老王学jsp之http500错误

今天在编写网页计算器时总是出现一个莫名其妙的错误

个人代码是这样的

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

结果报错：

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

后来经过查找解决，应该是<%!%>使用来定义全局变量，函数，类的

这里的filename不是常量，定之后须要计算才能得出，因此会出问题，改成《%%》

或者将filename改成一个常量的字符串便不会出现问题了。

若是仍是须要用到filename的全局变量；

能够先在定义一个全局的null值的变量，而后写一个函数进行赋值操做便可。

jsp老是会有一些莫名其妙的错误，并且不告诉你在多少行，真是烦。