Unable to determine session size using Session.getSizeInBytes() - wicket

Question

I am trying to determine the size of my session, which I want to track as I start to make a development change. My application runs fine normally with no serialisation errors - except when I try to get the session size.
In Wicket 1.4, I recall that I had no problem, but now I cannot do it. I get the same error if I get the session size in my PageBase constructor.
Can anyone see what I am doing wrong?
Java code
public class PageBase extends WebPage
{
private static final long serialVersionUID = 1L;
...
#Override
protected void onAfterRender()
{
super.onAfterRender();
Session ssnSession = Session.get();
long loSize = ssnSession.getSizeInBytes(); <-- Exception is raised here
...
}
...
}
Stack trace
11-Jul-2012 19:30:11 org.apache.wicket.util.lang.WicketObjects$SerializingObjectSizeOfStrategy sizeOf
WARNING: Unable to determine object size: [...].MySession#d2f5f1
java.io.NotSerializableException: java.lang.Object
at java.io.ObjectOutputStream.writeObject0(Unknown Source)
at java.io.ObjectOutputStream.defaultWriteFields(Unknown Source)
at java.io.ObjectOutputStream.writeSerialData(Unknown Source)
at java.io.ObjectOutputStream.writeOrdinaryObject(Unknown Source)
at java.io.ObjectOutputStream.writeObject0(Unknown Source)
at java.io.ObjectOutputStream.writeObject(Unknown Source)
at org.apache.wicket.util.lang.WicketObjects$SerializingObjectSizeOfStrategy.sizeOf(WicketObjects.java:124)
at org.apache.wicket.util.lang.WicketObjects.sizeof(WicketObjects.java:416)
at org.apache.wicket.Session.getSizeInBytes(Session.java:439)
at [...].PageBase.onAfterRender(PageBase.java:212)
at org.apache.wicket.Component.afterRender(Component.java:962)
at org.apache.wicket.Component.render(Component.java:2326)
at org.apache.wicket.Page.renderPage(Page.java:1035)
at org.apache.wicket.request.handler.render.WebPageRenderer.renderPage(WebPageRenderer.java:105)
...
My software environment
Web framework: Wicket 1.5.7
Java: 1.6.0_33; Java HotSpot(TM) Client VM 20.8-b03
Web server system: Google App Engine for Java version 1.7.0
Operating system: Microsoft Windows XP Home Edition version 2002 SP3 (version 5.1 running on x86; Cp1252; en_GB)

Answer 1

I have managed to fix my problem using
dev_appserver.cmd --jvm_flag=-Dsun.io.serialization.extendedDebugInfo=true
to highlight the offending data member. In my WebSession instances I used
private final Object m_objLockItemPaidStatus = new Object();
to lock processing in a synchronized block. I have changed this to use an instance of
/**
* This class is the simplest possible class which can be used as an object
* to lock processing in a <code>synchronized</code> block
* and which can also be serialised.
* (This class extends <code>java.lang.Object</code> solely to implement the
* <code>java.io.Serializable</code> interface.)
*/
public static class LockObject extends Object implements Serializable
{
private static final long serialVersionUID = 1L;
public LockObject()
{
super();
}
}
instead, in order to ensure that my WebSession instances can be serialised (and so I can find their approximate sizes too using WebSession#getSizeInBytes()).
One thing strange to me is that since I expect Wicket to serialise my WebSession instances in my Google App Engine for Java (GAE/J) environment, why was this not picked up already using the GAE/J development web server?