java.lang.NullPointerException?at serviceImpl.UsersDAOImpl.userLogin(UsersDAOImpl.java:19)?at action.UsersAction.login(UsersAction.java:22)?at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)?at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)?at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)?at java.lang.reflect.Method.invoke(Method.java:606)?at com.opensymphony.xwork2.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:452)?at com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:291)?at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:254)?at com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:176)?at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)?at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)?at com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(ValidationInterceptor.java:263)?at org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept(AnnotationValidationInterceptor.java:68)?at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)?at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)?at com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.intercept(ConversionErrorInterceptor.java:133)?at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)?at com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:207)?at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)?at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)?at com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:207)?at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)?at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)?at com.opensymphony.xwork2.interceptor.StaticParametersInterceptor.intercept(StaticParametersInterceptor.java:190)?at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)?at org.apache.struts2.interceptor.MultiselectInterceptor.intercept(MultiselectInterceptor.java:75)?at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)?at org.apache.struts2.interceptor.CheckboxInterceptor.intercept(CheckboxInterceptor.java:94)?at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)?at org.apache.struts2.interceptor.FileUploadInterceptor.intercept(FileUploadInterceptor.java:243)?at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)?at com.opensymphony.xwork2.interceptor.ModelDrivenInterceptor.intercept(ModelDrivenInterceptor.java:100)?at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)?at com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor.intercept(ScopedModelDrivenInterceptor.java:141)?at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)?at org.apache.struts2.interceptor.debugging.DebuggingInterceptor.intercept(DebuggingInterceptor.java:267)?at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)?at com.opensymphony.xwork2.interceptor.ChainingInterceptor.intercept(ChainingInterceptor.java:142)?at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)?at com.opensymphony.xwork2.interceptor.PrepareInterceptor.doIntercept(PrepareInterceptor.java:166)?at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)?at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)?at com.opensymphony.xwork2.interceptor.I18nInterceptor.intercept(I18nInterceptor.java:176)?at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)?at org.apache.struts2.interceptor.ServletConfigInterceptor.intercept(ServletConfigInterceptor.java:164)?at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)?at com.opensymphony.xwork2.interceptor.AliasInterceptor.intercept(AliasInterceptor.java:190)?at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)?at com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor.intercept(ExceptionMappingInterceptor.java:187)?at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)?at org.apache.struts2.impl.StrutsActionProxy.execute(StrutsActionProxy.java:52)?at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:485)?at org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(ExecuteOperations.java:77)?at org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:91)?at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:192)?at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165)?at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:198)?at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:108)?at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:522)?at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140)?at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)?at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:620)?at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)?at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:349)?at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:1102)?at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)?at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:788)?at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.run(NioEndpoint.java:1485)?at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)?at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)?at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)?at java.lang.Thread.run(Thread.java:744)怎么解決呢？代碼如下?@Override?public boolean userLogin(Users u) {??Transaction tx = null;//創建事務??String hql = "";??try{???Session session = HibernateSessionFactory.getSessionFactory().getCurrentSession();???tx = session.beginTransaction();???hql = "from Users where username=? and password=? ";???Query query = session.createQuery(hql);???query.setParameter(0,u.getUsername());???query.setParameter(1,u.getPassword());???List list = query.list();???tx.commit();//提交事務???if(list.size()>0){????return true;???}???else{????return false;???}?????}catch(Exception ex){???ex.printStackTrace();???return false;??}finally{???if(tx!=null){????tx = null;???}??}?}