z2-Environment

h1. The Vaadin Add-on

The Vaadin add-on provides the Vaadin libraries and some minimal supporting functionality:

* The *ContextClassLoader* ("source":https://redmine.z2-environment.net/projects/z2-addons/repository/z2-addons-vaadin/revisions/master/entry/com.vaadin/java/src.api/com/zfabrik/vaadin/ContextClassLoader.java, "javadoc":http://www.z2-environment.net/javadoc/com.vaadin!2Fjava/api/com/zfabrik/vaadin/ContextClassLoader.html) utility helps loading your Vaadin application class from the right context.

* An extensibility utility. The *ExtensionComponentsUtil* ("source":https://redmine.z2-environment.net/projects/z2-addons/repository/z2-addons-vaadin/revisions/master/entry/com.zfabrik.vaadin/java/src.api/com/zfabrik/vaadin/ExtensionComponentsUtil.java, "javadoc":http://www.z2-environment.net/javadoc/com.zfabrik.vaadin!2Fjava/api/com/zfabrik/vaadin/ExtensionComponentsUtil.html) help you to split a Vaadin web application across modules.

h2. More details on the ContextClassLoader

In non-modular application environments like the typical Java Web Application Server, you will typically use a build tool like ANT or Maven to package all libraries and resources that are required to run your Web application into one Web application archive (WAR). At runtime, a Web container like Apache Tomcat serves all code-like resources of the Web app from one classloading scope. 

Toolkits like Vaadin that need to load user classes by name now need to decide what class loader to refer to for loading of user-defined classes, such as the actual application class in the Vaadin case. The right and de-facto standard choice is to use the context class loader associated with the current thread. Unfortunately Vaadin uses the class loader that loaded the Vaadin classes however. 

In modular environments, such as Z2 but also OSGi, the Vaadin implementation will typically be shared among many Web applications. In that case, the application may see the Vaadin types but not vice versa.

In order to make Vaadin work as expected, add the following init parameter to the Vaadin servlet config in the @web.xml@ file of your Web app:

<pre><code class="xml">

<init-param>

	<param-name>ClassLoader</param-name>

	<param-value>com.my.package.ContextClassLoader</param-value>

</init-param>

</code></pre>

Read on here:

* "web.xml":https://redmine.z2-environment.net/projects/z2-samples/repository/z2-samples-vaadin-spring-hibernate/revisions/master/entry/com.zfabrik.samples.vaadin-spring-hibernate.web/web/WebContent/WEB-INF/web.xml of Vaadin sample app

* "Ticket":http://dev.vaadin.com/ticket/9809 with Vaadin

* A "blog article":http://www.z2-environment.net/blog/2012/07/for-techies-protecting-java-in-a-modular-world-context-classloaders/ on class loader topics

h2. More details on the ExtensionComponentsUtil

While the context class loader from above is a necessity to use Vaadin, the extension components util (ECU) is a great but not strictly required addition. It implements the following formula

(Vaadin is all Java) + (Z2 for modular Java applications) => Vaadin on Z2 for modular user interfaces

More specifically, when applications get bigger, user interface implementations, like other parts of your implementation, have a tendency to become too big to maintain as single, monolithic modules. Using the ECU you can rather easily break your user interface implementation into modules that may have their completely private set of dependencies. This does not only allow to split the code base into well-managable pieces, it does also allow to cleanly separate concerns and to implement general user interface extensibility.