SQLiteHelper 的 onCreate() 方法永远不会被调用

Question

我正在使用 SQLite 在 android 中创建一个数据库。

我编写代码以在 onCreate() 方法中创建一个名为 products 的表。

然后当我调用 add() 方法向表中添加一些值时，我收到一个错误，即没有名为 products 的表。

这是我的 SQLiteHelper 类：

public class MySQLiteHelper extends SQLiteOpenHelper {

    //variable declarations and some code
    ...

    @Override
    public void onCreate(SQLiteDatabase db) {

        // SQL statement to create a products table
        String CREATE_PRODUCTS_TABLE = "CREATE TABLE "+TABLE_NAME+ " ( " +
                "id INTEGER PRIMARY KEY AUTOINCREMENT, " +
                "url TEXT, " +
                "title TEXT, " +
                "price INTEGER );";

        // create products table
        db.execSQL(CREATE_PRODUCTS_TABLE);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // Drop older table if existed
        db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME);

        // Create tables again
        onCreate(db);
    }


void add(String url, String title, String price) {


    this.url = url;
    this.title = title;
    this.price = price;

    // 1. get reference to writable DB
    SQLiteDatabase db = this.getWritableDatabase();

    // 2. create ContentValues to add key "column"/value
    ContentValues values = new ContentValues();
    values.put(KEY_URL, url);
    values.put(KEY_TITLE, title);
    values.put(KEY_PRICE, price);

    // 3. insert
    db.insert(TABLE_NAME, // table
            null, //nullColumnHack
            values); // key/value -> keys = column names/ values = column values

    // 4. close
    db.close();
}

我从一个类似的问题中读到 onCreate() 不是构造函数，它仅在数据库不存在时调用，但在我的情况下数据库也不存在，那么为什么不调用它呢？

我还了解到我们需要致电getWritableDatabase() 或getReadableDatabase()

但我不确定在我的代码中的何处进行此类调用。

我尝试将onCreate() 中的“表创建代码”放入我的add() 方法中。第一次运行时，我什么也没得到，但从下一次开始，我不断收到“表已存在”的错误。

现在，在调用add() 方法插入一些值之前，如何确保只正确创建表一次。有什么帮助吗？

编辑：

我的 Logcat 显示：

03-07 00:59:22.684    2107-2147/com.example.nikhil.amazon1 W/EGL_emulation﹕ eglSurfaceAttrib not implemented
03-07 00:59:22.684    2107-2147/com.example.nikhil.amazon1 W/OpenGLRenderer﹕ Failed to set EGL_SWAP_BEHAVIOR on surface 0xa5683060, error=EGL_SUCCESS
03-07 00:59:24.922    2107-2120/com.example.nikhil.amazon1 I/art﹕ Background sticky concurrent mark sweep GC freed 1464(89KB) AllocSpace objects, 7(2MB) LOS objects, 7% free, 9MB/9MB, paused 75.596ms total 203.616ms
03-07 00:59:25.338    2107-2413/com.example.nikhil.amazon1 E/SQLiteLog﹕ (1) no such table: products
03-07 00:59:25.339    2107-2413/com.example.nikhil.amazon1 E/SQLiteDatabase﹕ Error inserting price=   24,599.00 title=Google Nexus 5 D821 (16GB, Black) url=http://www.amazon.in/Google-Nexus-D821-16GB-Black/dp/B00GC1J55C/ref=sr_1_1?s=electronics&ie=UTF8&qid=1421161258&sr=1-1&keywords=Google
    android.database.sqlite.SQLiteException: no such table: products (code 1): , while compiling: INSERT INTO products(price,title,url) VALUES (?,?,?)
            at android.database.sqlite.SQLiteConnection.nativePrepareStatement(Native Method)
            at android.database.sqlite.SQLiteConnection.acquirePreparedStatement(SQLiteConnection.java:889)
            at android.database.sqlite.SQLiteConnection.prepare(SQLiteConnection.java:500)
            at android.database.sqlite.SQLiteSession.prepare(SQLiteSession.java:588)
            at android.database.sqlite.SQLiteProgram.<init>(SQLiteProgram.java:58)
            at android.database.sqlite.SQLiteStatement.<init>(SQLiteStatement.java:31)
            at android.database.sqlite.SQLiteDatabase.insertWithOnConflict(SQLiteDatabase.java:1469)
            at android.database.sqlite.SQLiteDatabase.insert(SQLiteDatabase.java:1341)
            at com.example.nikhil.amazon1.MySQLiteHelper.add(MySQLiteHelper.java:86)
            at com.example.nikhil.amazon1.FirstParsing$1.run(FirstParsing.java:71)
            at java.lang.Thread.run(Thread.java:818)

我的问题与以下内容完全相同： 1)Android SQLiteOpenHelper : onCreate() method is not called. Why?

2) SQLiteOpenHelper failing to call onCreate?

3)SQLiteOpenHelper "onCreate" is not called? (the DB does not exist)

但我发现很难理解它并将其应用到我的代码中。

Answer 1

这可能有助于未来的发展

MySQLiteHelper - 定义数据库、表等

public class MySQLiteHelper extends SQLiteOpenHelper {

    private static final String FILE_NAME = "application.db";
    private static final int DB_VERSION = 1;
    private final String TAG = MySQLiteHelper.class.getCanonicalName();
    private static SQLiteDatabase database = null;

    public MySQLiteHelper(Context context) {
        super(context, FILE_NAME, null, DB_VERSION);
        // TODO Auto-generated constructor stub
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        // TODO Auto-generated method stub
        //version 1
        database.execSQL(ProductsTbl.CREATE_SQL);
        //can create new tables if necessary, make sure to increase database version

    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        // TODO Auto-generated method stub
        Log.i(TAG, "Upgrading from version " + oldVersion + " to version "
                                + newVersion);
        for (int i = oldVersion + 1; i <= newVersion; i++) {
            Log.i(TAG, "version is becoming current " + i);
            switch (i) {
                case 2:
                    db.execSQL(ProductsTbl.CREATE_SQL);
                    break;
            //add more cases for each additional table added

            }
        }
    }

    public static class ProductsTbl {
        public static final String TABLE_NAME = "products";
        public static final String ID = "_id";
        public static final String URL = "url";
        public static final String TITLE = "title";
        public static final String PRICE = "price";
        public static final String[] TABLE_COLUMNS = { ID, URL, TITLE, PRICE };
        public static final String CREATE_SQL = "create table " + TABLE_NAME
                                                                                        + "(" + ID + " INTEGER PRIMARY KEY AUTOINCREMENT,"
                                                                                        + URL + " TEXT,"
                                                                                        + TITLE + " TEXT,"
                                                                                        + PRICE + " INTEGER);";
    }

}

MySQLiteDatasource - 定义访问方法、插入、删除等

public class MySQLiteDatasource {

    //Database fields
    private SQLiteDatabase database;
    private MySQLiteHelper dbHelper;

    public MySQLiteDatasource(Context context) {
        dbHelper = new MySQLiteHelper(context);
    }

    public void open() throws SQLException {
        database = dbHelper.getWritableDatabase();
    }

    public void close() {
        dbHelper.close();
    }

    public void insertProduct(String url, String title, String price) {
        ContentValues values = new ContentValues();
        values.put(MySQLiteHelper.ProductsTbl.URL, url);
        values.put(MySQLiteHelper.ProductsTbl.TITLE, title);
        values.put(MySQLiteHelper.ProductsTbl.PRICE, price);

        try {
            database.insertOrThrow(MySQLiteHelper.ProductsTbl.TABLE_NAME, null, values);
        } catch (SQLiteConstraintException e) {
            //System.out.println(e);
            Log.e("SQLite Database", "Unable to INSERT into Database, possible duplicate topic already exists.");
        }
    }

}

这是您如何在一个活动或类似的东西中使用这 2 个类的方法。

  public static void addProductToDB(String url, String title, String price) {
    MySQLiteDatasource datasource = new MySQLiteDatasource(this);
    datasource.open();
    datasource.insertProduct(url, title, price);
    datasource.close();
  }

您可能还想考虑定义一个 Product 对象来保存稍后从数据库中获取的数据。