这是个做登录窗口的servlet程序，servlet和html编的都没问题啊
这是web.xml
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns=" http://java.sun.com/xml/ns/j2ee" xmlns:xsi=" http://www.w3.org/2001/XMLSchema-instance" version="2.4" xsi:schemaLocation=" http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
<servlet>
<servlet-name>XFireServlet</servlet-name>
<servlet-class>org.codehaus.xfire.transport.http.XFireConfigurableServlet</servlet-class>
<load-on-startup>0</load-on-startup>
</servlet>
<servlet>
<description>This is the description of my J2EE component</description>
<display-name>This is the display name of my J2EE component</display-name>
<servlet-name>FormServlet</servlet-name>
<servlet-class>servlet.FormServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>XFireServlet</servlet-name>
<url-pattern>/services/*</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>FormServlet</servlet-name>
<url-pattern>/FormServlet</url-pattern>
</servlet-mapping>
<welcome-file-list>
<welcome-file>index.jsp</welcome-file>
</welcome-file-list>
</web-app>
我按这个输入http://localhost:8080/FirstServlet/MyHtml.html是对的，
但是怎么不能直接写servlet的地址呢http://localhost:8080/FirstServlet/FormServlet
console出现的错误信息是：
2009-10-13 18:41:56 org.apache.catalina.core.StandardWrapperValve invoke
严重: Servlet.service() for servlet FormServlet threw exception
java.lang.NullPointerException
at servlet.FormServlet.doGet(FormServlet.java:51)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:627)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:269)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:188)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:172)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:117)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:108)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:174)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:873)
at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:665)
at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:528)
at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:81)
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:689)
at java.lang.Thread.run(Unknown Source)
我是菜鸟，刚学，希望高手耐心教导我，谢谢拉！！！