嚴重: Servlet.service() for servlet default threw exceptionjava.lang.NullPointerExceptionat service.LoginAction.execute(LoginAction.java:43)at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)at java.lang.reflect.Method.invoke(Method.java:597)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)public class LoginAction extends ActionSupport{  private String username;  private String password;  private PersonDAO personManager;  /*其中的seter和geter方法省略了*/@Override  public String execute() throws Exception {  String flag = "";  Person per = new Person();  per.setUname(this.username);  per.setUpwd(this.password);  personManager.save(per);return flag;  } } save函數的的代碼public class PersonDAO extends HibernateDaoSupport{public void save(Person transientInstance) {log.debug("saving Person instance");try {getHibernateTemplate().save(transientInstance);log.debug("save successful");} catch (RuntimeException re) {log.error("save failed", re);throw re;}}還有什么需要的請各位大俠提出來謝謝誒<bean id="PersonDAO" class="user.PersonDAO"><property name="sessionFactory"><ref bean="sessionFactory" /></property>