Question

运行jsp文件时tomcat提示的错，求大神帮忙看看，想知道具体解决步骤

java.lang.NullPointerException
at com.hxl.model1.UserBeanCl.checkUser(UserBeanCl.java:41)
at org.apache.jsp.logincl_jsp._jspService(logincl_jsp.java:101)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:432)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:395)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:339)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:505)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:423)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1079)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:620)
at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.doRun(AprEndpoint.java:2476)
at org.apache.tomcat.util.net.AprEndpoint$SocketProcessor.run(AprEndpoint.java:2465)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Thread.java:745)

Answer 1

java.lang.NullPointerException
at com.hxl.model1.UserBeanCl.checkUser(UserBeanCl.java:41)这说明UserBeanCl的checkUser方法抛空指针异常了，看看UserBeanCl的41行，那个对象或变量是null，你还去调它的方法什么的，比如user.getName(),如果user是null，就会报这个错吧。

追问

本来想贴出程序的，但是超过字数了，你说的我都知道，但是并没有找到错误，作为初学者，不太懂得调试方法，想知道一般遇到类似问题的调试方法有那些，具体的步骤是什么，谢谢！

Answer 2

空指针异常。这个情况一般是出在数据库的链接上面。你还是看下你的驱动链接和链接语句。但部分是出在这。

追问

检查过了，之前运行都没有错，后来把程序分层后是拷贝过来的，也检查了，并没有错。。。

追答

那你说没错就没错吧。