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
[font color=#0000FF]HTTP Status 500 - [/font]

[font color=#0000FF]type[/font] Exception report

[font color=#0000FF]message[/font]

[font color=#0000FF]description[/font] The server encountered an internal error () that prevented it from fulfilling this request.
 
[font color=#0000FF]exception[/font]

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)


[font color=#0000FF]root cause [/font]

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)


[font color=#0000FF]note[/font] The full stack trace of the root cause is available in the Apache Tomcat/6.0.20 logs.
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