no Session问题，即延迟加载
在开发中，相信很多同学都遇到过这个问题，也就是说当我们在前台页面用表达式语言取值的时候，会发现程序后台报错了，显示的错误时no session，其实这个就是延迟加载的问题，下面进行简单的谈一谈，并提出几点解决的建议。
延迟加载的问题是指当我们调用完action中的某个方法，在jsp页面要显示我们想要的信息的时候，发现在dao中打开的session已经关闭了。
如下图，第一个箭头表示的是我们通过前台页面返回action，action会通过service层调用dao去访问数据库，当从数据库中把值取出来之后返回到action中，再返回到前台页面中去。我们知道，只有我们在调用某个类getter()方法的时候才会打开session，可惜的是，在hibernateTemplate中注入的sessionFactory在action返回到前台页面前已经关闭了，也就是这个时候再前台页面调用getter()方法，想要打开session，但是sessionFactory已经关闭了，所以后台就会报错，显示no session。意思就是找不到session。
这个时候可以采用以下几种方式解决：
1、 将pojo类中的fetch的lazy改成eager，这个时候如果你的操作页面中牵扯了多个外键关系的话，就要把这几个外键关系的fetch的lazy都改成eager。这里的lazy是代表懒加载的意思，也就是说在查询一个表的信息的时候，不会把关联的表的信息查找出来。而如果改成eager的话，就代表说把关联的表的信息也全部加载出来。很明显，这样的操作会加载一些我们压根就不需要的操作，也可能我们只需要加载某张表的信息的时候，却自动的把关联的表加载出来。所以这种操作性能是很差的，不建议使用。
2、 配置过滤器openSessionInViewFilter，顾名思义，这个过滤器是指把在页面中打开session的服务，也就是说延迟session打开的状态，在前台页面中调用完方法之后再关闭session，配置如下：
<filter>
<filter-name>OpenSessionInView</filter-name>
<filter-class>
org.springframework.orm.hibernate3.support.OpenSessionInViewFilter
</filter-class>
</filter>
<filter-mapping>
<filter-name>OpenSessionInView</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
要注意的是这个过滤器要配置到struts的过滤器的前面，不然就不起作用了，因为配置在struts过滤器的后面的话，数据已经接收完毕了，也就是说sessionFactory已经关闭了，这个时候你再来配置说把session延长到view中，已经没作用了，所以要把openSessionInViewFilter配置在struts前面。但是要注意的是，上面的配置并不是完整的配置，完整的配置应该是在filter里面配置如下信息：
<filter>
<filter-name>OpenSessionInView</filter-name>
<filter-class>
org.springframework.orm.hibernate3.support.OpenSessionInViewFilter
</filter-class>
<init-param>
<param-name>singleSession</param-name>
<param-value>true</param-value>
</init-param>
</filter>
<filter-mapping>
<filter-name>OpenSessionInView</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
上面配置的意思是表示配置成单例的session。但是如果按以上配置的话，在后续session的操作中是会出现问题的，而如果不配置成单例的话，性能是很低的。所以这个方法也不建议。
3、 手工操作（建议）
a) 在action中操作
在action中操作指的是我们在传递对象到前台页面（这里主要是jsp）的时候，多传递一些对象到前台去。比如在action中我们需要传递question对象到前台中进行展示，但是同时也需要查找出这个问题所属的类型type，这个时候就可以在页action中除了传递question对象之外，再根据这个question的其他属性（如tid）来查找到对应的type，再把type对象传递到页面中。不过这样做比较麻烦的是除了自己要在dao中加方法外，如果一个页面管理着好几个数据表的话，传递的对象属性相对就比较多了。
b) 在daoimpl中操作（强烈建议）
在daoimpl中进行手工设置是本人觉得最合适的方式，也是比较推崇的一种。用这种方法的话，在daoimpl的操作中，就要用到hibernateCallback的内部类方法了。也就是说调用super.getHibernateTemplate().execute()方法，编写内部类，在得到一个list集合之后，取出集合的对象，调用要用到的关联类的getter()方法，就可以打开session了，一般不用调用主键的getter()方法。
范例（只是部分代码，无法运行，仅供参考）：
1 public Question findById(final Integer id) throws Exception { 2 return super.getHibernateTemplate().execute( 3 new HibernateCallback<Question>() { 4 @SuppressWarnings("unchecked") 5 public Question doInHibernate(Session session) 6 throws HibernateException, SQLException { // 用hibernateCallBack 7 String hql = "From Question AS q WHERE q.qid=?"; // 构建hql语句 8 Query query = session.createQuery(hql); // 创建query 9 query.setInteger(0, id); // 设置占位符10 List<Question> all = query.list(); // 得到具体的list集合，里面只存放一个question对象11 if (all.size() > 0) { // 集合不为空12 Question vo = all.get(0); // 取出question对象13 vo.getType().getTitle(); // 调用type的getTitle方法，这样会把type相关的session打开，在jsp调用时数据还可以调用14 vo.getAnswers().size(); // 调用getAnswers()方法，把session打开，消除延迟加载15 if (vo.getAnswer() != null) { // 如果存在最佳答案16 vo.getAnswer().getContent(); // 消除延迟加载17 }18 return vo; // 返回vo19 }20 return null;21 }22 });23 }
错误
21:22:57,444 DEBUG Loader:384 - Result set row: 0
21:22:57,445 DEBUG Loader:1514 - Result row: EntityKey[cn.itheima.domain.Order#1]
21:22:57,446 DEBUG Loader:384 - Result set row: 1
21:22:57,447 DEBUG Loader:1514 - Result row: EntityKey[cn.itheima.domain.Order#2]
21:22:57,448 DEBUG TwoPhaseLoad:144 - Resolving associations for [cn.itheima.domain.Order#1]
21:22:57,448 DEBUG TwoPhaseLoad:274 - Done materializing entity [cn.itheima.domain.Order#1]
21:22:57,449 DEBUG TwoPhaseLoad:144 - Resolving associations for [cn.itheima.domain.Order#2]
21:22:57,449 DEBUG TwoPhaseLoad:274 - Done materializing entity [cn.itheima.domain.Order#2]
21:22:57,450 DEBUG TransactionImpl:62 - committing
21:22:57,450 DEBUG AbstractFlushingEventListener:132 - Processing flush-time cascades
21:22:57,451 DEBUG AbstractFlushingEventListener:174 - Dirty checking collections
21:22:57,451 DEBUG Collections:184 - Collection found: [cn.itheima.domain.Customer.orders#3], was: [cn.itheima.domain.Customer.orders#3] (uninitialized)
21:22:57,452 DEBUG AbstractFlushingEventListener:106 - Flushed: 0 insertions, 0 updates, 0 deletions to 3 objects
21:22:57,452 DEBUG AbstractFlushingEventListener:113 - Flushed: 0 (re)creations, 0 updates, 0 removals to 1 collections
21:22:57,453 DEBUG EntityPrinter:102 - Listing entities:
21:22:57,453 DEBUG EntityPrinter:109 - cn.itheima.domain.Order{price=120.00, receiverInfo=南京, orderNum=2, customer=cn.itheima.domain.Customer#3}
21:22:57,454 DEBUG EntityPrinter:109 - cn.itheima.domain.Order{price=110.00, receiverInfo=北京, orderNum=1, customer=cn.itheima.domain.Customer#3}
21:22:57,454 DEBUG EntityPrinter:109 - cn.itheima.domain.Customer{id=3, cusImgsrc=null, cusPhone=567711131, cusName=fox, orders=<uninitialized>}
21:22:57,457 ERROR DefaultDispatcherErrorHandler:42 - Exception occurred during processing request: failed to lazily initialize a collection of role: cn.itheima.domain.Customer.orders, could not initialize proxy - no Session
org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: cn.itheima.domain.Customer.orders, could not initialize proxy - no Session
at org.hibernate.collection.internal.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollection.java:567)
at org.hibernate.collection.internal.AbstractPersistentCollection.withTemporarySessionIfNeeded(AbstractPersistentCollection.java:205)
at org.hibernate.collection.internal.AbstractPersistentCollection.initialize(AbstractPersistentCollection.java:546)
at org.hibernate.collection.internal.AbstractPersistentCollection.read(AbstractPersistentCollection.java:133)
at org.hibernate.collection.internal.PersistentSet.iterator(PersistentSet.java:163)
at com.alibaba.fastjson.serializer.CollectionCodec.write(CollectionCodec.java:74)
at com.alibaba.fastjson.serializer.JSONSerializer.writeWithFieldName(JSONSerializer.java:367)
at com.alibaba.fastjson.serializer.ASMSerializer_2_Customer.write(Unknown Source)
at com.alibaba.fastjson.serializer.JSONSerializer.writeWithFieldName(JSONSerializer.java:367)
at com.alibaba.fastjson.serializer.ASMSerializer_1_Order.write(Unknown Source)
at com.alibaba.fastjson.serializer.ListSerializer.write(ListSerializer.java:129)
at com.alibaba.fastjson.serializer.JSONSerializer.write(JSONSerializer.java:338)
at com.alibaba.fastjson.JSON.toJSONString(JSON.java:488)
at com.alibaba.fastjson.JSON.toJSONString(JSON.java:421)
at com.alibaba.fastjson.JSON.toJSONString(JSON.java:391)
at cn.itheima.action.OrderAction.findOrder(OrderAction.java:38)
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 ognl.OgnlRuntime.invokeMethod(OgnlRuntime.java:891)
at ognl.OgnlRuntime.callAppropriateMethod(OgnlRuntime.java:1293)
at ognl.ObjectMethodAccessor.callMethod(ObjectMethodAccessor.java:68)
at com.opensymphony.xwork2.ognl.accessor.XWorkMethodAccessor.callMethodWithDebugInfo(XWorkMethodAccessor.java:117)
at com.opensymphony.xwork2.ognl.accessor.XWorkMethodAccessor.callMethod(XWorkMethodAccessor.java:108)
at ognl.OgnlRuntime.callMethod(OgnlRuntime.java:1369)
at ognl.ASTMethod.getValueBody(ASTMethod.java:90)
at ognl.SimpleNode.evaluateGetValueBody(SimpleNode.java:212)
at ognl.SimpleNode.getValue(SimpleNode.java:258)
at ognl.Ognl.getValue(Ognl.java:494)
at ognl.Ognl.getValue(Ognl.java:458)
at com.opensymphony.xwork2.ognl.OgnlUtil$2.execute(OgnlUtil.java:309)
at com.opensymphony.xwork2.ognl.OgnlUtil.compileAndExecute(OgnlUtil.java:340)
at com.opensymphony.xwork2.ognl.OgnlUtil.getValue(OgnlUtil.java:307)
at com.opensymphony.xwork2.DefaultActionInvocation.invokeAction(DefaultActionInvocation.java:423)
at com.opensymphony.xwork2.DefaultActionInvocation.invokeActionOnly(DefaultActionInvocation.java:287)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:250)
at org.apache.struts2.interceptor.DeprecationInterceptor.intercept(DeprecationInterceptor.java:41)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:244)
at org.apache.struts2.interceptor.debugging.DebuggingInterceptor.intercept(DebuggingInterceptor.java:256)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:244)
at com.opensymphony.xwork2.interceptor.DefaultWorkflowInterceptor.doIntercept(DefaultWorkflowInterceptor.java:167)
at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:244)
at com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(ValidationInterceptor.java:265)
at org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept(AnnotationValidationInterceptor.java:76)
at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:244)
at com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.intercept(ConversionErrorInterceptor.java:138)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:244)
at com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:229)
at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:244)
at com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:229)
at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:244)
at com.opensymphony.xwork2.interceptor.StaticParametersInterceptor.intercept(StaticParametersInterceptor.java:191)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:244)
at org.apache.struts2.interceptor.MultiselectInterceptor.intercept(MultiselectInterceptor.java:73)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:244)
at org.apache.struts2.interceptor.DateTextFieldInterceptor.intercept(DateTextFieldInterceptor.java:125)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:244)
at org.apache.struts2.interceptor.CheckboxInterceptor.intercept(CheckboxInterceptor.java:91)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:244)
at org.apache.struts2.interceptor.FileUploadInterceptor.intercept(FileUploadInterceptor.java:253)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:244)
at com.opensymphony.xwork2.interceptor.ModelDrivenInterceptor.intercept(ModelDrivenInterceptor.java:100)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:244)
at com.opensymphony.xwork2.interceptor.ScopedModelDrivenInterceptor.intercept(ScopedModelDrivenInterceptor.java:141)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:244)
at com.opensymphony.xwork2.interceptor.ChainingInterceptor.intercept(ChainingInterceptor.java:145)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:244)
at com.opensymphony.xwork2.interceptor.PrepareInterceptor.doIntercept(PrepareInterceptor.java:171)
at com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:244)
at com.opensymphony.xwork2.interceptor.I18nInterceptor.intercept(I18nInterceptor.java:139)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:244)
at org.apache.struts2.interceptor.ServletConfigInterceptor.intercept(ServletConfigInterceptor.java:164)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:244)
at com.opensymphony.xwork2.interceptor.AliasInterceptor.intercept(AliasInterceptor.java:193)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:244)
at com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor.intercept(ExceptionMappingInterceptor.java:189)
at com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:244)
at org.apache.struts2.impl.StrutsActionProxy.execute(StrutsActionProxy.java:54)
at org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:564)
at org.apache.struts2.dispatcher.ng.ExecuteOperations.executeAction(ExecuteOperations.java:81)
at org.apache.struts2.dispatcher.ng.filter.StrutsPrepareAndExecuteFilter.doFilter(StrutsPrepareAndExecuteFilter.java:99)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:503)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:421)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1070)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:611)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:316)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
at java.lang.Thread.run(Thread.java:745)
