Wicket on Google App Engine

Google App Engine now supports Java. Let's get Wicket running on it, quickly.

Note: This post covers the basics of setting up Wicket on GAE. This subsequent post takes the next steps of setting up Spring and persistence with JDO and provides the source code of a sample application.

1. Use the Eclipse plugin to create a project
The Eclipse plugin can be installed via Eclipse Update Manager and it includes a Google App Engine SDK, so you don't need to install that separately. With the plugin, create a "New Web Application Project".


Uncheck "Use Google Web Toolkit" and leave "Use Google App Engine" activated. The project will contain a src folder and war folder. It contains a simple "Hello, World" example and you can run it immediately. But let's get Wicket running, quickly.



2. Add Wicket jars
Add the following jars to the war/WEB-INF/lib directory and add them as external jars to the eclipse project:

- wicket-1.3.5.jar
- slf4j-api-1.5.2.jar
- slf4j-log4j12-1.4.2.jar
- log4j-1.2.13.jar



3. Turn on logging
Turn on logging, so that you can see any strange things, that may occur. App Engine won't tell you much by default.

- log4j.properties: log4j.logger.org.apache.wicket=DEBUG, A1
- java6 logging.properties: .level = INFO

4. Create a Wicket Application and a Page
Add WicketApplication.java

package wicket;

public class WicketApplication extends WebApplication {
   public Class getHomePage() {
       return HomePage.class;
   }
}

and a simple HomePage.java and HomePage.html:

public class HomePage extends WebPage {
   public HomePage() {
       add(new Label("label", new Model("Hello, World")));
   }
}

5. Set up WicketFilter
Add the Wicket servlet filter to web.xml. Alternatively use WicketServlet:

 WicketFilter
 org.apache.wicket.protocol.http.WicketFilter
 
  applicationClassName
  wicket.WicketApplication
 



 WicketFilter
 /wicket/*

6. Add HTTP session support
Enable HTTP session support (by default it's disabled) by adding the following line to appengine-web.xml:

true

Now the app is ready to run, but there are still some issues.

7. Disable resource modification watching
When Wicket is started in development mode now, exceptions are raised because Wicket spawns threads to check resource files such as HTML files for modifications. Let's just disable resource modification checking by setting the resource poll frequency in our WicketApplication's init() method to null.

protected void init() {
   getResourceSettings().setResourcePollFrequency(null);
}

8. Disable SecondLevelCacheSessionStore
We cannot use the SecondLevelCacheSessionStore in the app engine, which is the default. At least not with a DiskPageStore, because this serializes the pages to the disk. But writing to the disk is not allowed in the app engine. You can use the simple HttpSessionStore implementation instead (this increases HTTP session size), by overriding newSessionStore() in our WicketApplication:

protected ISessionStore newSessionStore() {
   //return new SecondLevelCacheSessionStore(this, new DiskPageStore());
   return new HttpSessionStore(this);
}

9. Run
From the context menu of the project, call Run As -> Web Application. Open http://localhost:8080/wicket/ in your browser and see the app running.


Download the demo as an eclipse poject.