As an Apache Wicket user for more than five years I really enjoy its programming model. I recently played with Play Framework 2.1 and Scala and discovered that deploying to Heroku is as easy as a git push. I wondered how difficult it could be using this mechanism to deploy a Wicket application.

Existing attempts

My initial investigation led me to this blog post from Martijn Dashorst explaining how to deploy a Wicket 1.5 application to Heroku, the service has slightly evolved since and the quickstart no longer deploys (Maven repository is no longer available to the run environment).

Adding JPA in the mix

I adapted it to deploy and use Wicket 6 instead of 1.5. And as I am a CDI fan I completed the quickstart with the CDI Wicket module (inspired from this post from Igor Vaynberg) and made the necessary steps to use the heroku bundled PostgreSQL database as a JPA datasource.

The key to make the database works correctly on Heroku resides in the following code snippet, where we parse the provided environment variable to populate hibernate properties.

  try {
     URI dbUri = new URI(System.getenv("DATABASE_URL"));
     String username = dbUri.getUserInfo().split(":")[0];
     String password = dbUri.getUserInfo().split(":")[1];
     String dbUrl = "jdbc:postgresql://" + dbUri.getHost() 
                    + ':' + dbUri.getPort() + dbUri.getPath();
     System.setProperty("hibernate.connection.url", dbUrl);
     System.setProperty("hibernate.connection.user", username);
     System.setProperty("hibernate.connection.password", password);
} catch (Exception e) {
     LOGGER.error("Unable to extract database url");
}

Session replication

On Wicket mailing lists, a user recently asked the steps required to get the data store works correctly on Heroku (as the disk space is ephemeral), the key is using a NoSQL backend like Redis. So I wrote a simple and basic implementation of IDataStore using Redis (I think it can be optimized by someone familiar with Redis, pull requests are welcome).

Hands on

The quickstart can be found at the following address and deploys fine on a stack with Redis Cloud and PostgreSQL add ons enabled.
In the end you got :

• Wicket 6
• CDI via Weld
• JPA with PostgreSQL
• Redis datastore

You can see it live at the following address : http://wicket-6-sample.herokuapp.com/, the app can take a few seconds to start, as Heroku will stop it if it is idling for too long.

Apache Wicket is a great web framework, its clear separation between logic and markup allows to focus on what's need to be done.
As we use Wicket at SRMvision, we needed to send mails with rather rich templates to our users. The first implementation we used was relying on Wicket to generate these templates using Html. We finally don't use it (our mailing tasks are done by a background job, thus we don't have access to our webapp), but I though the code used in our fast proof of concept could help someone else.

import org.apache.wicket.markup.MarkupType;
import org.apache.wicket.markup.html.WebMarkupContainer;
import org.apache.wicket.markup.html.basic.Label;
import org.apache.wicket.markup.html.panel.Panel;
import org.apache.wicket.markup.repeater.RepeatingView;
import org.apache.wicket.model.PropertyModel;
import org.apache.wicket.request.Response;
import org.apache.wicket.response.StringResponse;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;


/**
 * Simple panel allowing to use the templating engine 
 *    provided by Wicket to generate HTML.
 * Typical use would be to generate mail content.
 *
 * User of this class will need to subclass it 
 *    and create a panel as usual. 
 * Then a call to getHtml() will return the generated Html.
 *
 * @author cedric at gatay.fr
 */
public class HtmlTemplaterPanel extends Panel {
    private static final Logger LOGGER = 
        LoggerFactory.getLogger(HtmlTemplaterPanel.class);

    /**
     * Default constructor takes no wicket:id > 
     *   the panel will not be added to any component
     */
    public HtmlTemplaterPanel(){
        super("dummy");
    }

    /**
     * Call this whenever you want to get
     *   the Html for this component
     * @return Html or empty string
     */
    public String getHtml(){
        final Response origResponse = getRequestCycle().getResponse();
        try{
            final StringResponse stringResponse = new StringResponse();
            getRequestCycle().setResponse(stringResponse);
            renderAssociatedMarkup("panel", "");
            return stringResponse.toString();
        }catch(Exception e){
            LOGGER.error("Unable to build HTML for panel : {}",
                         e.getMessage());
        }finally{
            getRequestCycle().setResponse(origResponse);
        }
        return "";
    }

    @Override
    protected boolean getStatelessHint() {
        return true;
    }

    @Override
    public MarkupType getMarkupType() {
        return MarkupType.HTML_MARKUP_TYPE;
    }
}

If you want to generate a template using Apache Wicket, you only need to create a Panel and its associated markup which inherits this simple class. Then, when you want to get the Html for your component, call getHtml().
The only drawback of this is that you need a RequestCycle to generate the markup (you'll find out why reading the code).