getWritableDatabase返回null

Question

我有Android服务,它使用SQLite数据库.我在数据库初始化方面遇到了一些麻烦.

我有DBHelper类:

public class DBHelper extends SQLiteOpenHelper{
public boolean db_loaded=false;
public DBHelper() {
    super(ContextBean.getLocalContext(), "resplugin1.db", null, 1);
}
@Override
public void onCreate(SQLiteDatabase db) {
    db.execSQL("create table tasks ("
            + "id integer primary key autoincrement,"
            + "title text);");
    db_loaded=true;
}

我有使用它的客户端类:

public class DataProvider {
private SQLiteDatabase database;
private DBHelper dbHelper;

public DataProvider(){
    dbHelper=new DBHelper();
    database = dbHelper.getWritableDatabase();
}
public void close(){
    dbHelper.close();
}
public TaskDescriptor getTaskById(int id){
    while(dbHelper.db_loaded==false){}
    String[] args={Integer.toString(id)};
    if(database==null){
        Log.d("ResPluginService1","DATABASE IS NULL");
    }
/*...*/
}

问题是数据库对象总是NULL. getWritableDatabase方法不会抛出异常,它只返回null.什么会导致如此奇怪的行为？