Java 休眠问题--NullPointerException

我在下面的Hibernate代码中得到一个NullPointerException。我已经用这个问题标出了界限。有人能帮忙吗

public List<Bulletin> searchForBulletins(String searchTerms) {
    StringTokenizer search = new StringTokenizer(searchTerms);
    String searchString = "from Bulletin where";
    /* Error in next line */
    SessionFactory sessionFactory = (SessionFactory) request.getSession()
            .getServletContext().getAttribute("sessionFactory");
    Session session = sessionFactory.openSession();
    while (search.hasMoreElements()) {
        String nextTerm = search.nextElement().toString();
        searchString += " subject like '%" + nextTerm
                + "%' or note like '%" + nextTerm + "%'";
        if (search.hasMoreElements()) {
            searchString += " or";
        }
    }
    try {
        Transaction tx = session.beginTransaction();
        Query myQuery = session.createQuery(searchString);
        tx.commit();
        return (List<Bulletin>) myQuery.list();
    } catch (HibernateException e) {
        e.printStackTrace();
        throw e;
    }
}


java.lang.NullPointerException
    dao.BulletinDAO.searchForBulletins(BulletinDAO.java:195)
    action.SearchBulletins.execute(SearchBulletins.java:44)
    sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    java.lang.reflect.Method.invoke(Unknown Source)
    com.opensymphony.xwork2.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:440)
    com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:279)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:242)
    com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:163)
    com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(ValidationInterceptor.java:251)
    org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept(AnnotationValidationInterceptor.java:68)
    com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.intercept(ConversionErrorInterceptor.java:122)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:195)
    com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:195)
    com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    com.opensymphony.xwork2.interceptor.StaticParametersInterceptor.intercept(StaticParametersInterceptor.java:148)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    org.apache.struts2.interceptor.MultiselectInterceptor.intercept(MultiselectInterceptor.java:75)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    org.apache.struts2.interceptor.CheckboxInterceptor.intercept(CheckboxInterceptor.java:94)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    org.apache.struts2.interceptor.FileUploadInterceptor.intercept(FileUploadInterceptor.java:243)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    com.opensymphony.xwork2.interceptor.ModelDrivenInterceptor.intercept(ModelDrivenInterceptor.java:89)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor.intercept(ScopedModelDrivenInterceptor.java:128)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    org.apache.struts2.interceptor.debugging.DebuggingInterceptor.intercept(DebuggingInterceptor.java:267)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    com.opensymphony.xwork2.interceptor.ChainingInterceptor.intercept(ChainingInterceptor.java:126)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    com.opensymphony.xwork2.interceptor.PrepareInterceptor.doIntercept(PrepareInterceptor.java:138)
    com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:87)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    com.opensymphony.xwork2.interceptor.I18nInterceptor.intercept(I18nInterceptor.java:164)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    org.apache.struts2.interceptor.ServletConfigInterceptor.intercept(ServletConfigInterceptor.java:164)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    com.opensymphony.xwork2.interceptor.AliasInterceptor.intercept(AliasInterceptor.java:128)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor.intercept(ExceptionMappingInterceptor.java:176)
    com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:236)
    org.apache.struts2.impl.StrutsActionProxy.execute(StrutsActionProxy.java:52)
    org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:485)
    org.apache.struts2.dispatcher.FilterDispatcher.doFilter(FilterDispatcher.java:395)

公共列表搜索公告（字符串搜索术语）{
StringTokenizer搜索=新的StringTokenizer（搜索术语）；
String searchString=“从公告中”；
/*下一行出错*/
SessionFactory SessionFactory=（SessionFactory）请求。getSession（）
.getServletContext（）.getAttribute（“sessionFactory”）；
Session Session=sessionFactory.openSession（）；
while（search.hasMoreElements（））{
字符串nexterm=search.nextElement（）.toString（）；
searchString+=“主题类似“%”+nexterm
+“%”或类似“%”+nexterm+“%”的注释；
if（search.hasMoreElements（））{
searchString+=“或”；
}
}
试一试{
事务tx=会话.beginTransaction（）；
Query myQuery=session.createQuery（searchString）；
tx.commit（）；
return（List）myQuery.List（）；
}捕获（休眠异常e）{
e、 printStackTrace（）；
投掷e；
}
}
java.lang.NullPointerException
BulletinDAO.searchForBulletins（BulletinDAO.java:195）
action.SearchBulletins.execute（SearchBulletins.java:44）
sun.reflect.NativeMethodAccessorImpl.invoke0（本机方法）
sun.reflect.NativeMethodAccessorImpl.invoke（未知源）
sun.reflect.DelegatingMethodAccessorImpl.invoke（未知源）
java.lang.reflect.Method.invoke（未知源）
com.opensymphony.xwork2.DefaultActionInvocation.invokeAction（DefaultActionInvocation.java:440）
com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly（DefaultActionInvocation.java:279）
com.opensymphony.xwork2.DefaultActionInvocation.invoke（DefaultActionInvocation.java:242）
com.opensymphony.xwork2.intercept.DefaultWorkflowIntercept.doIntercept（DefaultWorkflowIntercept.java:163）
com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept（MethodFilterInterceptor.java:87）
com.opensymphony.xwork2.DefaultActionInvocation.invoke（DefaultActionInvocation.java:236）
com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept（ValidationInterceptor.java:251）
org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept（AnnotationValidationInterceptor.java:68）
com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept（MethodFilterInterceptor.java:87）
com.opensymphony.xwork2.DefaultActionInvocation.invoke（DefaultActionInvocation.java:236）
com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.intercept（ConversionErrorInterceptor.java:122）
com.opensymphony.xwork2.DefaultActionInvocation.invoke（DefaultActionInvocation.java:236）
com.opensymphony.xwork2.interceptor.parameters-sinterceptor.doIntercept（parameters-sinterceptor.java:195）
com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept（MethodFilterInterceptor.java:87）
com.opensymphony.xwork2.DefaultActionInvocation.invoke（DefaultActionInvocation.java:236）
com.opensymphony.xwork2.interceptor.parameters-sinterceptor.doIntercept（parameters-sinterceptor.java:195）
com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept（MethodFilterInterceptor.java:87）
com.opensymphony.xwork2.DefaultActionInvocation.invoke（DefaultActionInvocation.java:236）
com.opensymphony.xwork2.interceptor.StaticParametersInterceptor.intercept（StaticParametersInterceptor.java:148）
com.opensymphony.xwork2.DefaultActionInvocation.invoke（DefaultActionInvocation.java:236）
org.apache.struts2.interceptor.MultiselectInterceptor.intercept（MultiselectInterceptor.java:75）
com.opensymphony.xwork2.DefaultActionInvocation.invoke（DefaultActionInvocation.java:236）
org.apache.struts2.interceptor.CheckboxInterceptor.intercept（CheckboxInterceptor.java:94）
com.opensymphony.xwork2.DefaultActionInvocation.invoke（DefaultActionInvocation.java:236）
org.apache.struts2.interceptor.FileUploadInterceptor.intercept（FileUploadInterceptor.java:243）
com.opensymphony.xwork2.DefaultActionInvocation.invoke（DefaultActionInvocation.java:236）
com.opensymphony.xwork2.interceptor.modeldriventerceptor.intercept（modeldriventerceptor.java:89）
com.opensymphony.xwork2.DefaultActionInvocation.invoke（DefaultActionInvocation.java:236）
com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor.intercept（ScopedModelDrivenInterceptor.java:128）
com.opensymphony.xwork2.DefaultActionInvocation.invoke（DefaultActionInvocation.java:236）
org.apache.struts2.interceptor.debug.DebuggingInterceptor.intercept（DebuggingInterceptor.java:267）
com.opensymphony.xwork2.DefaultActionInvocation.invoke（DefaultActionInvocation.java:236）
com.opensymphony.xwork2.interceptor.ChainingInterceptor.intercept（ChainingInterceptor.java:126）
com.opensymphony.xwork2.DefaultActionInvocation.invoke（DefaultActionInvocation.java:236）
com.opensymphony.xwork2.interceptor.PrepareInterceptor.doIntercept（PrepareInterceptor.java:138）
com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept（MethodFilterInterceptor.java:87）
com.opensymphony.xwork2.DefaultActionInvocation.invoke（DefaultActionInvocation.java:236）
com.opensymphony.xwork2.interceptor.I18nInterceptor.intercept（I18nInterceptor.java:164）
com.opensymphony.xwork2.DefaultActionInvocation.invoke（DefaultActionInvocation.java:236）
org.apache.struts2.interceptor.ServletConfigInterceptor.intercept（ServletConfigInterceptor.java:164）
com.opensymphony.xwork2.DefaultActionInvocation.invoke（DefaultActionInvocation.java:236）
com.opensymphony.xwork2.interceptor.AliasInterceptor.intercept（AliasInterceptor.java:128）
com.opensymphony.xwork2.DefaultActionInvocation.invoke（DefaultActionInvocation.java:236）
com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor.intercept（ExceptionMappingInterceptor.java:176）
com.opensymphony.xwork2
ServletContext s= request.getSession().getServletContext();
SessionFactory sf = s.getAttribute("sessionFactory");