Page History

Serialization

...

Wicket serializes pages to the session between requests. That's why entire component tree must be serializable - i.e., may not contain non-serializable objects. And, if such an object happens to be referenced at the moment of serialization by a field somewhere in a component tree, an WicketRuntimeException will be thrown. Unfortunately, in Tomcat stack trace displayed is not very helpful and the only useful information there is the class of WebPage that contains a non-serializable somewhere in the component tree. This faulty object may be found using the exclusion method - by removing components one by one and checking after each removal if the exception is no longer thrown.

Final variables

There is a pitfall concerning final variables and serialization. Consider the following fragment:

 public MyPanel(String id)
 {
     super(id);
     
     // ... 
     
     final NonSerializableObject nonSerializableObj = getNSO();
     
     // ...
     
     Link link = new Link("myLink")
     {
         public void onClick()
         {
             if (nonSerializableObj.getSomeBooleanProperty())
             {
                 // ...
             }
             
             // ...
         }
     }
     
     // ...
 }

Everything looks fine at the first sight, but let's look closer at how it works:

...