Java Null指针

Question

当我从MyServ2类调用getResultSet()方法时,有没有人知道为什么我得到一个空指针错误

这是我的DBClass(导入等省略)

public DBClass(){

    }

public Connection dbConnect(String db_connect_string,
        String db_userid, String db_password)
{

        try
        {
        DriverManager.registerDriver(new oracle.jdbc.OracleDriver());

        conn = DriverManager.getConnection(
        db_connect_string, db_userid, db_password);

        return conn;

        }
        catch (Exception e)
            {
                e.printStackTrace();

                return null;
            }
}



public ResultSet getResultSet(String query){

    try{
    stmt = conn.createStatement();
    result = stmt.executeQuery(query);  

    } catch(Exception e){
        e.printStackTrace();
        return null;
    }
    return result;

}



} 

这是我的MyServ2课程

public class MyServ2 extends HttpServlet {
    private static final long serialVersionUID = 1L;
       private DBClass db;

    public MyServ2() {
        super();
        db = new DBClass();

    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {


        ResultSet rs = db.getResultSet("Select * from ....ect");
        try {
            while(rs.next()){
                System.out.println(rs.getString(1).toString());
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

}

Answer 1

你没有打电话db.dbConnect(),所以db.conn将是空的.