我做了个网站，当写入用户名按登陆时出现这样的信息：
java.lang.NullPointerException
at greatech.dbcontrol.XmlParser.getValueByName(XmlParser.java:56)
at com.servlet.loginservlet.doPost(LoginServlet.java:117)
at javax.servlet.http.Httpservlet.service(HttpServlet.java:165)
at javax.servlet.http.Httpservlet.service(HttpServlet.java:103)
at com.caucho.server.dispatch.servletFilterChain.doFilter(servletFilterChain.java:113)
at com.servlet.PrivilegeFilter.doFilter(PrivilegeFilter.java:65)
at com.caucho.server.dispatch.FilterFilterChain.doFilter(FilterFilterChain.java:84)
at com.caucho.server.cache.cacheFilterChain.doFilter(cacheFilterChain.java:190)
at com.caucho.server.webapp.webAppFilterChain.doFilter(webAppFilterChain.java:177)
at com.caucho.server.dispatch.ServletInvocation.service(ServletInvocation.java:221)
at com.caucho.server.port.tcpConnection.run(tcpConnection.java:331)
at com.caucho.server.util.ThreadPool.runTasks(ThreadPool.java:464)
at java.lang.Thred.run(Thrad.java:536)
