Question

我创建了一个msacess db mydb.accdb 并指出了odbc源代码，我在lserv.java文件中有以下代码，所有tomcat的必要配置都已完成并且servlet已经完成没有响应，所以我在init方法中添加了一个打印错误，显示以下错误

javax.servlet.ServletException: Servlet.init() for servlet login threw exception
    org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
    org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293)
    org.apache.coyote.http11.Http11AprProcessor.process(Http11AprProcessor.java:879)
    org.apache.coyote.http11.Http11AprProtocol$Http11ConnectionHandler.process(Http11AprProtocol.java:600)
    org.apache.tomcat.util.net.AprEndpoint$Worker.run(AprEndpoint.java:1703)
    java.lang.Thread.run(Thread.java:722)
root cause

java.lang.NullPointerException
    lserv.init(lserv.java:23)
    javax.servlet.GenericServlet.init(GenericServlet.java:212)
    org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
    org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293)
    org.apache.coyote.http11.Http11AprProcessor.process(Http11AprProcessor.java:879)
    org.apache.coyote.http11.Http11AprProtocol$Http11ConnectionHandler.process(Http11AprProtocol.java:600)
    org.apache.tomcat.util.net.AprEndpoint$Worker.run(AprEndpoint.java:1703)
    java.lang.Thread.run(Thread.java:722)

java代码

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.sql.*;

public class lserv extends HttpServlet
{
Connection con;
ResultSet rs;
Statement st;
PrintWriter out;
public void init() 
{
try
{
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
con=DriverManager.getConnection("jdbc:odbc:mydb");
st=con.createStatement();

}
catch(Exception e)
{
out.println(e.getMessage());
}
}

public void doGet(HttpServletRequest req,HttpServletResponse res) 
{

try
{
out=res.getWriter();
res.setContentType("text/html");
if(req.getParameter("sub").equals("value"))
{
String s1=req.getParameter("t1");
String s2=req.getParameter("t2");
rs=st.executeQuery("SELECT * FROM login WHERE uname='"+s1.trim()+"AND pass='"+s2.trim()+"';" );
if(rs.next())
{
out.println("welcome");
}
else
{
out.println("sorry");
}

}
}
catch(Exception e){
out.println(e.getMessage());
}
}
}

html页面代码

<html>
<title>Login System</title>
<style type="text/css">
div.ex {
width:220px;
padding:10px;
border:5px solid gray;
margin:10px;
}
</style>

<body>
<div class="ex">
<strong>User Login</strong>
<form name=f1 method=get action="http://localhost:8080/login">
User &nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp<input type=text name=t1><br>
Password <input type=password name=t2><br>
<input type=submit name=sub value="Login">
</div>

</body>
</html>

我有一个名为login的表，字段为uname，并传递值。

有什么问题？请帮忙

Answer 1

我认为这应该非常明确;

java.lang.NullPointerException
    lserv.init(lserv.java:23)
    [...]

只需在代码的第23行修复NPE即可。：）

对我来说，这行似乎导致错误：

out.println(e.getMessage());

变量out未初始化，因此null。

基本Java登录Servlet不工作

1 个答案: