Question

Servlet.service() for servlet UserServlet threw exception

Servlet.service() for servlet UserServlet threw exception
java.lang.NullPointerException
at web.UserServlet.service(UserServlet.java:43)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:298)
at org.apache.coyote.http11.Http11AprProcessor.process(Http11AprProcessor.java:864)
at org.apache.coyote.http11.Http11AprProtocol$Http11ConnectionHandler.process(Http11AprProtocol.java:579)
at org.apache.tomcat.util.net.AprEndpoint$Worker.run(AprEndpoint.java:1665)
at java.lang.Thread.run(Thread.java:619)

给出异常信息中at web.UserServlet.service(UserServlet.java:43)指定的代码
if (forward) {
ServletContext application = getServletContext();
RequestDispatcher dispatcher = application
.getRequestDispatcher(nextPage);
dispatcher.forward(request, response);//提示此处报异常
} else {
response.sendRedirect(request.getContextPath() + nextPage);
}

Answer 1

1.java.lang.NullPointerException说明是空指针，出错在43行，可知是dispatcher为空值
2.javax.servlet. ServletContext的getNamedDispatcher(String name)得到名为name的一个Web资源，包括Servlet和JSP页面。这个资源的名字要在web.xml中指定。
3.nextPage如果是个路径的话应该是以"/"开头的，如果不是"/"开头，则路径不能超出servletContext的范围。

Answer 2

nextPage这个字符串指定的页面，无法生成dispatcher。
从而dispatcher为空，造成的错误。

检查一下nextPage这个字符串，放到地址栏里面能访问不。

Answer 3

请确定dispatcher是不是为空的句柄.
可用下面代码替换
request.getRequestDispatcher(nextPage).forward(request, response)

Answer 4

nextPage是啥东西? 上边怎么定义的? 贴过来看看