Question

嘿，我想从我的数据库中读取，所以我使用了这个类，所以它将我的数据库复制到它应该的位置，然后当我尝试查询它崩溃时：

public class DataBaseHelper extends SQLiteOpenHelper{
    //The Androids default system path of your application database. 

    private static String DB_PATH = "/data/data/omar.gestore/databases/";  
    private static String DB_NAME = "test1.db";
    private SQLiteDatabase myDataBase;

    private final Context myContext;

    /** * Constructor * Takes and keeps a reference of the passed context in order to access to the application assets and resources. * @param context */
    public DataBaseHelper(Context context) {
        super(context, DB_NAME, null, 1); 
        this.myContext = context; 
    }

/** * Creates a 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 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.getReadableDatabase();
            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 + DB_NAME; 
            checkDB = SQLiteDatabase.openDatabase(myPath, null,
                                SQLiteDatabase.OPEN_READONLY);
        }catch(SQLiteException e){

        //database does't exist yet.
        }

        if(checkDB != null){
            checkDB.close();
        }

        return checkDB != null ? true : false; 
    }

    /** * 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 transfering 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 + DB_NAME;

        //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 + DB_NAME; 
        myDataBase = SQLiteDatabase.openDatabase(myPath, null,
                                SQLiteDatabase.OPEN_READONLY);
    }

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

    @Override public void onCreate(SQLiteDatabase db) {

    }

    @Override public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

    }

    public Cursor myQuery(String mquery){
        try
        {
            Cursor c =myDataBase.rawQuery(mquery,null);
            return c;
        }
        catch(Exception x)
        {
            return null;
        }
        // Add your public helper methods to access and get content from the database. 
        // You could return cursors by doing "return myDataBase.query(....)" so itd be easy 
        // to you to create adapters for your views.
    }
}

在我的主课程中：

DataBaseHelper myDatabase=new DataBaseHelper(this);

try {
    myDatabase.createDataBase();
} catch (IOException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
}
myDatabase.myQuery("SELECT * FROM adab;");

但因为空指针而崩溃..谁能告诉我为什么？感谢。

EDIT1：崩溃在这一行：Cursor c = myDataBase.rawQuery（mquery，null）;在myQuery函数中

Answer 1

您已创建数据库并从资产中复制了一些数据。但是您尚未打开复制的数据库，因此myDataBase为null。在openDatabase()

之后致电myDatabase.createDataBase();

尝试从SQLite数据库中读取时崩溃

1 个答案: