Android开发：FragmentCategory中SharedPref空指针无法获取ApiUrl

我是Android开发新手，在FragmentCategory中尝试从SharedPref获取ApiUrl时遇到空指针异常，报错信息如下：

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String com.kannada.newspaper.india.utils.SharedPref.getApiUrl()' on a null object reference

多次尝试仍未解决，相关代码如下：

FragmentCategory 核心代码 #

@Override
public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
    super.onViewCreated(view,savedInstanceState);

    this.callbackCall = RestAdapter.createAPI(sharedPref.getApiUrl()).getHome(AppConfig.REST_API_KEY);
    this.callbackCall.enqueue(new Callback<CallbackHome>() {
        public void onResponse(Call<CallbackHome> call, Response<CallbackHome> response) {
            CallbackHome responseHome = response.body();
            if (responseHome == null || !responseHome.status.equals("ok")) {
                return;
            }
            displayData(responseHome);
        }

        private void displayData(CallbackHome responseHome) {
            displayCategory(responseHome.category);
        }

        public void onFailure(Call<CallbackHome> call, Throwable th) {
            Log.e("onFailure", th.getMessage());
            if (!call.isCanceled()) {
            }
        }
    });
}

SharedPref 类代码 #

public class SharedPref {

    private Context context;
    private SharedPreferences sharedPreferences;
    private SharedPreferences.Editor editor;
    private static final String IS_FIRST_TIME_LAUNCH = "IsFirstTimeLaunch";

    public SharedPref(Context context) {
        this.context = context;
        sharedPreferences = context.getSharedPreferences("setting", Context.MODE_PRIVATE);
        editor = sharedPreferences.edit();
    }

    public Boolean getIsDarkTheme() {
        return sharedPreferences.getBoolean("theme", false);
    }

    public void setIsDarkTheme(Boolean isDarkTheme) {
        editor.putBoolean("theme", isDarkTheme);
        editor.apply();
    }

    public void setFirstTimeLaunch(boolean isFirstTime) {
        editor.putBoolean(IS_FIRST_TIME_LAUNCH, isFirstTime);
        editor.commit();
    }

    public boolean isFirstTimeLaunch() {
        return sharedPreferences.getBoolean(IS_FIRST_TIME_LAUNCH, true);
    }

    public void setDefaultFilterRecipes(int i) {
        editor.putInt("filter", i);
        editor.apply();
    }

    public Integer getCurrentFilterRecipes() {
        return sharedPreferences.getInt("filter", 0);
    }

    public void updateFilterRecipes(int position) {
        editor.putInt("filter", position);
        editor.apply();
    }

    public Integer getRecipesViewType() {
        return sharedPreferences.getInt("recipes_list", RECIPES_GRID_2_COLUMN);
    }

    public void updateRecipesViewType(int position) {
        editor.putInt("recipes_list", position);
        editor.apply();
    }

    public void setYoutubeApiKey() {
        editor.putInt("youtube_api_key", 0);
        editor.apply();
    }

    public void saveConfig(String api_url, String application_id) {
        editor.putString("api_url", api_url);
        editor.putString("application_id", application_id);
        editor.apply();
    }

    public String getApiUrl() {
        return sharedPreferences.getString("api_url", "https://kalviaruvi.com/demoandroid/your_recipes_app/");
    }

    public String getApplicationId() {
        return sharedPreferences.getString("application_id", "com.app.yourrecipesapp");
    }

    public void saveCredentials(String youtube_api_key, String fcm_notification_topic, String onesignal_app_id, String more_apps_url) {
        editor.putString("youtube_api_key", youtube_api_key);
        editor.putString("fcm_notification_topic", fcm_notification_topic);
        editor.putString("onesignal_app_id", onesignal_app_id);
        editor.putString("more_apps_url", more_apps_url);
        editor.apply();
    }

    public String getYoutubeAPIKey() {
        return sharedPreferences.getString("youtube_api_key", "0");
    }

    public String getFcmNotificationTopic() {
        return sharedPreferences.getString("fcm_notification_topic", "your_recipes_app_topic");
    }

    public String getOneSignalAppId() {
        return sharedPreferences.getString("onesignal_app_id", "0");
    }

    public String getMoreAppsUrl() {
        return sharedPreferences.getString("more_apps_url", "https://play.google.com/store/apps/developer?id=Solodroid");
    }
}

解决方案 #

核心原因 #

你在FragmentCategory中使用的sharedPref对象未初始化，处于空引用状态，调用其getApiUrl()方法时触发空指针异常。

修复步骤 #

1. 在FragmentCategory中声明SharedPref成员变量：

   private SharedPref sharedPref;
   

2. 在onViewCreated方法开头初始化sharedPref（推荐用requireContext()确保Context非空）：

   sharedPref = new SharedPref(requireContext());
   

3. 完整修改后的onViewCreated方法：

   @Override
   public void onViewCreated(@NonNull View view, @Nullable Bundle savedInstanceState) {
       super.onViewCreated(view, savedInstanceState);
       // 初始化SharedPref实例
       sharedPref = new SharedPref(requireContext());
   
       this.callbackCall = RestAdapter.createAPI(sharedPref.getApiUrl()).getHome(AppConfig.REST_API_KEY);
       this.callbackCall.enqueue(new Callback<CallbackHome>() {
           public void onResponse(Call<CallbackHome> call, Response<CallbackHome> response) {
               CallbackHome responseHome = response.body();
               if (responseHome == null || !responseHome.status.equals("ok")) {
                   return;
               }
               displayData(responseHome);
           }
   
           private void displayData(CallbackHome responseHome) {
               displayCategory(responseHome.category);
           }
   
           public void onFailure(Call<CallbackHome> call, Throwable th) {
               Log.e("onFailure", th.getMessage());
               if (!call.isCanceled()) {
                   // 可添加失败后的UI提示或重试逻辑
               }
           }
       });
   }
   

优化建议（可选） #

如果多个页面需要使用SharedPref，建议将其改造为单例模式，避免重复初始化并减少内存泄漏风险：

public class SharedPref {
    private static SharedPref instance;
    private Context context;
    private SharedPreferences sharedPreferences;
    private SharedPreferences.Editor editor;
    private static final String IS_FIRST_TIME_LAUNCH = "IsFirstTimeLaunch";

    // 私有构造方法，避免外部直接实例化
    private SharedPref(Context context) {
        this.context = context.getApplicationContext(); // 使用ApplicationContext避免内存泄漏
        sharedPreferences = context.getSharedPreferences("setting", Context.MODE_PRIVATE);
        editor = sharedPreferences.edit();
    }

    // 获取单例实例
    public static synchronized SharedPref getInstance(Context context) {
        if (instance == null) {
            instance = new SharedPref(context);
        }
        return instance;
    }

    // 原有方法保持不变...
}

在Fragment中使用单例：

sharedPref = SharedPref.getInstance(requireContext());

内容的提问来源于stack exchange，提问作者atozcodes