Question

我无法理解如何初始化数据库错误：

java.lang.NullPointerException
        at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224)
        at android.database.sqlite.SQLiteOpenHelper.getReadableDatabase(SQLiteOpenHelper.java:188)
        at com.example.stopcall.app.dal.PhoneDal.getItem(PhoneDal.java:105)
        at com.example.stopcall.app.ItemDetailFragment$1.onClick(ItemDetailFragment.java:87)
        at android.view.View.performClick(View.java:4654)
        at android.view.View$PerformClick.run(View.java:19438)
        at android.os.Handler.handleCallback(Handler.java:733)

代码落在这一行：SQLiteDatabase db = this.getWritableDatabase();

来自此代码：

public class PhoneDal extends SQLiteOpenHelper {

    // Database Version
    private static final int DATABASE_VERSION = 1;
    // Database Name
    private static final String DATABASE_NAME = Constants.DB_NAME;

    private static final String BLOCKED_PHONES_TABLE = "BLOCKED_PHONES_TABLE";
    private static final String COMMENTS_TABLE = "COMMENTS_TABLE";

    public PhoneDal(Context context) {
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        // SQL statement to create book table

        String CREATE_BLOCKED_PHONES_TABLE = "CREATE TABLE "
                + BLOCKED_PHONES_TABLE + " ( "
                + "id INTEGER PRIMARY KEY AUTOINCREMENT, " + "phone TEXT, "
                + "isBlocked BIT )";

        db.execSQL(CREATE_BLOCKED_PHONES_TABLE);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        if (newVersion > oldVersion) {
            Log.w("MyAppTag", "Updating database from version " + oldVersion
                    + " to " + newVersion + " .Existing data will be lost.");
            // Drop older books table if existed
            db.execSQL("DROP TABLE IF EXISTS " + BLOCKED_PHONES_TABLE);

            // create fresh books table
            this.onCreate(db);
        }

    }



    private static final String KEY_ID = "id";
    private static final String KEY_PHONE = "KEY_PHONE";
    private static final String KEY_IS_BLOCKED = "KEY_IS_BLOCKED";

    public void addItem(Phone phone) {
        // 1. get reference to writable DB
        SQLiteDatabase db = this.getWritableDatabase();

初始化代码：

public class ItemDetailFragment extends Fragment {
    /**
     * The fragment argument representing the item ID that this fragment
     * represents.
     */
    public static final String ARG_ITEM_ID = "item_id";

    /**
     * The dummy content this fragment is presenting.
     */
    private DummyContent.DummyItem mItem;
    private PhoneDal phoneDal;
    private CommentDal commentDal;

    /**
     * Mandatory empty constructor for the fragment manager to instantiate the
     * fragment (e.g. upon screen orientation changes).
     */
    public ItemDetailFragment() {
        this.mItem = mItem;
        this.phoneDal = new PhoneDal(getActivity());
        this.commentDal = new CommentDal(getActivity());
    }

    @SuppressLint("ValidFragment")
    public ItemDetailFragment(DummyContent.DummyItem mItem, PhoneDal phoneDal, CommentDal commentDal) {
        this.mItem = mItem;
        this.phoneDal = phoneDal;
        this.commentDal = commentDal;
    }

Answer 1

public ItemDetailFragment() {
    this.phoneDal = new PhoneDal(getActivity());

片段构造函数太早，无法调用getActivity() - 它将返回null。在Context构造函数中将null作为SQLiteOpenHelper传递会在调用get...Database()时导致异常。

将您的PhoneDal初始化推迟到片段生命周期中的onAttach()或更高版本。

Answer 2

您正在使用null作为上下文初始化PhoneDal，这会在以后导致NPE。问题是你在Fragment的构造函数中调用getActivity()，此时它将返回null。

不要为View和Fragment等组件实现构造函数！

相反，使用像onCreate，OnStart，OnAttach这样的专用方法来初始化和操作它们。

在Fragment的构造函数的Javadoc中也提到了这一点：

应用程序通常不应该实现构造函数。首先将应用程序代码放置在可以使用片段的运行中在onAttach（Activity）中，片段实际上就是这个点与其活动有关。

有关Fragment生命周期的更多信息：Fragments

顺便提一下，建议使用应用程序上下文初始化SQLiteOpenHelper：Avoid memory leaks

android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked（SQLiteOpenHelper.java:224）中的java.lang.NullPointerException

2 个答案: