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.
There is a pitfall concerning final variables and serialization. Consider the following fragment:
Everything looks fine at the first sight, but let's look closer at how it works:
- nonSerializableObj is actually unaccessible from our Link anonymous subclass
- To make it accessible, compiler:
- Creates an anonymous field of the NonSerializableObject class and an accessor to it
- Assigns value to it simultaneously with assignment to nonSerializableObj
- Makes nonSerializableObj inside the Link anonymous subclass a local variable
- Initializes this local variable through forementioned accessor
- That's why after compilation we have nonSerializableObj stored in a FIELD instead of LOCAL VARIABLE
- 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.