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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:

1. nonSerializableObj is actually unaccessible from our Link anonymous subclass
2. To make it accessible, compiler:
   1. Creates an anonymous field of the NonSerializableObject class and an accessor to it
   2. Assigns value to it simultaneously with assignment to nonSerializableObj
   3. Makes nonSerializableObj inside the Link anonymous subclass a local variable
   4. Initializes this local variable through forementioned accessor
3. That's why after compilation we have nonSerializableObj stored in a FIELD instead of LOCAL VARIABLE
4. And because it is non-serializable, serialization of entire component tree fails

How this can be worked around? Extract this property as a local variable and reference it inside the inner class instead of the entire object. If this property is a Serializable then its conversion to field won't do any harm and therefore serializableness will be preserved.