4147)
                                                   at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4200)
                                                   at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4166)
                                                   at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:4293)
                                                   at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4174)
                                                   at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:4350)
                                                   at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4147)
                                                   at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:4200)
                                                   at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:4166)
                                                   at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:4174)
                                                   at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:4147)
                                                   at android.view.ViewRootImpl.deliverInputEvent(ViewRootImpl.java:6661)
                                                   at android.view.ViewRootImpl.doProcessInputEvents(ViewRootImpl.java:6635)
                                                   at android.view.ViewRootImpl.enqueueInputEvent(ViewRootImpl.java:6596)
                                                   at android.view.ViewRootImpl$WindowInputEventReceiver.onInputEvent(ViewRootImpl.java:6764)
                                                   at android.view.InputEventReceiver.dispatchInputEvent(InputEventReceiver.java:186)
                                                   at android.os.MessageQueue.nativePollOnce(Native Method)
                                                   at android.os.MessageQueue.next(MessageQueue.java:325)
                                                   at android.os.Looper.loop(Looper.java:142)
                                                   at android.app.ActivityThread.main(ActivityThread.java:6494)

我最终使用了：

@Override
public void onPageSelected(int position) {

    Fragment fragment = adapter.getFragment(position);
    if (fragment != null) {
        fragment.onResume();
    }
}

在MainActivity中，并在每个片段中使用onResume（）。此解决方案适用于适配器： 在创建此片段的所有视图之前，将调用onResumeFragment（）。 首先尝试重新创建新实例，然后在活动中调用

FragmentLifeCycle

接口的

onResumeFragment

您应该在

onCreateView

中膨胀布局，但不应该使用

onCreateView

中的

findViewById

初始化其他视图

下面是来自FragmentManager的代码

  // This calls onCreateView()
f.mView = f.performCreateView(f.getLayoutInflater(f.mSavedFragmentState), null, f.mSavedFragmentState);

// Null check avoids possible NPEs in onViewCreated
// It's also safe to call getView() during or after onViewCreated()
if (f.mView != null) {
    f.mView.setSaveFromParentEnabled(false);
    if (f.mHidden) f.mView.setVisibility(View.GONE);
    f.onViewCreated(f.mView, f.mSavedFragmentState);
}

最好在

onViewCreated

中为字段指定子视图。这是因为框架为您自动执行空检查，以确保片段的视图层次结构已正确创建和膨胀（如果使用XML布局文件）

---

创建视图后，初始化视图。

问题是，您试图过早访问视图：此时尚未创建视图层次结构

如果您发布了将在下一帧上发生的事件，则可以保证已设置视图层次结构：

@Override
public void onResumeFragment() {
    new Handler().post(new Runnable() {
        @Override
        public void run() {
            refreshData();
        }
    });
}

---

ViewPager

将两侧的多个项目连接在一起（即碎片恢复），但是

FragmentPagerAdapter

使用

Fragment.setUserVisibleHint

指示哪个项目是当前项目。相反，利用这一点

以下是如何利用用户可见提示：

删除页面更改侦听器上的

放弃

FragmentLifecycle

界面

将片段设置为：

（在科特林，但你会得到要点）

更多信息

FragmentPagerAdapter.getItem

是一种工厂方法。它总是应该返回一个片段的新实例。如果您试图缓存它们，请删除缓存（1），不要使用

getItem

yourself（2）

有时崩溃，有时不崩溃的代码是需要调试的。这可能是由于在不需要的情况下重用片段造成的

新片段实例未附加，没有理由创建视图，并且在您离开

onPageSelected

后将被垃圾收集

---

您正在错误地使用OnPageChangeListener。这不是控制视图生命周期事件的安全方法。您需要将PagerAdapter与ViewPager结合使用，并覆盖其实例化Item/destroyItem回调

请参见此示例：

PagerAdapter对ViewPager的作用就像ListAdapter对ListView的作用一样，要使系统正常工作，您需要两者

使用

onViewCreated（）

fragment的回调方法来更新数据，这样您就可以确保所有视图的布局都是完美的

@Override
public void onViewCreated(View view, @Nullable Bundle savedInstanceState) {
   refreshData();

}

---

使用

处理程序仍然有风险，因为您无法确定视图是否膨胀。
问题

片段的生命周期是独立的。您无法确定当注册
onPageSelected（）
时，该片段是否已被放置。这是一个异步事件。所以你不能依赖这个回调

但另一方面，您也不能仅依赖于
onResume（）
，因为在ViewPager中，与当前可见页面相邻的页面是预加载的

解决方案

当片段对用户可见并正在运行时，主要需要
refreshData（）
。该词的定义也是如此：

当片段对用户可见并正在运行时调用。（……）

因此，只需在片段的
onResume（）
中调用
refreshData（）
，如果您注意到在ViewPager未真正显示此页面时调用了此函数，请不要担心。
添加此签入refreshData（）方法：

正如大多数人所说，您需要确保您的片段是活动的，并且对用户可见。我也有类似的问题。我使用onHiddenChanged来决定何时重新加载数据

@Override
public void onHiddenChanged(boolean hidden) {
    super.onHiddenChanged(hidden);
    if (!hidden) {
        refreshData();
    }
}

当我为ViewPager实现自定义生命周期时，我也遇到了同样的问题。我认为您正在使用FragmentStatePagerAdapter使用ViewPager填充片段。正如我们所知，碎片状态页面雷达在碎片失去焦点时会摧毁所有碎片。我们需要使用单例模式为每个页面提供相同的对象

在您的代码中，对于单例模式，片段创建可以如下所示

private SummaryFragment mInstance;

private SummaryFragment() {
        // Required empty public constructor
    }

public static SummaryFragment newInstance(String param1, String param2) {
        if(mInstance == null)
        mInstance = new SummaryFragment();
        return mInstance;
    }

这样做解决了我的问题。如果这不适合你？你能分享你的PagerAdapter课程吗。
你为什么不把
totalTimeFullTv
选为TextView@vm345如果OP使用appcompat，则不需要强制转换。appcompat将Android更高版本中的一个新更新修改为片段和活动，这使得在使用appcompat导入时强制转换变得多余。您是否尝试在onResumeFragment（）中延迟50-100毫秒？尝试了10次，效果似乎好多了，但仍有几次崩溃。不要试着用500！但这似乎不是正确的解决方案。这只是一个解决方法。您需要将
refreshData（）
放在
onActivityCreated（）
方法中，以避免这种情况。您的视图没有被创建。它大大减少了崩溃，但它们仍然偶尔发生……有时仍然发生。目前正在尝试500毫秒！但这似乎不是正确的编码解决方案。在我的日志中，我可以肯定地看到OnCreate发生在onResumeFragment之前。OnCreate中的视图不为null。这很奇怪。这不是推荐的解决方案。这基本上是希望页面是在设定的毫秒数之后布局的。在某些情况下，这种延迟可能“太早”，