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This page will contain the accumulated thoughts on what the wicket best practices are. It will also list situations that cause erroneus behaviours and are hard to track down.

Table of contents

Best Practices

Html Template Declaration

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" 
     "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 
 <html  
      xmlns="http://www.w3.org/1999/xhtml"  
      xmlns:wicket="http://wicket.sourceforge.net/"  
      xml:lang="en"  
      lang="en">

Wicket Servlet Mapping

Wicket 1.x

It is best to map the servlet as /app/* instead of /*. The /app/* mapping allows static resources to be handled by the webserver as opposed to the wicket servlet.

You may experience difficulties with form POSTs if you map to /*. For example, if a GET request on a Wicket form produces a URL like http://www.examplehost.com/examplecontext/?wicket:interface=wicket-0:2:3, you will find that Wicket writes an action="/examplecontext?wicket:interface=wicket-0:2:3:someForm:2:IFormSubmitListener" attribute to the |<form\> element. Because of the missing / before the ?, the container may not recognise the resulting POST as a request for the Wicket servlet. This behaviour has been observed with:

  • Jetty 6
  • mod_jk when mounted to accept requests for a subdirectory beneath the root web directory (i.e. "JkMount /* exampleworker" works but not "JkMount /examplesubdirectory/* exampleworker")

Wicket 2.x

To avoid this issue with Wicket 2.0, the recommandation has been changed to use a ServletFilter rather than a servlet - See the Migrate-2.0 page for details of the changes required.

More info

(Thanks, Igor)
Usually static resources are handled by the application server. The server knows it is serving a file and thus sets the last modified date of the resource to the last modified date of a file.

So, for example, let's say you have /myapp/images/foo.gif and you map your wicket servlet to /*

What happens is that when a request comes in for /myapp/images/foo.gif it will match the wicket servlet - so now it is wicket servlet's job to serve this file to the browser. now we are nice enough to provide support for this - but obviously we cannot do as good a job as the application server which has a lot more context.

So for 1.x, we recommend mapping the servlet to something like /app/* so that foo.gif will be processed by the application server and only wicket-specific requests are processed by the servlet.

For 2.x, what we do is instead of using a servlet, use a filter.

The advantage of a filter is that, unlike a servlet, it can choose not to process the request and let whatever is next in chain try. So when using a wicket filter and a request comes in for foo.gif the filter can choose not to process it because it knows it is not a wicket-related request. Since the filter didnt process it, it falls on to the application server to try, and then it works.

Anonymous Inner classes

Don't do this:

class MyPage extends WebPage {
    private MyVeryLargeObject subject;
    // ....
    public MyPage() {
        // get my very large object from database
        subject = MyVeryLargeObjectDao.get();
        // ...
        form.add(new TextField("name", new PropertyModel(MyPage.this, "some.very.large.navigational.structure.name"));
    }
}

The 'subject' instance of the MyVeryLargeObjectDao class will be serialized into the session with each page version. This can get out of hand, because with some business object models, the attached object can become very large. For this we introduced DetachableModels, which will retrieve the data from the database when needed, and will clean up when the data is not needed (at the end of the request for instance).

The other thing to avoid is anonymous or nested instances of IModel. Usually you share an instance of a model between two page instances. If you create an anonymous or nested instance of IModel, then you automatically get a 'this' reference to the class that surrounds it. This will usually be the page, but can also be the form or a listview. Anyway, because the reference is /final/, you will copy that reference to the old page, with the model to the new page, thus duplicating the component tree (it gets versioned etc.). This will eventually lead to OutOfMemoryError.

Search in the mailinglist for outofmemoryerror for other descriptions of this behaviour. I doubt that I have done the subject justice.

''Martijn Dashorst - Extracted from wicket-user mailing list

Gotchas

Empty URL Parameter in CSS Style ( background-image: url(""); )

I ran into this problem where clicking on links in DataTable's generated an internal error in wicket and an error log indicating that the requested component was not found. Strangely, the case was only with Firefox and I had no problem with IE.
After debugging and digging everywhere, I found the cause was an empty image style attribute in my page causing firefox to re-generate a request to the page after the page has been shown, which would in turn re-render the component tree and invalidate the previous components:

    <td style="background-image: url(''); ">

This kind of problem is tricky to detect and even if it isn't causing any errors it would generate an extra load on servers.

--Iman Rahmatizadeh

BODY:onLoad in Panel not added without wicket:head

The onLoad attribute will reference some javascript and that must be added to a header. And because the Panel requires the onLoad AND the javascript it is natural to add the javascript to the Panels header. Thus you are able to create completely independent components which have all the information required to properly execute. No need to remember that you have to add a javascrpt reference to your Page. The Panel will handle it for you.

Taken from the mailing list. Question by Joshua Lim, answer by Juergen Donnerstag

Adding wicket:head to a Page

<wicket:head> is intended for panels and borders only. Do not add this to a page.

Starting download after form submission (Wicket 1.1)

If you want to respond with a download to a form submission, you have to set the response page to null after writing your data into the response.

protected void onSubmit() {
   WebResponse r = (WebResponse)getRequestCycle().getResponse();
   r.setContentType("application/octet-stream");
   r.setHeader("Content-Disposition", "attachment; filename=\"data.abc\"");
   r.write("data");
   getRequestCycle().setResponsePage((WebPage)null);
}

See wicket.examples.displaytag.export.Export for a more complete example.

Starting download after form submission (Wicket 1.2)

The same as for 2.0, except that target.setFileName is from 1.2.3. You can override configure, cast to WebResponse, and call setAttachmentHeader on the response like this:

ResourceStreamRequestTarget target = new ResourceStreamRequestTarget(new FileResourceStream(f), d.getContentType())
{
	@Override
	protected void configure(Response arg0, IResourceStream arg1)
	{
		super.configure(arg0, arg1);
		WebResponse response = (WebResponse) arg0;
		response.setAttachmentHeader(filename);
	}
};

or do it like this:

 WebResource export = new WebResource() {
       @Override
       public IResourceStream getResourceStream() {
         CharSequence discounts = DataBase.getInstance()
             .exportDiscounts();
         return new StringResourceStream(discounts, "text/plain");
       }

       @Override
       protected void setHeaders(WebResponse response) {
         super.setHeaders(response);
         response.setAttachmentHeader("discounts.csv");
       }
     };
     export.setCacheable(false);

     new ResourceLink(this, "exportLink", export);

Starting download after form submission (Wicket 2.0)

This can be achieved using code like this (2.0):

Form form = new Form(this, "exportForm");
new Button<String>(form, "exportButton", new Model<String>(
    "export")) {

  @Override
  protected void onSubmit() {
    CharSequence export = DataBase.getInstance()
        .exportDiscounts();
    ResourceStreamRequestTarget target = new ResourceStreamRequestTarget(
        new StringResourceStream(export, "text/plain"));
    target.setFileName("discounts.csv");
    RequestCycle.get().setRequestTarget(target);
  }
};

PackageResources and relative paths

Assuming you have the following directory structure:

/com
/com/A.class
/com/sun/B.class
/com/sun/resource.txt

with the following Application.mountBookmarkablePage() rules:

mount("/A", A.class);
mount("/A/B", B.class);

Keep in mind that the path argument in the PackageResource methods ignores any Application mount() rules you might have applied. So it is legal to write:

PackageResource.get(com.sun.B.class, "resource.txt")

but it is not legal to write:

PackageResource.get(com.A.class, "B/resource.txt")

PBEWithMD5AndDES not found

[For the moment, I'll leave this, but I'm not convinced this actually belongs here at all - Gwyn 13:56, 23 Aug 2006 (BST)]

Not entirely clear what causes this.

INFO  - ClassCryptFactory          - using encryption/decryption object wicket.util.crypt.SunJceCrypt@17d26fc
ERROR - AbstractCrypt              - Unable to encrypt text ''
java.security.NoSuchAlgorithmException: Algorithm PBEWithMD5AndDES not available
	at javax.crypto.SunJCE_b.a(DashoA12275)
	at javax.crypto.SecretKeyFactory.getInstance(DashoA12275)
	at wicket.util.crypt.SunJceCrypt.generateSecretKey(SunJceCrypt.java:119)
	at wicket.util.crypt.SunJceCrypt.crypt(SunJceCrypt.java:95)
	at wicket.util.crypt.AbstractCrypt.encryptStringToByteArray(AbstractCrypt.java:204)
	at wicket.util.crypt.AbstractCrypt.encrypt(AbstractCrypt.java:107)
	at wicket.markup.html.form.PasswordTextField.getModelValue(PasswordTextField.java:97)
	at wicket.markup.html.form.FormComponent.getValue(FormComponent.java:387)
	at wicket.markup.html.form.TextField.onComponentTag(TextField.java:102)

A possible fix is to add another security provider like this:

[Comments on the mailing list suggest this has been seen twice only, and may be due to misconfigured/corrupted JDK - Suggest reinstalling the JDK before adding BouncyCastle... Gwyn 13:56, 23 Aug 2006 (BST)]

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