为什么session无法进行判断，请高手指教

Question

//源码：
<%
String SessionVB,SessionContent;
SessionVB=session.getAttribute("LoginStatus");
SessionContent="ok";if(!SessionVB.equals(SessionContent))
{
    out.print("<script>window.alert('超时或未登录');window.location.href='index.jsp'</script>");
}
%>//出错信息：
type Exception reportmessage description The server encountered an internal error () that prevented it from fulfilling this request.exception org.apache.jasper.JasperException: Unable to compile class for JSPAn error occurred at line: 0 in the jsp file: /www.kingvjsp.com/kingvsys/../include/session/sessionLoginStatus.jspGenerated servlet error:
    [javac] Compiling 1 source fileC:\Program Files\Apache Group\Tomcat 4.1\work\Standalone\localhost\kingv\www.kingvjsp.com\kingvsys\Main_System_jsp.java:49: incompatible types
found   : java.lang.Object
required: java.lang.String
SessionVB=session.getAttribute("LoginStatus");
                              ^
1 error

Answer 1

getAttribute（）方法返回的是一个字符串，不是一个session对象

Answer 2

SessionVB=(String)session.getAttribute("LoginStatus");

Answer 3

SessionVB=(String)session.getAttribute("LoginStatus");session.getAttribute方法返回的是Object对象 所以要强制转换

Answer 4

//我改为这样了：，但还是出错
<%
String SessionVB,SessionContent;
SessionVB=(String)session.getAttribute("LoginStatus");
SessionContent="ok";if(!SessionVB.equals(SessionContent))
{
    out.print("<script>window.alert('超时或未登录');window.location.href='index.jsp'</script>");
}
%>
//出错信息
HTTP Status 500 - --------------------------------------------------------------------------------type Exception reportmessage description The server encountered an internal error () that prevented it from fulfilling this request.exception org.apache.jasper.JasperException
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:207)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:240)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:187)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:809)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:200)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:146)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:209)
at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:596)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:433)
at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:948)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:144)
at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:596)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:433)
at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:948)
at org.apache.catalina.core.StandardContext.invoke(StandardContext.java:2358)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:133)
at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:596)
at org.apache.catalina.valves.ErrorDispatcherValve.invoke(ErrorDispatcherValve.java:118)
at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:594)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:116)
at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:594)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:433)
at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:948)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:127)
at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:596)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:433)
at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:948)
at org.apache.coyote.tomcat4.CoyoteAdapter.service(CoyoteAdapter.java:152)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:799)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.processConnection(Http11Protocol.java:705)
at org.apache.tomcat.util.net.TcpWorkerThread.runIt(PoolTcpEndpoint.java:577)
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:683)
at java.lang.Thread.run(Thread.java:536)
root cause java.lang.NullPointerException
at org.apache.jsp.Main_System_jsp._jspService(Main_System_jsp.java:52)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:92)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:809)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:162)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:240)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:187)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:809)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:200)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:146)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:209)
at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:596)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:433)
at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:948)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:144)
at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:596)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:433)
at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:948)
at org.apache.catalina.core.StandardContext.invoke(StandardContext.java:2358)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:133)
at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:596)
at org.apache.catalina.valves.ErrorDispatcherValve.invoke(ErrorDispatcherValve.java:118)
at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:594)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:116)
at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:594)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:433)
at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:948)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:127)
at org.apache.catalina.core.StandardPipeline$StandardPipelineValveContext.invokeNext(StandardPipeline.java:596)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:433)
at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:948)
at org.apache.coyote.tomcat4.CoyoteAdapter.service(CoyoteAdapter.java:152)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:799)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.processConnection(Http11Protocol.java:705)
at org.apache.tomcat.util.net.TcpWorkerThread.runIt(PoolTcpEndpoint.java:577)
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:683)
at java.lang.Thread.run(Thread.java:536)--------------------------------------------------------------------------------Apache Tomcat/4.1.31

Answer 5

继续改：if判断条件不足，SessionVB可能为空
<%
String SessionVB,SessionContent;
SessionVB=(String)session.getAttribute("LoginStatus");
SessionContent="ok";if(SessionVB!=null && !SessionVB.equals(SessionContent))
{
    out.print("<script>window.alert('超时或未登录');window.location.href='index.jsp'</script>");
}
%>

Answer 6

条件写错了，应该是
if(SessionVB==null || !SessionVB.equals(SessionContent))
{
    out.print("<script>window.alert('超时或未登录');window.location.href='index.jsp'</script>");
}

Answer 7

我对session.getParameter("LoginStatus")进行了输入，发现结果是null，为何？
对其进行赋值的语句如下：<%@ page contentType="text/html;charset=gb2312"%>
<%@ page import="java.sql.*"%>
<%
String checkt,checkh;
checkt=request.getParameter("my_check_t");
checkh=request.getParameter("my_check_h");
if(!checkt.equals(checkh))
{
    out.print("<script>window.alert('验证码不正确');window.location.href='index.jsp'</script>");
}%>
<body>
<%//链接数据库
String user="sa";
String password="";
Class.forName("com.microsoft.jdbc.sqlserver.SQLServerDriver").newInstance();
String url="jdbc:microsoft:sqlserver://localhost:1433;DatabaseName=kingvsql";
Connection conn= DriverManager.getConnection(url,user,password);
String User,Pwd;
User=request.getParameter("usr_id");
Pwd=request.getParameter("usr_pwd");
Statement stmt=conn.createStatement(ResultSet.TYPE_SCROLL_SENSITIVE,ResultSet.CONCUR_UPDATABLE);
String sql="select * from Kingv_Login where Kingv_Login_Username='"+User+"' and Kingv_Login_Password='"+Pwd+"'";
ResultSet rs=stmt.executeQuery(sql);
if (rs.next())
   {
      session.setAttribute("Login_Status","ok");
      response.sendRedirect("Main_System.jsp");
    }
else
{      response.sendRedirect("index.jsp");
}
rs.close();
stmt.close();
conn.close();
%>

Answer 8

在你的代码里打印调试语句，看看程序有没有进if(rs.next())

Answer 9

晕列，System.out.println("程序执行到此处");
在觉得可能出错的地方都打印一些东西
看看那些代码执行了，那些代码没有被执行
从而找出错误所在

Answer 10

if (rs.next())
   {
      session.setAttribute("Login_Status","ok");
      response.sendRedirect("Main_System.jsp");
    }
else
{
      session.setAttribute("Login_Status","ng");
      response.sendRedirect("index.jsp");
}这个试试

Answer 11

我什么都不知道，只知道在
out.print("<script>window.alert('验证码不正确');window.location.href='index.jsp'</script>");
后面最好加上return;

Answer 12

session.getAttribute("LoginStatus")  ?