Question

我是Android新手。我想访问assets文件夹中的数据库。我在项目中创建了一个DataBaseHelper类，并复制了下面的代码。现在我想知道如何在我的MainActivity中调用此类。我称之为

DataBaseHandler db = new DataBaseHandler（this）;

但是出错了

默认构造函数无法处理隐式超级构造函数抛出的异常类型IOException。必须定义显式构造函数

所以有人可以帮助我如何在我的项目中调用它并使用创建的数据库。

 public class DataBaseHelper extends SQLiteOpenHelper {
 private Context mycontext;

//private String DB_PATH = mycontext.getApplicationContext().getPackageName()+"/databases/";
private static String DB_NAME = "(datbasename).sqlite";//the extension may be .sqlite or .db
public SQLiteDatabase myDataBase;
/*private String DB_PATH = "/data/data/"
                    + mycontext.getApplicationContext().getPackageName()
                    + "/databases/";*/

public DataBaseHelper(Context context) throws IOException {
    super(context,DB_NAME,null,1);
    this.mycontext=context;
    boolean dbexist = checkdatabase();
    if (dbexist) {
        //System.out.println("Database exists");
        opendatabase(); 
    } else {
        System.out.println("Database doesn't exist");
        createdatabase();
    }
}

public void createdatabase() throws IOException {
    boolean dbexist = checkdatabase();
    if(dbexist) {
        //System.out.println(" Database exists.");
    } else {
        this.getReadableDatabase();
        try {
            copydatabase();
        } catch(IOException e) {
            throw new Error("Error copying database");
        }
    }
}   

private boolean checkdatabase() {
    //SQLiteDatabase checkdb = null;
    boolean checkdb = false;
    try {
        String myPath = DB_PATH + DB_NAME;
        File dbfile = new File(myPath);
        //checkdb = SQLiteDatabase.openDatabase(myPath,null,SQLiteDatabase.OPEN_READWRITE);
        checkdb = dbfile.exists();
    } catch(SQLiteException e) {
        System.out.println("Database doesn't exist");
    }
    return checkdb;
}

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 + DB_NAME;

    //Open the empty db as the output stream
    OutputStream myoutput = new FileOutputStream("/data/data/(packagename)/databases   /(datbasename).sqlite");

    // transfer byte to inputfile to 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 + DB_NAME;
    myDataBase = SQLiteDatabase.openDatabase(mypath, null, SQLiteDatabase.OPEN_READWRITE);
}

public synchronized void close() {
    if(myDataBase != null) {
        myDataBase.close();
    }
    super.close();
}

}

Answer 1

如果您只使用现有数据库，则无需从SQLiteOpenHelper扩展。

您的DatabaseHandler应该是这样的：

import android.content.Context;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

public class DataBaseHelper {
    private Context mycontext;

    private String DB_PATH = mycontext.getApplicationContext().getPackageName() + "/databases/";
    private static String DB_NAME = "your_db_name.sqlite";//the extension may be .sqlite or .db
    public SQLiteDatabase myDataBase;
/*private String DB_PATH = "/data/data/"
                    + mycontext.getApplicationContext().getPackageName()
                    + "/databases/";*/

    public DataBaseHelper(Context context) throws IOException {
        this.mycontext = context;
        boolean dbexist = checkdatabase();
        if (!dbexist) {
            System.out.println("Database doesn't exist");
            copydatabase();
        }
        opendatabase();
    }

    private boolean checkdatabase() {
        //SQLiteDatabase checkdb = null;
        boolean checkdb = false;
        String myPath = DB_PATH + DB_NAME;
        File dbfile = new File(myPath);
        //checkdb = SQLiteDatabase.openDatabase(myPath,null,SQLiteDatabase.OPEN_READWRITE);
        checkdb = dbfile.exists();
        return checkdb;
    }

    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 + DB_NAME;

        //Open the empty db as the output stream
        OutputStream myoutput = new FileOutputStream(outfilename);

        // transfer byte to inputfile to 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 + DB_NAME;
        myDataBase = SQLiteDatabase.openDatabase(mypath, null, SQLiteDatabase.OPEN_READWRITE);
    }

    public synchronized void close() {
        if (myDataBase != null) {
            myDataBase.close();
        }
    }
}

请注意，上述代码不安全，仅用于学习。

访问资产中的数据库并从MainActivity

1 个答案: