数据库采用MySQL
服务器采用Tomcat 
打开网页出现这种情况，
还望大侠帮助~不胜感激
HTTP Status 500 - 

type Exception report

message

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 /ch23/dynamicTree.jsp at line 17

14: <div id="load" style="display:none"> <img src="images/loading.gif">Loading data..</div>
15: <ul class="tree">
16: <%
17: Category topCate = new Category(0);
18: TreeviewElement[] top = topCate.getChildren();
19: for(int i= 0;i<top.length;i++)
20: out.println(treeviewRender.renderTreeViewAjax(top[i],false));


Stacktrace:
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:505)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:416)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:342)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:267)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
book.util.EncodingFilter.doFilter(EncodingFilter.java:21)


root cause 

java.lang.NullPointerException
book.ch23.Category.FromDb(Category.java:45)
book.ch23.Category.<init>(Category.java:19)
org.apache.jsp.ch23.dynamicTree_jsp._jspService(dynamicTree_jsp.java:71)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:374)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:342)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:267)
javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
book.util.EncodingFilter.doFilter(EncodingFilter.java:21)


note The full stack trace of the root cause is available in the Apache Tomcat/6.0.20 logs.