org.hibernate.exception.JDBCConnectionException: could not load an entity:

javax.servlet.ServletException: org.hibernate.exception.JDBCConnectionException: could not load an entity: [cn.sdau.wxxyjw.domain.Announce#4]
org.apache.struts2.dispatcher.Dispatcher.serviceAction(Dispatcher.java:515)
org.apache.struts2.dispatcher.FilterDispatcher.doFilter(FilterDispatcher.java:419)
com.opensymphony.module.sitemesh.filter.PageFilter.parsePage(PageFilter.java:119)
com.opensymphony.module.sitemesh.filter.PageFilter.doFilter(PageFilter.java:55)
org.apache.struts2.dispatcher.ActionContextCleanUp.doFilter(ActionContextCleanUp.java:99)
root cause org.hibernate.exception.JDBCConnectionException: could not load an entity: [cn.sdau.wxxyjw.domain.Announce#4]
org.hibernate.exception.SQLStateConverter.convert(SQLStateConverter.java:74)
org.hibernate.exception.JDBCExceptionHelper.convert(JDBCExceptionHelper.java:43)
org.hibernate.loader.Loader.loadEntity(Loader.java:1874)
com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(ValidationInterceptor.java:150)
org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept(AnnotationValidationInterceptor.java:48)
com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:86)
.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:221)
com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:167)
.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:221)

root cause com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException: No operations allowed after connection closed.Connection was implicitly closed due to underlying exception/error:
** BEGIN NESTED EXCEPTION ** com.mysql.jdbc.exceptions.jdbc4.CommunicationsException
MESSAGE: The last packet successfully received from the server was39484 seconds ago.The last packet sent successfully to the server was 39484 seconds ago, which  is longer than the server configured value of 'wait_timeout'. You should consider either expiring and/or testing connection validity before use in your application, increasing the server configured values for client timeouts, or using the Connector/J connection property 'autoReconnect=true' to avoid this problem.STACKTRACE:com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: The last packet successfully received from the server was39484 seconds ago.The last packet sent successfully to the server was 39484 seconds ago, which  is longer than the server configured value of 'wait_timeout'. You should consider either expiring and/or testing connection validity before use in your application, increasing the server configured values for client timeouts, or using the Connector/J connection property 'autoReconnect=true' to avoid this problem.
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
at com.mysql.jdbc.Util.handleNewInstance(Util.java:406)
.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
at java.lang.Thread.run(Thread.java:619)
Caused by: java.net.SocketException: Broken pipe
at java.net.SocketOutputStream.socketWrite0(Native Method)
at java.net.SocketOutputStream.socketWrite(SocketOutputStream.java:92)
at java.net.SocketOutputStream.write(SocketOutputStream.java:136)
at java.io.BufferedOutputStream.flushBuffer(BufferedOutputStream.java:65)
at java.io.BufferedOutputStream.flush(BufferedOutputStream.java:123)
at com.mysql.jdbc.MysqlIO.send(MysqlIO.java:3227)
... 142 more
** END NESTED EXCEPTION **
.apache.struts2.dispatcher.ActionContextCleanUp.doFilter(ActionContextCleanUp.java:99)
将项目运行在tomcat下面开始时候正常 不会出现上面的错误 但是过了一天左右的时间就会出现上面的错误，重启tomcat就行了
不知道是什么原因啊  不知道是不是二级缓存的事情？？？希望知道的指点迷津