jsp 链接MySql 500错误，空指针问题

Question

错误信息：
type Exception reportmessage description The server encountered an internal error () that prevented it from fulfilling this request.exception javax.servlet.ServletException: Servlet.init() for servlet CreateDBServlet threw exception
org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:856)
org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.processConnection(Http11Protocol.java:744)
org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527)
org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80)
org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
java.lang.Thread.run(Thread.java:619)
root cause java.lang.NullPointerException
java.lang.Class.forName0(Native Method)
java.lang.Class.forName(Class.java:169)
org.sunxin.lesson.jsp.bookstore.CreateDBServlet.init(CreateDBServlet.java:22)
javax.servlet.GenericServlet.init(GenericServlet.java:212)
org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:856)
org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.processConnection(Http11Protocol.java:744)
org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527)
org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80)
org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
java.lang.Thread.run(Thread.java:619)
note The full stack trace of the root cause is available in the Apache Tomcat/5.5.9 logs.源代码：
import javax.servlet.*;
import java.io.*;
import javax.servlet.http.*;
import java.sql.*;public class CreateDBServlet extends HttpServlet
{
    private String url;
    private String user;
    private String password;
    
    public void init() throws ServletException
    {
        String driverClass=getInitParameter("driverClass");
        url=getInitParameter("url");
        user=getInitParameter("user");
        password=getInitParameter("password");
        try
        {
            Class.forName(driverClass);
        }
        catch(ClassNotFoundException ce)
        {
            throw new UnavailableException("加载数据库驱动失败！");
        }
    }
    
    public void doGet(HttpServletRequest req, HttpServletResponse resp)
               throws ServletException,IOException
    {
        Connection conn=null;
        Statement stmt=null;
        try
        {
            conn=DriverManager.getConnection(url,user,password);
            stmt=conn.createStatement();
            stmt.executeUpdate("create database bookstore");
            stmt.executeUpdate("use bookstore");
            stmt.executeUpdate("create table bookinfo(id INT not null primary key,title VARCHAR(50) not null,author VARCHAR(50) not null,bookconcern VARCHAR(100) not null,publish_date DATE not null,price FLOAT(4,2) not null,amount SMALLINT,re VARCHAR(200)) ENGINE=InnoDB");
            stmt.addBatch("insert into bookinfo values(1,'Java从入门到精通','张三','张三出版社','2004-6-1',34.00,35,null)");
            stmt.addBatch("insert into bookinfo values(2,'JSP深入编程','李四','李四出版社','2004-10-1',56.00,20,null)");
            stmt.addBatch("insert into bookinfo values(3,'J2EE高级编程','王五','王五出版社','2005-3-1',78.00,10,null)");
            stmt.executeBatch();
            
            PrintWriter out=resp.getWriter();
            out.println("success!");
            out.close();
        }
        catch(SQLException se)
        {
            se.printStackTrace();
        }
        finally
        {
            if(stmt!=null)
            {
                try
                {
                    stmt.close();
                }
                catch(SQLException se)
                {
                    se.printStackTrace();
                }
                stmt=null;
            }
            if(conn!=null)
            {
                try
                {
                    conn.close();
                }
                catch(SQLException se)
                {
                    se.printStackTrace();
                }
                conn=null;
            }
        }
    }
}
望高手解答一下，小弟的困惑，小弟刚开始学习JSP

Answer 1

lz是NullPointerException 异常，你把driverClass打印出来看看是什么？会不会是null

Answer 2

java.lang.NullPointerException 
java.lang.Class.forName0(Native Method) 
web.xml中相应初始值是否都正确

Answer 3

<?xml version="1.0" encoding="gb2312"?><web-app xmlns="http://java.sun.com/xml/ns/j2ee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"
    version="2.4">
    
    <init-param>
    <param-name>driverClass</param-name>
    <param-value>com.mysql.jdbc.Driver</param-value>
    </init-param>
    <init-param>
    <param-name>url</param-name>
    <param-value>jdbc:mysql://localhost:3306/mysql</param-value>
    </init-param>
    <init-param>
    <param-name>user</param-name>
    <param-value>root</param-value>
    </init-param>
    <init-param>
    <param-name>password</param-name>
    <param-value>2130445lc</param-value>
    </init-param>    <servlet>
        <servlet-name>CreateDBServlet</servlet-name>
        <servlet-class>org.lincheng.lesson.jsp.bookstore.CreateDBServlet</servlet-class>
    </servlet>
    
    <servlet-mapping>
        <servlet-name>CreateDBServlet</servlet-name>
        <url-pattern>/createdb</url-pattern>
    </servlet-mapping>
    
    </web-app>

Answer 4

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Answer 5

你在String driverClass=getInitParameter("driverClass"); 
后面用System.out.println(driverClass);把driverClass打出来看一下是不是取到了值先，看看取到值没

Answer 6

在你的 ide里面（我用eclipse） 看root cause 就是异常堆栈最下面的那一条