java - 在两个不同的 Activity 中使用 SQLiteOpenHelper 的单个实例的最佳方法是什么？

Question

我第一次在 Android (Java) 中使用 sqlite 数据库编写应用程序。

两个 Activity 必须保存一些信息，因此我在两个 Activity 中都使用 MySQLiteHelper 来访问数据库。

我读到here将 SQLiteOpenHelper 构建为静态数据成员可能是一个很好的实践，所以我这样做了。

The static factory method ensures that there exists only one DatabaseHelper instance at any time.

我在每个 Activity 中创建一个使用getWritableDatabase()方法的SQLiteOpenHelper，但我不知道在哪里使用close() > 方法。

我应该在每次修改之后还是在 Activity 结束时放置此方法？

谢谢=)

Answer 1

您需要创建一个类，在其中放置所有常用方法、常量、变量等。

然后你必须在这个类中移动“getWritableDatabase()”，请。我建议您始终记住关闭数据库调用。与“close()”。

但这里实际使用的解决方案如下:

在我的应用程序中，我有不同的数据库适配器，这只是一个示例:

package com.app.android;

import android.content.ContentValues;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteException;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;

public class DBAdapter {

    public static final String KEY_ROWID = "id";
    public static final String KEY_NAME = "name";
    public static final String KEY_EMAIL = "email";
    public static final String TAG = "DBAdapter";

    //public static final String DATABASE_NAME = "my_db";
    //public static final String DATABASE_TABLE = "contacts";
    //public static final int DATABASE_VERSION = 1;

    public static final String START_TBL_CREATION = "create table "+Appiah.DATABASE_TABLE+" (_id integer primary key autoincrement, ";

    public static final String [] TABLE_COLUMNS_TO_BE_CREATED = new String []{
        KEY_NAME+" text not null, ",
        KEY_EMAIL+" text not null"
    };

    public static final String END_TBL_CREATION = ");";

    private static final String DATABASE_CREATE = START_TBL_CREATION
            + TABLE_COLUMNS_TO_BE_CREATED[0]
            + TABLE_COLUMNS_TO_BE_CREATED[1]
    + END_TBL_CREATION;

    private final Context context;

    private DatabaseHelper DBHelper;
    private SQLiteDatabase db;

    public DBAdapter (Context ctx){
        this.context = ctx;
        DBHelper = new DatabaseHelper(context);//there would be an error initially but just keep going...
    }

    private static class DatabaseHelper extends SQLiteOpenHelper{//after importing for "SQLiteOpenHelper", Add unimplemented methods

        DatabaseHelper(Context context){
            super (context, Appiah.DATABASE_NAME, null, Appiah.DATABASE_VERSION);//pls. note : "Appiah" is the class in which all the common methods, variables, etc. are sitting.
        }

        @Override
        public void onCreate(SQLiteDatabase db) {

            try{
                db.execSQL(DATABASE_CREATE);
            }catch(SQLException e){
                e.printStackTrace();
            }

        }

        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
            Log.w(TAG, "Upgrading database from version "+ oldVersion + " to "
                    + newVersion + ", which will destroy all old data");
            db.execSQL("DROP TABLE IF EXISTS contacts");
            onCreate(db);
        }
    }

    //opens the database
    public DBAdapter open() throws SQLiteException{
        db = DBHelper.getWritableDatabase();
        return this;
    }

    //closes the database
    public void close(){
        DBHelper.close();
    }

    //insert a contact into the database
    public long insertContact(String name, String email){
        ContentValues initialValues = new ContentValues();
        initialValues.put(KEY_NAME, name);
        initialValues.put(KEY_EMAIL, email);
        return db.insert(Appiah.DATABASE_TABLE, null, initialValues);
    }

    //deletes a particular contact
    public boolean deleteContact(long rowId){
        String whereClause = KEY_ROWID + "=" + rowId;
        String[] whereArgs = null;
        return db.delete(Appiah.DATABASE_TABLE, whereClause, whereArgs) > 0;
    }

    //retrieves all the contacts
    public Cursor getAllContacts(){
        String[] columns = new String[]{KEY_ROWID, KEY_NAME, KEY_EMAIL};
        String selection = null;
        String[] selectionArgs = null;
        String groupBy = null;
        String having = null;
        String orderBy = null;
        return db.query(Appiah.DATABASE_TABLE, columns, selection, selectionArgs, groupBy, having, orderBy);
    }

    //retrieve a particular contact with ID as input
    public Cursor getContact_with_ID(long rowId) throws SQLException {
        boolean distinct = true;
        String table = Appiah.DATABASE_TABLE;
        String [] columns = new String []{KEY_ROWID, KEY_NAME, KEY_EMAIL};
        String selection = KEY_ROWID + "=" + rowId;
        String [] selectionArgs = null;
        String groupBy = null;
        String having = null;
        String orderBy = null;
        String limit = null;
        Cursor mCursor = db.query(distinct, table, columns, selection, selectionArgs, groupBy, having, orderBy, limit);

        if(mCursor != null){
            mCursor.moveToFirst();
        }

        return mCursor;

    }

    public Cursor getContact_with_nameEntered(String name_str) throws SQLException {
        boolean distinct = true;
        String table = Appiah.DATABASE_TABLE;
        String [] columns = new String []{KEY_ROWID, KEY_NAME, KEY_EMAIL};
        String selection = KEY_NAME + "=" + name_str;//check again and do "%" thing to expand scope and increase chances of a name getting found or populated
        String [] selectionArgs = null;
        String groupBy = null;
        String having = null;
        String orderBy = null;
        String limit = null;
        Cursor mCursor = db.query(distinct, table, columns, selection, selectionArgs, groupBy, having, orderBy, limit);

        if(mCursor != null){
            mCursor.moveToFirst();
        }

        return mCursor;

    }

    //update a contact
    public boolean updateContact(long rowId, String name, String email){
        ContentValues args = new ContentValues();
        args.put(KEY_NAME, name);
        args.put(KEY_EMAIL, email);

        String table = Appiah.DATABASE_TABLE;
        ContentValues values = args;
        String whereClause = KEY_ROWID + "=" + rowId;
        String []whereArgs = null;

        return db.update(table, values, whereClause, whereArgs) > 0;
    }

    /*

     TO USE ANY OF THE ABOVE METHODS :
     1. type this before in your "onCreate()" : DBAdapter db = new DBAdapter(this);
     2. in the special case of getting all contacts to display : do the ff : 

        db.open();
        Cursor c = db.getAllContacts();
        if(c.moveToFirst()){
            do{
                textView.setText("ID : " + c.getString(0) + "\nName : " + c.getString(1) + "\nEmail Address : " + c.getString(2) );
            }while(c.moveToNext());//the "while" added ensures that, the looping process occurs
        }
        db.close();

     */

}

我希望这有帮助。它可以变得更深入，但我确实希望这会有所帮助。祝一切顺利。