


In contrast to Struts, Wicket pages and components are no singletons, they are stateful and session-scoped. This enables us to store user-specific information within pages and components. The information should be stored in fields. This way you can access the information within a class while avoiding long method signatures only for passing the same information around. Instances of components can exist for several requests. For example, a page with a form which gets submitted and produces validation errors uses the same page instance. Furthermore the same page instance gets used when the user presses the back button of the browser and resubmits this form again. Information which gets passed by the constructor should be assigned to fields (normally this must be models). When storing information in fields you should consider that the information is serializable, because the pages are stored using Java serialization. By default Wicket stores pages on the hard disk. A non-serializable object leads to _NullPointerExceptions_ and _NonSerializableExceptions_. Additionally, big data (like binary stuff) should not be stored directly in fields because this can cause performance losses and memory leaks during serialization and deserialization. In this case, you should use the _LoadableDetachableModel_ which can be assigned to a field because this provides an efficient mechanism to load and detach data.
