Question

当我从MyServ类调用getName时，我得到null，但是当我从DBClass本地调用它们时，它们返回字符串。谁知道我做错了什么？

package DB;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;

public class DBClass{

    private Statement stmt;
    private Connection conn;
    private ResultSet result;
    public String name, surname;

    public DBClass(){
    }

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

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

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

            stmt = conn.createStatement();
            result = stmt.executeQuery("Select * from .....etc");

            if(result.next()){
                name = result.getString(1).toString();
                surname = result.getString(2).toString();
            }
            return conn;

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

            return null;
        }
    }

    public String getName(){
        return name;
    }

    public String getSurname(){
        return surname;
    }

}




package DB;

import java.io.IOException;
import java.sql.Connection;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class MyServ
 */
public class MyServ extends HttpServlet{

    private static final long serialVersionUID = 1L;
    private final DBClass dbclass;
    private final String name, surname;

    private final Connection conn;

    public MyServ(){

        dbclass = new DBClass();
        final DBClass db = new DBClass();

        conn =
            db.dbConnect("jdbc:oracle:thin:@elanweb:1510:xxxxx", "xxxxx",
                "xxxxxx");

        name = dbclass.getName();
        surname = dbclass.getSurname();
    }

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

    }

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

        System.out.println("MyServer -- " + name + " " + surname);
        response.sendRedirect("http://localhost:8080/DWP/");

    }
}

Answer 1

当然你的意思是：

name = db.getName();

而不是：

name = dbclass.getName();

您似乎在构造函数中引用了错误的变量，没有打开的连接（dbclass）。

Answer 2

只是因为值 null（是一个字符串，类型）。

查看代码的相关位。在MyServ构造函数中，您可以：

dbclass = new DBClass();

根据DBClass构造函数，这会将所有字段初始化为默认值（字符串为null）。

然后在调用之前不再使用此变量：

dbclass.getName();

正确地查找name变量 - 它是null，因为它在构造对象时被隐式赋值为null - 并将其返回给你。

也许更好的问题是，你期望它返回什么，以及为什么？

Java getter方法返回null而不是字符串

2 个答案: