wicket-user-guide/src/docs/guide/bestpractices/bestpractices_12.gdoc

The factory pattern is useful, but nevertheless not suitable for Wicket components.

*Listing 15:*

{code}
public class CmsFactory {
   public Label getCmsLabel(String markupId, final String url) {
       IModel<String> fragment = new AbstractReadOnlyModel<String>() {
          @Override
          public String getObject() {
             return loadSomeContent(url);
          }
       };
       Label result = new Label(markupId, fragment);
       result.setRenderBodyOnly(true);
       result.setEscapeModelStrings(false);
       return result;
   }

   public String loadContent(String url) {
      // load some content
   }
}

// create the component within the page:
public class MyPage extends WebPage {
   @SpringBean
   CmsFactory cmsFactory;

   public MyPage() {
      add(cmsFactory.getCmsLabel("id", "http://url.to.load.from"));
   }
}
{code}

This approach for adding a label from the @CmsFactory@ to a page seems to be okay at first glance, but it comes with some disadvantages. There is no possibility to use inheritance anymore. Furthermore, there is no possibility to override @isVisible()@ and @isEnabled()@. The factory could also be a Spring service which instanciates the component. A better solution is to create a @CmsLabel@.

*Listing 16:*

{code}
public class CmsLabel extends Label {
   @SpringBean
   CmsResource cmsResource;
   public CmsLabel(String id, IModel<String> urlModel) {
      super(id, urlModel);
      IModel<String> fragment = new AbstractReadOnlyModel<String>(){
         @Override
         public String getObject() {
            return cmsResource.loadSomeContent(urlModel.getObject());
         }
      };
      setRenderBodyOnly(true);
      setEscapeModelStrings(false);
   }
}

// create the component within a page
public class MyPage extends WebPage {
   public MyPage() {
      add(new CmsLabel("id", Model.of("http://url.to.load.from")));
   }
}
{code}

The label in listing 16 is clearly encapsulated in a component without using a factory. Now you can easily create inline implementations and override @isVisible()@ or other stuff. Naturally, you might claim "I need a factory to initialize some values in the component, e.g. a Spring service.". For this you can create a implementation of @IComponentInstantiationListener@. This listener gets called on the super-constructor of every component. The most popular implementation of this interface is the @SpringComponentInjector@ which injects Spring beans in components when the fields are annotated with @\@SpringBean@. You can easliy write and add your own implementation of @IComponentInstantiationListener@. So there is no reason for using a factory anymore. More information about the instanciation listener is located in Wicket's JavaDoc.