即使会话未关闭，Hibernate`会话也会被关闭

Question

我有一个Coupons数据库，使用Hibernate我试图获得5行。

这是代码：

public List<Coupon> getPage(int pageNum, boolean isActive) {
    session = factory.openSession();
    session.beginTransaction();
    Query query;
    if(isActive) {
        query = session.createQuery("from Coupon where (NOW() < fadeoutDate)");
    } else {
        query = session.createQuery("from Coupon");
    }
    query.setFirstResult((pageNum - 1) * numCouponsPerPage);
    query.setMaxResults(numCouponsPerPage);
    List<Coupon> page = (List<Coupon>) query.list();
    closeSession();
    return page;
}

（numCouponsPerPage是一个等于5的常量）

尝试获取页面时：

List<Coupon> p = cm.getPage(dpage,active.equalsIgnoreCase("true"));（dpage = 1，第二个参数返回false），

我收到了这个错误：

Dec 14, 2013 4:11:06 AM org.apache.catalina.core.StandardWrapperValve invoke
SEVERE: Servlet.service() for servlet [jsp] in context with path [/firstXDw] threw exception [An exception occurred processing JSP page /views/system/loadPage.jsp at line 30

27:     
28:     if (whatToLoad.equalsIgnoreCase("coupons")) {
29: 
30:         List<Coupon> p = cm.getPage(dpage,active.equalsIgnoreCase("true"));
31:         for (Coupon c : p) {
32:             out.println("<tr>");
33:             out.println("<td>" + c.getId() + "</td>");


Stacktrace:] with root cause
org.hibernate.SessionException: Session is closed!
    at org.hibernate.impl.AbstractSessionImpl.errorIfClosed(AbstractSessionImpl.java:72)
    at org.hibernate.impl.SessionImpl.createQuery(SessionImpl.java:1768)
    at implementations.Coupons.getPage(Coupons.java:130)
    at org.apache.jsp.views.system.loadPage_jsp._jspService(loadPage_jsp.java:82)
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
    at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:419)
    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:391)
    at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:304)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:240)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:164)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:462)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:164)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:562)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:395)
    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:250)
    at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:188)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:302)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)

为什么抛出异常Session is closed？我在函数的第一行打开会话..

问题是什么以及如何解决？