严重: Servlet.service() for servlet default threw exception 求救

严重: Servlet.service() for servlet default threw exception
java.lang.NullPointerException
at org.bookstore.action.UserAction.register(UserAction.java:22)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.opensymphony.xwork2.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:404)
at com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:267)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:229)
at com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:221)
at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:86)
at com.opensymphony.xwork2.DefaultActionInvocation$2.doProfiling(DefaultActionInvocation.java:224)
at com.opensymphony.xwork2.DefaultActionInvocation$2.doProfiling(DefaultActionInvocation.java:223)
at com.opensymphony.xwork2.util.profiling.UtilTimerStack.profile(UtilTimerStack.java:455)UserAction类里是这样子  public String register()throws Exception{
   System.out.println(usertable.getUsername());
   UserTable usertable1=new UserTable();
   usertable1.setUsername(usertable.getUsername());
   usertable1.setAge(usertable.getAge());
   usertable1.setPassword(usertable.getPassword());
   usertable1.setSex(usertable.getSex());
   userService.saveUserTable(usertable1);
  
   return SUCCESS;
   }
调试在 System.out.println(usertable.getUsername());
就出错UserTable类里值这样子public class UserTable implements java.io.Serializable { // Fields private Integer userid;
private String username;
private String password;
private String sex;
private Integer age;
    private Set orderses=new HashSet(0);