老王學jsp之http500錯誤

今天在編寫網頁計算器時總是出現一個莫名其妙的錯誤

個人代碼是這樣的

<%!
    String filename=this.getServletContext().getRealPath("/");
%>

結果報錯：

HTTP Status 500 - java.lang.NullPointerException

--------------------------------------------------------------------------------

type Exception report

message java.lang.NullPointerException

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

exception 

org.apache.jasper.JasperException: java.lang.NullPointerException
    org.apache.jasper.servlet.JspServletWrapper.getServlet(JspServletWrapper.java:177)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:369)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:390)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:728)


root cause 

java.lang.NullPointerException
    javax.servlet.GenericServlet.getServletContext(GenericServlet.java:125)
    org.apache.jsp.count_jsp.<init>(count_jsp.java:24)
    sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
    sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
    java.lang.reflect.Constructor.newInstance(Unknown Source)
    java.lang.Class.newInstance0(Unknown Source)
    java.lang.Class.newInstance(Unknown Source)
    org.apache.jasper.servlet.JspServletWrapper.getServlet(JspServletWrapper.java:172)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:369)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:390)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:728)


note The full stack trace of the root cause is available in the Apache Tomcat/7.0.42 logs.


--------------------------------------------------------------------------------

Apache Tomcat/7.0.42

後來經過查找解決，應該是<%!%>使用來定義全局變量，函數，類的

這裏的filename不是常量，定之後須要計算才能得出，因此會出問題，改成《%%》

或者將filename改成一個常量的字符串便不會出現問題了。

若是仍是須要用到filename的全局變量；

能夠先在定義一個全局的null值的變量，而後寫一個函數進行賦值操做便可。

jsp老是會有一些莫名其妙的錯誤，並且不告訴你在多少行，真是煩。