SharedPreferences/Context 抛出 NullPointerException

Question

我正在尝试将一些值从活动传递到非活动接收器类以创建通知。我不明白为什么我会在这个问题上出错。我的意思是，我传递了上下文，检索它，并相应地使用它。我阅读了一些关于这个问题的帖子并尽我所能解决它，但这是我能走多远。我在这条线上遇到了错误：

SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(applicationContext);

DailyForecastActivity.java

public class DailyForecastActivity extends AppCompatActivity {

    private Daily[] mDays;
    public static Context contextOfApplication;
    @Bind(android.R.id.list)
    ListView mListView;
    @Bind(android.R.id.empty)
    TextView mEmptyTextView;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_daily_forecast);
        ButterKnife.bind(this);
        contextOfApplication = getApplicationContext();

        GPSTracker gpsTracker = new GPSTracker(this);
        TextView mLocationLabel = (TextView) findViewById(R.id.locationLabel);
        Intent intent = getIntent();

        Parcelable[] parcelables = intent.getParcelableArrayExtra(MainActivity.DAILY_FORECAST);
        mDays = Arrays.copyOf(parcelables, parcelables.length, Daily[].class);
        DayAdapter adapter = new DayAdapter(this, mDays);
        mListView.setAdapter(adapter);
        mListView.setEmptyView(mEmptyTextView);
        mListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                String dayOfTheWeek = mDays[position].getDayOfTheWeek();
                String condition = mDays[position].getSummary();
                String tempMin = mDays[position].getTempMin() + "";
                String tempMax = mDays[position].getTempMax() + "";
                int icon = mDays[position].getIconId();

                String message = String.format("%s Summary: %s", dayOfTheWeek, condition);
                Toast.makeText(DailyForecastActivity.this, message, Toast.LENGTH_SHORT).show();
                SharedPreferences.Editor editor = getPreferences(MODE_PRIVATE).edit();
                editor.putString("summary", condition);
                editor.putString("tempMin", tempMin);
                editor.putString("tempMax", tempMax);
                editor.putString("dayOfTheWeek", dayOfTheWeek);
                editor.putInt("icon", icon);
                editor.apply();
            }
        });

        String area = gpsTracker.getSubLocality(this);
        String city = gpsTracker.getAdminArea(this);
        String country = gpsTracker.getCountryName(this);
        mLocationLabel.setText(area + ", " + city + ", " + country);
    }
    public static Context getContextOfApplication(){
    return contextOfApplication;
    }
}

Receiver.java

public class Receiver extends BroadcastReceiver{

    Context applicationContext = DailyForecastActivity.getContextOfApplication();
    SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(applicationContext);
    String summary = preferences.getString("summary", null);
    String tempMin = preferences.getString("tempMin", null);
    String tempMax = preferences.getString("tempMax", null);
    String dayOfTheWeek = preferences.getString("dayOfTheWeek", null);
    int icon = preferences.getInt("icon", 0);

    @Override
    public void onReceive(Context context, Intent intent) {
        this.applicationContext = context;
        PendingIntent pi = PendingIntent.getActivity(context, 0, new Intent(context, MainActivity.class), 0);
        NotificationCompat.Builder mBuilder = new NotificationCompat.Builder(context)
            .setSmallIcon(icon)
            .setContentTitle(dayOfTheWeek + " Weather")
            .setContentText(summary + "Temperature: " + tempMax + "/" + tempMin)
            .setTicker("Daily Weather");

        mBuilder.setContentIntent(pi);
        mBuilder.setDefaults(NotificationCompat.DEFAULT_SOUND);
        mBuilder.setAutoCancel(true);

        NotificationManager mNotificationManager = (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);
        mNotificationManager.notify(1, mBuilder.build());


    }
}

logcat

java.lang.RuntimeException: Unable to instantiate receiver com.theoc.stormy.Receiver: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
                                                                at android.app.ActivityThread.handleReceiver(ActivityThread.java:2565)
                                                                at android.app.ActivityThread.access$1700(ActivityThread.java:148)
                                                                at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1360)
                                                                at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                at android.os.Looper.loop(Looper.java:135)
                                                                at android.app.ActivityThread.main(ActivityThread.java:5272)
                                                                at java.lang.reflect.Method.invoke(Native Method)
                                                                at java.lang.reflect.Method.invoke(Method.java:372)
                                                                at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:909)
                                                                at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:704)
                                                             Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
                                                                at android.preference.PreferenceManager.getDefaultSharedPreferencesName(PreferenceManager.java:374)
                                                                at android.preference.PreferenceManager.getDefaultSharedPreferences(PreferenceManager.java:369)
                                                                at com.theoc.stormy.Receiver.<init>(Receiver.java:42)
                                                                at java.lang.reflect.Constructor.newInstance(Native Method)
                                                                at java.lang.Class.newInstance(Class.java:1572)
                                                                at android.app.ActivityThread.handleReceiver(ActivityThread.java:2560)
                                                                at android.app.ActivityThread.access$1700(ActivityThread.java:148) 
                                                                at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1360) 
                                                                at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                at android.os.Looper.loop(Looper.java:135) 
                                                                at android.app.ActivityThread.main(ActivityThread.java:5272) 
                                                                at java.lang.reflect.Method.invoke(Native Method) 
                                                                at java.lang.reflect.Method.invoke(Method.java:372) 
                                                                at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:909) 
                                                                at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:704) 

编辑

我解决了 NPE 错误，但现在我的 SharedPreferences 值得到了“null”。我在 DailyForecastActivity 中进行了调试，我在其中放置了值，那里似乎没有问题。我能够输入我想要的所有值。看起来我无法在 Receive.java 中检索 SharedPreferences。另外作为旁注，由于某种原因，我收到了通知声音，但根本没有收到任何通知。

Answer 1

在onReceive中初始化SharedPreference

preferences = PreferenceManager.getDefaultSharedPreferences(context);

当您的BroadcastReceiver 被调用时，您的活动可能为空

Answer 2

尝试这样做：

首先，通过链接到一个类在清单中实例化您的应用程序

<application
    android:name = ".App"
    android:allowBackup="true"
    android:icon="@drawable/icon"
    android:label="@string/app_name" >

然后在您的应用程序类中实现以下方法

public class App extends Application {
private static final String TAG = ".App";
private static Context context = null;

public void onCreate(){
    super.onCreate();
    context = getApplicationContext();
}

@SuppressWarnings("unused")
public static Context getContext() {
    if (context != null) {
        return context;
    } else {
        Log.e(TAG, "App was not successfully created");
        throw new Error("App was not successfully created");
    }
}

}

Answer 3

我总是通过以下方式获得我的共同偏好：

context.getSharedPreferences(PREFERENCE_FILENAME, Context.MODE_PRIVATE);

您还可以验证是否在您的应用程序数据/目录中创建了首选项文件。 请仔细阅读共享首选项在 android 中的工作原理。

Answer 4

致电DailyForecastActivity.java

SharedPreferences.Editor editor = getPreferences(MODE_PRIVATE).edit();

在Receiver.java

SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(applicationContext);

您正在调用SharedPreferences 的两个不同实例。您正在保存一个，然后您尝试从没有保存任何内容的第二个中获取，因此您将获得 null 的所有值。你必须联合它并在第一种和第二种情况下使用相同的，比如：

SharedPreferences sharedPref = getPreferences("MyPref", Context.MODE_PRIVATE);

在DailyForecastActivity 和

SharedPreferences sharedPref = context.getPreferences("MyPref", Context.MODE_PRIVATE);

在Receiver，这将导致获得相同的实例。

您也可以尝试使用相同的Context。 Context 从getActivity() 获取不一样Context 从getApplicationContext() 获取。所以在这两种情况下都使用getApplicationContext()，或者在activity的构造函数中通过参数传递Context。