tomcat发布web项目JSP页面打开报空指针异常解决方法。

访问JSP页面直接报：

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: java.lang.NullPointerException
	org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:538)
	org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:370)
	org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313)
	org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
	org.springframework.web.servlet.view.InternalResourceView.renderMergedOutputModel(InternalResourceView.java:238)
	org.springframework.web.servlet.view.AbstractView.render(AbstractView.java:250)
	org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1047)
	org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:817)
	org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:719)
	org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:669)
	org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:574)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
	org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:70)
	org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:88)
	org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)
root cause
java.lang.NullPointerException
	org.apache.jsp.WEB_002dINF.pages.imagecheck.test_jsp._jspInit(test_jsp.java:22)
	org.apache.jasper.runtime.HttpJspBase.init(HttpJspBase.java:52)
	org.apache.jasper.servlet.JspServletWrapper.getServlet(JspServletWrapper.java:164)
	org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:340)
	org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313)
	org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
	org.springframework.web.servlet.view.InternalResourceView.renderMergedOutputModel(InternalResourceView.java:238)
	org.springframework.web.servlet.view.AbstractView.render(AbstractView.java:250)
	org.springframework.web.servlet.DispatcherServlet.render(DispatcherServlet.java:1047)
	org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:817)
	org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:719)
	org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:669)
	org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:574)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
	org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:70)
	org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:88)
	org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:76)
note The full stack trace of the root cause is available in the Apache Tomcat/6.0.35 logs.
 
后面百度查了原因，是项目lib下面的jsp-api.jar和tomcat 下的jsp-api.jar有冲突导致的。因为我项目里用到了一个user library里包含了jsp-api.jar 和servlet-api.jar两个包，然后通过myeclips发布的时候将这两个包复制到webroot/lib下去了，而tomcat的lib下面本来就有这两个包，导致了冲突，这就是这个错误产生的主要原因。
解决方案是，编辑自定义user library,将user library设置为 system library,这样在通过myeclips发布时就不会自动拷贝这两个JAR包。
或者通过在tomcat_home/conf/catalina/下面加一个配置文件指定webroot来发布项目，避免不必要的自动拷贝.
以前都是用后一种方式发布的项目，今天突然测试一个代码，然后偷懒用myeclipse上的发布按钮发布到tomcat的webapps目录下的发布方式。结果就出现了今天的这个问题，还奇怪的怎么好好的代码就不好用了，整了好几个小时，各种百思不得其解啊。整半天原来是JAR包冲突导致的。
spring mvc架子搭好了，可以访问jsp了，泪流满面啊。码代码去- -#
  
Apache Tomcat/6.0.35