getWritableDatabase抛出NullPointerException

Question

我已经搜索了现有的NPE/数据库线程，但没有找到任何直接相关的东西……

我有一个OnClickListener类(独立于关联的Activity类)，它调用下面代码中的createFromPreset方法。下面方法代码的最后一行(this.getWritableDatabase)抛出了一个NPE。当我调试代码并在该行中断时，"this“显示为"TableControllerMealItem”(不是我所期望的null )。

这段代码以前工作得很好。这可能是巧合，但我刚刚创建了一个Application类(并在Android Manifest中注册了它)。这会不会影响到什么呢？？

public class TableControllerMealItem extends MySQLiteHelper {
    public TableControllerMealItem(Context context) {
        super(context);
    }
public long createFromPreset(meal_item meal_item, long meal_id, long preset_item_id) {
        String presetDesc = "";
        int presetMinutes = 0;

        //Get details from the preset_item (description and minutes)
        String sql = "SELECT * FROM preset_item WHERE _id = " + preset_item_id;
        SQLiteDatabase db = this.getWritableDatabase();

下面是LogCat：

05-27 11:58:39.613: E/AndroidRuntime(3021): java.lang.NullPointerException
05-27 11:58:39.613: E/AndroidRuntime(3021):     at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224)
05-27 11:58:39.613: E/AndroidRuntime(3021):     at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:164)
05-27 11:58:39.613: E/AndroidRuntime(3021):     at com.ian.mealtimer.TableControllerMealItem.createFromPreset(TableControllerMealItem.java:56)
05-27 11:58:39.613: E/AndroidRuntime(3021):     at com.ian.mealtimer.OnClickListenerSelectPresetItem.onClick(OnClickListenerSelectPresetItem.java:40)
05-27 11:58:39.613: E/AndroidRuntime(3021):     at android.view.View.performClick(View.java:4438)
05-27 11:58:39.613: E/AndroidRuntime(3021):     at android.view.View$PerformClick.run(View.java:18422)
05-27 11:58:39.613: E/AndroidRuntime(3021):     at android.os.Handler.handleCallback(Handler.java:733)
05-27 11:58:39.613: E/AndroidRuntime(3021):     at android.os.Handler.dispatchMessage(Handler.java:95)
05-27 11:58:39.613: E/AndroidRuntime(3021):     at android.os.Looper.loop(Looper.java:136)
05-27 11:58:39.613: E/AndroidRuntime(3021):     at android.app.ActivityThread.main(ActivityThread.java:5017)
05-27 11:58:39.613: E/AndroidRuntime(3021):     at java.lang.reflect.Method.invokeNative(Native Method)
05-27 11:58:39.613: E/AndroidRuntime(3021):     at java.lang.reflect.Method.invoke(Method.java:515)
05-27 11:58:39.613: E/AndroidRuntime(3021):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
05-27 11:58:39.613: E/AndroidRuntime(3021):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
05-27 11:58:39.613: E/AndroidRuntime(3021):     at dalvik.system.NativeStart.main(Native Method)

MySQLite帮助器类如下所示：

public class MySQLiteHelper extends SQLiteOpenHelper {
    private static final String TAG = "MySQLiteHelper";
    public static final String TABLE_PRESET_ITEM = "preset_item";
    public static final String COLUMN_ID = "_id";
    public static final String COLUMN_PRESET_DESC = "preset_desc";
    public static final String COLUMN_PRESET_MINUTES = "preset_minutes";
    protected static final String DATABASE_NAME = "mealtimersqlite.db";
    private static final int DATABASE_VERSION = 1;

    public MySQLiteHelper(Context context) {
        super(context, "mealtimersqlite.db", null, 5);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        // TODO Auto-generated method stub
        String SQLCommand;

        SQLCommand = "CREATE TABLE preset_item( _id INTEGER PRIMARY KEY, "
                + "preset_desc TEXT NULL, preset_minutes INTEGER NULL );";
        db.execSQL(SQLCommand);

        SQLCommand = "CREATE TABLE meal(_id INTEGER PRIMARY KEY,"
                + "meal_desc TEXT NULL, meal_ready_time INTEGER NULL, meal_reminders_flag INTEGER NULL);";
        db.execSQL(SQLCommand);

        SQLCommand = "CREATE TABLE meal_item(_id INTEGER PRIMARY KEY, "
                + "meal_item_desc TEXT NULL, meal_item_minutes INTEGER NULL, meal_item_start_time INTEGER NULL, "
                + "meal_item_ready_time INTEGER NULL, fk_meal_id INTEGER NOT NULL, fk_preset_item_id INTEGER NULL );";
        db.execSQL(SQLCommand);
        Log.d(TAG, "Mealtimer Database Created");
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int arg1, int arg2) {
        // TODO Auto-generated method stub
        // Drop older table if existed
        db.execSQL("DROP TABLE IF EXISTS preset_item;");
        db.execSQL("DROP TABLE IF EXISTS meal;");
        db.execSQL("DROP TABLE IF EXISTS meal_item;");
        Log.d(TAG, "Mealtimer Database Tables Dropped;");
        // Create tables again
        onCreate(db);
    }

}

下面是调用createFromPreset方法的类：

public class OnClickListenerSelectPresetItem implements OnClickListener {
    public final static String EXTRA_MEAL_ID = "com.ian.mealtimer.MEAL_ID";
    private long glbMealId = 1;
    Context context;
    private Activity activity;

    public OnClickListenerSelectPresetItem(Activity activity){
        this.activity = activity;
        this.glbMealId = ((MealTimerApplication) activity.getApplication()).getMealId();
        }
    public void onCreate(long meal_id) {
    }

    public void onClick(View view) {
        //INSERT a meal_item using then preset_item_id of clicked item
        //then return to the MealItemDetail Activity

        //First get the id of the selected item
        Long selectedPresetItem = Long.valueOf(view.getTag().toString());

        //Now insert the meal_item record

        // Save new meal_item record
        meal_item meal_item = new meal_item();
        long newRecord = new TableControllerMealItem(
                context).createFromPreset(meal_item, glbMealId, selectedPresetItem);

Answer 1

传递给SQLiteOpenHelper构造函数的Context是null。

Answer 2

OnClickListenerSelectPresetItem中的context成员变量永远不会初始化，它的默认值为null。

将Context的null传递给SQLiteOpenHelper构造函数会导致getDatabaseLocked()中的NPE，例如在帮助器上调用getWritableDatabase()。

Answer 3

您需要通过传递给OnClickListenerSelectPresetItem()的Activity参数初始化本地成员Context

public OnClickListenerSelectPresetItem(Activity activity) {
  this.activity = activity;
  this.context = activity;  // Activity is derived from Context
  this.glbMealId = ((MealTimerApplication) activity.getApplication()).getMealId();
}

这是因为Activity is dervied from (a subclass of) Context。