如何在Singleton和SharedPreferences类中传递应用程序上下文

时间:2014-03-30 11:20:39

标签: android nullpointerexception singleton sharedpreferences android-context

如何将应用程序上下文从Singleton类传递给SharedPreferences?我在 onActivityCreated(Bundle savedInstanceState)方法中有一个片段和一个GridView,在项目单击时,我在logcat中得到NullPointerException:

03-30 05:12:54.784: E/AndroidRuntime(1950): FATAL EXCEPTION: main
03-30 05:12:54.784: E/AndroidRuntime(1950): java.lang.NullPointerException
03-30 05:12:54.784: E/AndroidRuntime(1950):     at android.preference.PreferenceManager.getDefaultSharedPreferencesName(PreferenceManager.java:374)
03-30 05:12:54.784: E/AndroidRuntime(1950):     at android.preference.PreferenceManager.getDefaultSharedPreferences(PreferenceManager.java:369)
03-30 05:12:54.784: E/AndroidRuntime(1950):     at com.example.duaapp.utility.SharedPreferencesSupplication.save(SharedPreferencesSupplication.java:35)

Singleton Class

public class SingletonClass {

    public static Context applicationContext;

    public static int fontSizeMin = 17;
    public static int fontSizeMax = 35;

public static final String keySelVerseFromList = "keySelVerseFromList";
    public static final String keyFavVerses = "keyFavVerses";
    public static final String keyListOfVerses = "keyListOfVerses";

    public static final String keyIsFavSelected = "keyIsFavSelected";

}

SharedPreferences

  public class SharedPreferencesSupplication {


        public void save(String valueKey, String value) {

            SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(SingletonClass.applicationContext);
            SharedPreferences.Editor edit = prefs.edit();
            edit.putString(valueKey, value);
            edit.commit();
        }

        public void save(String valueKey, int value) {

            SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(SingletonClass.applicationContext);
            SharedPreferences.Editor edit = prefs.edit();
            edit.putInt(valueKey, value);
            edit.commit();
        }

        public void save(String valueKey, boolean value) {

            SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(SingletonClass.applicationContext);
            SharedPreferences.Editor edit = prefs.edit();
            edit.putBoolean(valueKey, value);
            edit.commit();
        }

        public void save(String valueKey, long value) {

            SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(SingletonClass.applicationContext);
            SharedPreferences.Editor edit = prefs.edit();
            edit.putLong(valueKey, value);
            edit.commit();
        }
    }

在gridview_item_Click上,每当调用 new SharedPreferencesSupplication()。save(SingletonClass.keyIsFavSelected,false); 时,应用程序崩溃并在logcat中引发nullpointer异常。我哪里错了?

2 个答案:

答案 0 :(得分:6)

从不在Android中使用对Context的静态引用,这会导致重要的内存泄漏,因为Context引用了所有应用程序资源。

大多数Android UI类都有一个对上下文的动态引用,您可以在getActivity()的片段中,在视图中getContext()考虑使用这些而不是上下文单例

有关此here

的更多信息

答案 1 :(得分:3)

虽然Guillermo Merino关注有效,但单例类中使用的应用程序Context不应自动被视为内存泄漏。

调用context.getApplicationContext()无论访问何处或如何访问,都将始终从您的流程中返回相同的实例。这将是"活着"至少与单身人士一样长,因此对其进行引用不应该造成伤害。

也就是说,执行某些操作可能还不够,正如戴夫史密斯在他的blog post中所描述的那样。

由于OP尝试访问默认SharedPreferences,因此只需使用应用Context就可以实现,例如:

// Custom Application class.
public class MyApplication extends Application {
    @Override
    public void onCreate() {
        super.onCreate();
        SingletonClass.INSTANCE.init(getApplicationContext());
    }
}

// Singleton.
public enum SingletonClass {
    INSTANCE;

    private Context applicationContext;

    public static int fontSizeMin = 17;
    public static int fontSizeMax = 35;

    public static final String keySelVerseFromList = "keySelVerseFromList";
    public static final String keyFavVerses = "keyFavVerses";
    public static final String keyListOfVerses = "keyListOfVerses";
    public static final String keyIsFavSelected = "keyIsFavSelected";

    // Make sure you only call this once - from MyApplication.
    public void init(final Context context) {
        applicationContext = context.getApplicationContext();
    }

    public Context getApplicationContext() {
        if (null == applicationContext) {
            throw new IllegalStateException("have you called init(context)?");
        }

        return applicationContext;
    }

}

我不相信这会导致任何内存泄漏,但如果我弄错了,请纠正我。