Wicket in Action

Wicket 1.5 - Mounting resources

27 July 2011, by martin-g

In the previous article I described how to mount pages at nice looking urls. Today I'll show you how to mount resource.

Recently there were several people asking in the mailing lists how to serve images loaded from a database so this will be the functionality of the demo application for this article.

To mount a resource you should do:
MyApp.java

public void init() {
  super.init();
  mountResource("/mount/path", new SomeResourceReference());
  // #mountResource() is a shortcut for:
  // mount(new ResourceMapper("/mount/path", new com.example.SomeResourceReference()));
}

With this setup requests to http://host/mount/path will be handled by com.example.SomeResourceReference. The extraction of the request parameters is the same as with pages. A request to http://host/mount/path/indexed?queryNamed=queryValue will have one indexed and one named parameters. They can be read with Attributes.getParameters().get(0) and .get("queryNamed") respectfully. Named indexed parameters also work the same way as with pages.

Now since we know how to mount resources let's see how to create links to them.

MyApp.java

public void init() {
  super.init();
  mountResource("/images/${name}", new ImageResourceReference());
  mountPage("imagesPage", ImageResourcesPage.class);
}

Here we mounted the resource reference that will find the images and a page which will make use of them.

The important thing when creating the resource reference is to give it unique ResourceReference.Key and provide implementation of #getResource() method:
ImageResourceReference.java

public ImageResourceReference() {
    // this creates a Key with scope 'ImageResourceReference.class' 
    // and a name 'imagesDemo'
    super(ImageResourceReference.class, "imagesDemo");
}

@Override
public IResource getResource() {
    return new ImageResource();
}

The key is needed by ResourceMapper to be able to recognize the resource reference when asked to create urls to it.
ImageResource is an implementation of IResource which will serve the images.

ImageResource.java:

private static class ImageResource extends DynamicImageResource {

     @Override
     protected byte[] getImageData(Attributes attributes) {

         PageParameters parameters = attributes.getParameters();
         StringValue name = parameters.get("name");
            
         byte[] imageBytes = null;
            
         if (name.isEmpty() == false) {
             imageBytes = getImageAsBytes(name.toString());
         }
         return imageBytes;
     }

     private byte[] getImageAsBytes(String label) {...}

     @Override
     public boolean equals(Object that) {
         return that instanceof ImageResource;
     }
}

Since we need to serve images we take advantage of Wicket's DynamicImageResource which cares about HTTP headers and stuff and leave us only to load the image's bytes. The parameter Attributes gives us access to the current request, response and the extracted request parameters. The override of #equals() is the second requirement needed by ResourceMapper to fully recognize the mapped resource. For our need any ImageResource instance will do the job.

To create urls to images you should use the following code:
ImageResourcesPage.java

ResourceReference imagesResourceReference = new ImageResourceReference();
 PageParameters imageParameters = new PageParameters();
        
 String imageName = "anyName.jpg";
 imageParameters.set("name", imageName);
 CharSequence urlForImage = getRequestCycle().urlFor(imagesResourceReference, imageParameters);
 ExternalLink link = new ExternalLink("link", urlForImage.toString());

The link will produce urls like /images/anyName.jpg.

With this the task is done.

The full source of the demo application can be found in my Github account.

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