tomcat报这个错误是什么意思啊

type Exception report

message An exception occurred processing JSP page /student/list.jsp at line 31

description The server encountered an internal error that prevented it from fulfilling this request.

exception
org.apache.jasper.JasperException: An exception occurred processing JSP page /student/list.jsp at line 31

28:

29: <%
30: StudentDao sd = new StudentDaoImpl();
31: List list = sd.getAll();
32: %>
33:
34:

Stacktrace:
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:575)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:477)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:395)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:339)
javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
cn.itcauc.filter.GenericEncodingFilter.doFilter(GenericEncodingFilter.java:29)
org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:96)

root cause
java.lang.NullPointerException
cn.itcauc.dao.impl.StudentDaoImpl.getAll(StudentDaoImpl.java:63)
org.apache.jsp.student.list_jsp._jspService(list_jsp.java:125)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:439)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:395)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:339)
javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
cn.itcauc.filter.GenericEncodingFilter.doFilter(GenericEncodingFilter.java:29)
org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:96)

note The full stack trace of the root cause is available in the Apache Tomcat/7.0.65 logs.

学号：

这个方法调用产生了空指针了，看看代码里面有什么对象没哟判空。

list.jsp这个jsp的第31行调用方法时报错