struts2小白有关问题

struts2小白问题
我是一刚刚开始自学struts2的小白 自己写了一小段代码，一直有错误不知道怎么调 请各位帮忙

错误提示：
Struts Problem Report

Struts has detected an unhandled exception:
# Messages: Unable to instantiate Action, userData.action.UserAction, defined for 'execute' in namespace '/'null
File: userData/action/UserAction.java
Line number: 11
Stacktraces
Unable to instantiate Action, userData.action.UserAction, defined for 'execute' in namespace '/'null

  com.opensymphony.xwork2.DefaultActionInvocation.createAction(DefaultActionInvocation.java:318)
  com.opensymphony.xwork2.DefaultActionInvocation.init(DefaultActionInvocation.java:399)
  com.opensymphony.xwork2.DefaultActionProxy.prepare(DefaultActionProxy.java:198)
  org.apache.struts2.impl.StrutsActionProxy.prepare(StrutsActionProxy.java:61)
  org.apache.struts2.impl.StrutsActionProxyFactory.createActionProxy(StrutsActionProxyFactory.java:39)
  com.opensymphony.xwork2.DefaultActionProxyFactory.createActionProxy(DefaultActionProxyFactory.java:58)
  org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:475)
  org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(ExecuteOperations.java:77)
  org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:91)
  org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
  org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
  org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
  org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
  org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
  org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
  org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
  org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:286)
  org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:845)
  org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
  org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
  java.lang.Thread.run(Thread.java:662)
   

java.lang.NullPointerException

  userData.action.UserAction.(UserAction.java:11)
  sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
  sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
  sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
  java.lang.reflect.Constructor.newInstance(Constructor.java:513)
  java.lang.Class.newInstance0(Class.java:355)
  java.lang.Class.newInstance(Class.java:308)
  com.opensymphony.xwork2.ObjectFactory.buildBean(ObjectFactory.java:130)
  com.opensymphony.xwork2.ObjectFactory.buildBean(ObjectFactory.java:161)
  com.opensymphony.xwork2.ObjectFactory.buildBean(ObjectFactory.java:150)
  com.opensymphony.xwork2.ObjectFactory.buildAction(ObjectFactory.java:120)
  com.opensymphony.xwork2.DefaultActionInvocation.createAction(DefaultActionInvocation.java:299)
  com.opensymphony.xwork2.DefaultActionInvocation.init(DefaultActionInvocation.java:399)
  com.opensymphony.xwork2.DefaultActionProxy.prepare(DefaultActionProxy.java:198)
  org.apache.struts2.impl.StrutsActionProxy.prepare(StrutsActionProxy.java:61)
  org.apache.struts2.impl.StrutsActionProxyFactory.createActionProxy(StrutsActionProxyFactory.java:39)
  com.opensymphony.xwork2.DefaultActionProxyFactory.createActionProxy(DefaultActionProxyFactory.java:58)