在片段中使用dagger-2注入首选项时使用NullPointerException

Question

我在注入首选项时遇到错误：

找不到问题所在。

下面是我正在使用的一些类：

AppModule类

 @Module
    public class AppModule {
        private final MyApplication app;

        public AppModule(MyApplication app) {
            this.app = app;
        }

        @Provides
        @Singleton
        public Context provideContext() {
            return app;
        }

        @Provides
        @Singleton
        SharedPreferences providesSharedPreference() {
            return PreferenceManager.getDefaultSharedPreferences(app);
        }
    }

// The AppComponent interface

    @Singleton
    @Component(
            modules = {
                    AppModule.class,
            }
    )

    public interface AppComponent {

        void inject(DisplayFragment fragment);

    }

应用程序类

public class MyApplication extends Application {

    private static MyApplication instance = new MyApplication();
    private static AppComponent appComponent;

    public static MyApplication getInstance() {
        return instance;
    }

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

    public AppComponent getAppComponent() {
        if (appComponent == null) {
            appComponent = DaggerAppComponent.builder()
                    .appModule(new AppModule(this))
                    .build();
        }
        return appComponent;
    }
}

最后，在我正在做的一个片段中：

public class DisplayFragment extends Fragment implements View.OnClickListener {
...
    @Inject
    SharedPreferences prefs;

    @Inject
    public DisplayFragment() {
        MyApplication.getInstance().getAppComponent().inject(this);
    }
..
}

我收到这个错误，应用程序崩溃了。

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
                                                       at android.content.ContextWrapper.getPackageName(ContextWrapper.java:133)
                                                       at android.preference.PreferenceManager.getDefaultSharedPreferencesName(PreferenceManager.java:390)
                                                       at android.preference.PreferenceManager.getDefaultSharedPreferences(PreferenceManager.java:385)
                                                       at dagger.module.AppModule.providesSharedPreference(AppModule.java:34)
                                                       at dagger.module.AppModule_ProvidesSharedPreferenceFactory.get(AppModule_ProvidesSharedPreferenceFactory.java:23)
                                                       at dagger.module.AppModule_ProvidesSharedPreferenceFactory.get(AppModule_ProvidesSharedPreferenceFactory.java:8)
                                                       at dagger.internal.ScopedProvider.get(ScopedProvider.java:47)
                                                       at ui.DisplayFragment_MembersInjector.injectMembers(DisplayFragment_MembersInjector.java:29)
                                                       at ui.DisplayFragment_MembersInjector.injectMembers(DisplayFragment_MembersInjector.java:8)
                                                       at dagger.component.DaggerAppComponent.inject(DaggerAppComponent.java:45)
                                                       at ui.DisplayFragment.<init>(DisplayFragment.java:37)
                                                       at ui.DisplayFragment.newInstance(DisplayFragment.java:41)
                                                       at ui.MainActivity$MyPagerAdapter.getItem(MainActivity.java:86)
                                                       at android.support.v4.app.FragmentPagerAdapter.instantiateItem(FragmentPagerAdapter.java:101)
                                                       at android.support.v4.view.ViewPager.addNewItem(ViewPager.java:1034)
                                                       at android.support.v4.view.ViewPager.populate(ViewPager.java:1182)
                                                       at android.support.v4.view.ViewPager.populate(ViewPager.java:1116)
                                                       at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1642)
                                                       at android.view.View.measure(View.java:19324)
                                                       at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:825)
                                                       at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:511)
                                                       at android.view.View.measure(View.java:19324)
                                                       at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6113)
                                                       at android.widget.FrameLayout.onMeasure(FrameLayout.java:223)
                                                       at android.support.v7.widget.ContentFrameLayout.onMeasure(ContentFrameLayout.java:139)
                                                       at android.view.View.measure(View.java:19324)
                                                       at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6113)
                                                       at android.support.v7.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:393)
                                                       at android.view.View.measure(View.java:19324)
                                                       at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6113)
                                                       at android.widget.FrameLayout.onMeasure(FrameLayout.java:223)
                                                       at android.view.View.measure(View.java:19324)
                                                       at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6113)
                                                       at android.widget.LinearLayout.measureChildBeforeLayout(LinearLayout.java:1723)
                                                       at android.widget.LinearLayout.measureVertical(LinearLayout.java:788)
                                                       at android.widget.LinearLayout.onMeasure(LinearLayout.java:648)
                                                       at android.view.View.measure(View.java:19324)
                                                       at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:6113)
                                                       at android.widget.FrameLayout.onMeasure(FrameLayout.java:223)
                                                       at com.android.internal.policy.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2694)
                                                       at android.view.View.measure(View.java:19324)
                                                       at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2545)
                                                       at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1454)
                                                       at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1709)
                                                       at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1335)
                                                       at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6816)
                                                       at android.view.Choreographer$CallbackRecord.run(Choreographer.java:894)
                                                       at android.view.Choreographer.doCallbacks(Choreographer.java:696)
                                                       at android.view.Choreographer.doFrame(Choreographer.java:631)
                                                       at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:880)
                                                       at android.os.Handler.handleCallback(Handler.java:815)
                                                       at android.os.Handler.dispatchMessage(Handler.java:104)
                                                       at android.os.Looper.loop(Looper.java:207)
                                                       at android.app.ActivityThread.main(ActivityThread.java:5896)
                                                    at ja

谁来帮帮忙。

Answer 1

您可能需要将providesSharedPreference()更改为：

@Provides
@Singleton
SharedPreferences providesSharedPreference(Context context) {
    return PreferenceManager.getDefaultSharedPreferences(context);
}

并在onCreateView()方法中进行注入，因为当前的设置不能保证在调用片段构造函数时Context已经初始化。

Answer 2

我不确定你的应用程序单例是如何工作的(MyApplication instance = new MyApplication())，因为安卓为你创建了应用程序。这也是崩溃的原因:您将这个自制的、未初始化的实例传递给PreferenceManager。

在您的代码片段中，尝试注入onCreateView()，例如：

public class DisplayFragment extends Fragment {
    @Nullable
    @Override
    public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
        ((MyApplication) getActivity().getApplication()).getAppComponent().inject(this);
        return ...; // your fragment view
    }
}