Java EE, JDBC and Tomcat

Question

我正在尝试使用 JSP 从数据库中获取数据，但在服务器上运行时出现以下错误。谁能告诉我出了什么问题并告诉我如何解决它？

type Rapport d''exception
message java.lang.NullPointerException
description Le serveur a rencontré une erreur interne qui l''a empêché de satisfaire la requête.
exception
org.apache.jasper.JasperException: java.lang.NullPointerException
    org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:549)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:470)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:390)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51)
cause mère
java.lang.NullPointerException
    org.apache.jsp.vrf_jsp._jspService(vrf_jsp.java:84)
    org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:432)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:390)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51)
note La trace complète de la cause mère de cette erreur est disponible dans les fichiers journaux de Apache Tomcat/7.0.47.

这是我的代码：

package utilitaire;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
public class Database 
{
    public Connection con;
    public Connection createConnection()
    {
        try
        {
            Class.forName("com.mysql.jdbc.driver");
            this.con = DriverManager.getConnection("jdbc:mysql://localhost:3306/scoop_phase2p2","root","");
        }catch(ClassNotFoundException e){
            System.out.println("driver introuvable");
        }catch(SQLException e){
            e.getErrorCode();
        }
        return this.con;
    }

}

/***

*

<%@ page language = "java" %>
<%@ page import = "utilitaire.Admin" %>
<%@ page import = "utilitaire.Database" %>
<%@ page import = "java.sql.SQLException" %>
<%@ page import = "java.sql.ResultSet" %>
<%@ page import = "java.sql.PreparedStatement" %>
<%@ page import = "java.sql.Connection" %>
<%
    String login = request.getParameter("login");
    String mdp = request.getParameter("mdp");
    Database db = new Database();
    Connection conn = db.createConnection();
    try
    {
        PreparedStatement query = conn.prepareStatement("SELECT * FROM admin  WHERE login= ? AND DECODE(mdp, 'key')=?");
        query.setString(1, login);
        query.setString(2, mdp);
        ResultSet rs = query.executeQuery(); 
        for(int i=0; rs.next(); i++)
        if(i!=0)
            out.print("yup");
        else
            out.print("nope");
    }catch(SQLException e){
        e.getCause();
    }
%>

Answer 1

首先：

Class.forName("com.mysql.jdbc.driver");

这里的驱动程序应该是驱动程序。Java 类名区分大小写。出于这个原因，你的代码抛出 ClassNotFoundException：

} catch(ClassNotFoundException e){
        System.out.println("driver introuvable");
    }catch(SQLException e){
        e.getErrorCode();
    }
    return this.con;

这里有几个问题。首先，无论发生什么，您都会返回"con"。即使它仍然为空。SQLException 处理程序实际上什么都不做。至少您应该记录堆栈跟踪。

   Database db = new Database();
Connection conn = db.createConnection();
try
{
    PreparedStatement query = conn.prepareStatement("SELECT * FROM admin  WHERE login= ? AND DECODE(mdp, 'key')=?");

这里可能发生的情况：conn 保持空，你不检查它。然后尝试在空引用上调用 prepareStatement 并获取 NullPointerException。

修复"驱动程序 -> 驱动程序"问题，而不是返回 null 并吞下异常，而是重新抛出它，以便您有机会查看问题所在：

   try
    {
        Class.forName("com.mysql.jdbc.Driver");
        this.con = DriverManager.getConnection("jdbc:mysql://localhost:3306/scoop_phase2p2","root","");
        return this.con;
    }catch(ClassNotFoundException e){
        throw new IllegalStateException(e);
    }catch(SQLException e){
        throw new IllegalStateException(e);
    }