出现这个错误是怎么回事呢，老师

An exception occurred processing JSP page /details.jsp at line 41
org.apache.jasper.JasperException: An exception occurred processing JSP page /details.jsp at line 41

38:                     PetDAO petDao = new PetDAO();  
39:                     
40:                     
41:                     Pet pet = petDao.getPetByPid(Integer.parseInt(request.getParameter("pid"))); 
42:                     if(pet!= null)  
43:                     {  
44:                     


Stacktrace:
	org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:574)
	org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:476)
	org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:396)
	org.apache.jasper.servlet.JspServlet.service(JspServlet.java:340)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
	org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)