如何使用 SQLite 数据库在 Android 中预填充数据库

通常，需要使用现有数据库启动 Android 应用程序。这称为预填充数据库。在本文中，我们将看到如何使用 SQLite 数据库在 Android 中预填充数据库。本例中使用的数据库可以下载为Demo Database。

方法：

在build.gradle文件中添加支持库，并在依赖项部分添加 Recycler View 依赖项。
```
dependencies{
       implementation 'androidx.recyclerview:recyclerview:1.1.0'
}
```
确保在资产文件夹中添加数据库。要创建资产文件夹，请右键单击app directory->new->Folder->(select)Assets Folder 。然后只需将您的.db 文件粘贴到资产文件夹中。
在activity_main.xml 中，添加以下代码。
```
  
    
  
```
使用以下代码创建一个新的custom_layout.xml文件。
```
    
```

创建一个DatabaseHelper 类并添加以下代码。

package org.geeksforgeeks.dictionary;
  
import android.app.Activity;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteException;
import android.database.sqlite.SQLiteOpenHelper;
import android.util.Log;
  
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;
  
public class DatabaseHelper
    extends SQLiteOpenHelper {
  
    // The Android's default system path
    // of your application database.
    private static String DB_PATH = "";
    private static String DB_NAME = "database.db";
    private SQLiteDatabase myDataBase;
    private final Context myContext;
    private SQLiteOpenHelper sqLiteOpenHelper;
  
    // Table name in the database.
    public static final String
        ALGO_TOPICS
        = "algo_topics";
  
    /**
     * Constructor
     * Takes and keeps a reference of 
     * the passed context in order 
     * to access the application assets and resources. */
    public DatabaseHelper(Context context)
    {
  
        super(context, DB_NAME, null, 1);
        this.myContext = context;
        DB_PATH = myContext.getDatabasePath(DB_NAME)
                      .toString();
    }
  
    // Creates an empty database
    // on the system and rewrites it
    // with your own database.
    public void createDataBase()
        throws IOException
    {
  
        boolean dbExist = checkDataBase();
  
        if (dbExist) {
            // do nothing - database already exist
        }
        else {
            // By calling this method and
            // the empty database will be
            // created into the default system
            // path of your application
            // so we are gonna be able
            // to overwrite that database
            // with our database.
            this.getWritableDatabase();
            try {
                copyDataBase();
            }
            catch (IOException e) {
                throw new Error(
                    "Error copying database");
            }
        }
    }
    // Check if the database already exist
    // to avoid re-copying the file each
    // time you open the application
    // return true if it exists
    // false if it doesn't.
    private boolean checkDataBase()
    {
        SQLiteDatabase checkDB = null;
        try {
            String myPath = DB_PATH;
            checkDB
                = SQLiteDatabase
                      .openDatabase(
                          myPath, null,
                          SQLiteDatabase.OPEN_READONLY);
        }
        catch (SQLiteException e) {
  
            // database doesn't exist yet.
            Log.e("message", "" + e);
        }
        if (checkDB != null) {
            checkDB.close();
        }
        return checkDB != null;
    }
  
    /**
     * Copies your database from your
     * local assets-folder to the just 
     * created empty database in the
     * system folder, from where it 
     * can be accessed and handled.
     * This is done by transferring bytestream.
     * */
    private void copyDataBase()
        throws IOException
    {
        // Open your local db as the input stream
        InputStream myInput
            = myContext.getAssets()
                  .open(DB_NAME);
  
        // Path to the just created empty db
        String outFileName = DB_PATH;
  
        // Open the empty db as the output stream
        OutputStream myOutput
            = new FileOutputStream(outFileName);
  
        // transfer bytes from the
        // inputfile to the outputfile
        byte[] buffer = new byte[1024];
        int length;
        while ((length = myInput.read(buffer)) > 0) {
            myOutput.write(buffer, 0, length);
        }
  
        // Close the streams
        myOutput.flush();
        myOutput.close();
        myInput.close();
    }
  
    public void openDataBase()
        throws SQLException
    {
        // Open the database
        String myPath = DB_PATH;
        myDataBase = SQLiteDatabase
                         .openDatabase(
                             myPath, null,
                             SQLiteDatabase.OPEN_READONLY);
    }
  
    @Override
    public synchronized void close()
    {
        // close the database.
        if (myDataBase != null)
            myDataBase.close();
        super.close();
    }
  
    @Override
    public void onCreate(SQLiteDatabase db)
    {
        // It is an abstract method
        // but we define our own method here.
    }
  
    @Override
    public void onUpgrade(SQLiteDatabase db,
                          int oldVersion,
                          int newVersion)
    {
        // It is an abstract method which is
        // used to perform different task
        // based on the version of database.
    }
  
    // This method is used to get the
    // algorithm topics from the database.
    public List getAlgorithmTopics(
        Activity activity)
    {
        sqLiteOpenHelper
            = new DatabaseHelper(activity);
        SQLiteDatabase db
            = sqLiteOpenHelper
                  .getWritableDatabase();
  
        List list
            = new ArrayList<>();
  
        // query help us to return all data
        // the present in the ALGO_TOPICS table.
        String query = "SELECT * FROM " + ALGO_TOPICS;
        Cursor cursor = db.rawQuery(query, null);
  
        if (cursor.moveToFirst()) {
            do {
                list.add(cursor.getString(1));
            } while (cursor.moveToNext());
        }
        return list;
    }
}

创建一个MyAdapter。 Java类并添加以下代码。

package org.geeksforgeeks.dictionary;
  
import android.app.Activity;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
  
import androidx.annotation.NonNull;
import androidx.recyclerview.widget.RecyclerView;
import java.util.List;
  
public class MyAdapter
    extends RecyclerView.Adapter {
  
    private List data;
    Activity activity;
  
    public MyAdapter(Activity activity,
                     List data)
    {
        this.data = data;
        this.activity = activity;
    }
  
    // This method is used to attach
    // custom layout to the recycler view
    @NonNull
    @Override
    public ViewHolder onCreateViewHolder(
        @NonNull ViewGroup parent,
        int viewType)
    {
        LayoutInflater LI
            = activity.getLayoutInflater();
        View vw = LI.inflate(
            R.layout.custom_layout, null);
        return new ViewHolder(vw);
    }
  
    // This method is used to set the action
    // to the widgets of our custom layout.
    @Override
    public void onBindViewHolder(
        @NonNull ViewHolder holder,
        int position)
    {
        holder.topic_name
            .setText(data.get(position));
    }
  
    @Override
    public int getItemCount()
    {
        return data.size();
    }
  
    class ViewHolder
        extends RecyclerView.ViewHolder {
        TextView topic_name;
        public ViewHolder(View itemView)
        {
            super(itemView);
            this.topic_name
                = itemView.findViewById(R.id.textView);
        }
    }
}

最后，在MainActivity 中。 Java添加如下代码。

package org.geeksforgeeks.algorithmTopics;
  
import androidx.appcompat.app.AppCompatActivity;
import androidx.recyclerview.widget.DefaultItemAnimator;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;
import android.os.Bundle;
import java.util.ArrayList;
import java.util.List;
  
public class MainActivity
    extends AppCompatActivity {
  
    private static RecyclerView.Adapter adapter;
    private static RecyclerView recyclerView;
    public static List data;
    DatabaseHelper db;
  
    @Override
    protected void onCreate(Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        recyclerView
            = findViewById(R.id.my_recycler_view);
  
        db = new DatabaseHelper(this);
        recyclerView.setLayoutManager(
            new LinearLayoutManager(this));
        data = new ArrayList<>();
        fetchData();
    }
  
    public void fetchData()
    {
        // Before fetching the data
        // directly from the database.
        // first we have to creates an empty
        // database on the system and
        // rewrites it with your own database.
        // Then we have to open the
        // database to fetch the data from it.
        db = new DatabaseHelper(this);
        try {
            db.createDataBase();
            db.openDataBase();
        }
        catch (Exception e) {
            e.printStackTrace();
        }
  
        data = db.getAlgorithmTopics(this);
        adapter = new MyAdapter(this, data);
        recyclerView.setAdapter(adapter);
    }
}

输出：