做的第一个JSF程序~遇到问题~在线等

在我JSF运行的时候抛出以下异常:
javax.servlet.ServletException: Cannot find FacesContext
org.apache.jasper.runtime.PageContextImpl.doHandlePageException(PageContextImpl.java:825)
org.apache.jasper.runtime.PageContextImpl.handlePageException(PageContextImpl.java:758)
org.apache.jsp.info_jsp._jspService(info_jsp.java:75)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:94)
javax.servlet.http.HttpServlet.service(HttpServlet.java:810)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:298)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:292)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:236)
javax.servlet.http.HttpServlet.service(HttpServlet.java:810)
root cause javax.servlet.jsp.JspException: Cannot find FacesContext
javax.faces.webapp.UIComponentTag.doStartTag(UIComponentTag.java:405)
com.sun.faces.taglib.jsf_core.ViewTag.doStartTag(ViewTag.java:105)
org.apache.jsp.info_jsp._jspx_meth_f_view_0(info_jsp.java:90)
org.apache.jsp.info_jsp._jspService(info_jsp.java:66)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:94)
javax.servlet.http.HttpServlet.service(HttpServlet.java:810)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:298)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:292)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:236)
javax.servlet.http.HttpServlet.service(HttpServlet.java:810)我就郁闷了~
我是照着书上做的,问了很多人也没有答案~
想知道为什么会报出这个异常还有异常的解决方法