The GuestBook application allows users to enter comments that appear on a
page like a weblog. Drawing the list of comments is very easy with the Wicket
ListView component. This example also gives an impression of what form
handling is like.
As with all examples, you have to put all files in the same package
directory. This means putting the markup files and the java files next to one
another. It is possible to alter this behavior, but that is beyond the scope
of this example.
The Comment POJO model is very straightforward:
In the file GuestBook.java we have put the Java component code for the
guestbook page. This is the homepage for the guestbook application. The page
consists of a form for entering new items to the guestbook and a list of
repeating markup for showing the guestbook entries.
The GuestBook constructor adds a CommentForm and a ListView of
the comments. Notice how the model is passed in as the second argument to the
Then as the view renders, the populateItem method is called passing in a
ListItem container for the current row in the list.
The implementation below obtains the Comment POJO from the list item and
adds label components for the date and text of the Comment. This is all
accomplished in just a few lines of code.
When the CommentForm is submitted, the onSubmit() method is called.
Notice that nothing gets the value of the TextArea that was added in the
CommentForm constructor. This is because the comment is the model and the
third parameter to the TextArea constructor specified the property of the
model to update. So all onSubmit() has to do is create a new comment from
the model that was updated and add it to the comment list. When the page
redraws, the new list will be rendered.
We use a synchronized list as our shared static model used by commentListView
(commentList) to ensure that it is only updated by one thread at a time.
Remember, this is a multi-user application with a shared model!
Finally, you may notice the call to commentListView.modelChanged(). This
informs the list view that its model has been modified. In more advanced
usage scenarios, this would allow Wicket to expire stale pages accessed with
the browser’s back button.
In the HTML below, notice the way that the TextArea component is being
nested inside the CommentForm. Wicket is able to keep everything straight
because the Java Component.add() calls have to result in the same nesting
structure as the HTML.
Finally, notice the <wicket:remove> block. This is simply markup that is
there for previewing purposes only. When the page renders, it is stripped
For completeness, we’ve included the GuestBookApplication class, and as a
final treat the modifications to the web.xml file.
Add the following two sections (servlet and servlet-mapping) to your web.xml
file for running this application.