Exploring the Errai Framework

One of the prominent features of Java EE 6 was the introduction of Contexts and Dependency Injection (JSR 299). Built on top of Dependency Injection for Java (JSR 330), CDI allowed Dependency Injection (DI) and Aspect Oriented Programming (AOP) to become standardized giving developers the flexibility to implement a native solution rather than having to utilize a third party library such as Spring. CDI makes it easy to inject resources such as managed beans and services. It has done wonders for server side components, but if there is one thing web 2.0 has taught us, more and more functionality is being leveraged on the client side. Java based web frameworks have also moved in this direction as well with implementations such as Google Web Toolkit (GWT) compiling Java code into highly optimized JavaScript. Until now, CDI was restricted to being a server side technology, but with the help of the Errai Framework, CDI can now be leveraged on the browser. Errai is more than a dependency injection toolkit but rather a set of technologies built on top of the ErraiBus messaging framework for developing rich web applications using GWT. In this post, we will demonstrate several components of the Errai framework and how they can be used to build powerful web applications.

To help illustrate the various components of the Errai framework, we will walk through a sample application. This application allows users to perform basic mathematic operations such as addition, subtraction, multiplication and division among others. After a successful submission, the result of the operation is displayed below along with a log of all of the operations they have previously performed. Also included next to each submission is an icon indicating the users’ operating system and browser.

This application was designed specifically for showcasing various Errai components. The source code for this application is found at https://github.com/sabre1041/errai-math which will be useful when we begin discussing the implementation. The application can also be accessed directly as it is deployed on Red Hat’s OpenShift PaaS by navigating to http://erraimath.andyserver.com. The application can be deployed using GWT development mode, JBoss EAP 6 or OpenShift. It utilizes the following Errai components:

• Errai JAX-RS
• Errai CDI
• Errai UI
• Errai Data Binding

We will discuss each component in detail moving forward.

Before we dive into the details of the application, let’s briefly discuss how Errai applications fit into the conventions of a GWT application. A GWT application is configured into Modules which are defined in xml files. These files are placed at the top of the project hierarchy. In our application, the module file is named ErraiMath.gwt.xml. Standalone GWT applications need to also define an EntryPoint class which denotes the starting point for the application in their module file. Errai applications can forgo this requirement by instead annotating a class with the @EntryPoint annotation. Our EntryPoint class is called ErraiMath in the com.redhat.errai.math.client.local package. All client side files will be placed in this package.