Using Spring DI in your Gaelyk projects
Since I started using Gaelyk, one of the features I missed most (coming from a Grails background) is Spring's dependency injection. Until recently I didn't even know if it was possible to use Spring in Google App Engine, so I decided to do a little investigation on the subject, and found out that it's very easy indeed.
Here's a little tutorial on how to configure Spring in your Gaelyk project. I'm assuming you have basic knowledge of Spring, Gaelyk and Maven.
First, let's create a Gaelyk project. The easiest way is using the excelent maven-gaelyk archetype:
mvn archetype:generate -DarchetypeGroupId=org.codeconsole -DarchetypeArtifactId=gaelyk-archetype -DarchetypeVersion=0.5.5 -DarchetypeRepository=http://maven-gaelyk.googlecode.com/svn/repository/ -DartifactId=gaelyk-spring -DgroupId=com.deluan.gaelyk -DgaeApplicationName=gaelyk-spring
Now open the project in your favorite IDE, so we can edit the configuration files. First, add the Spring dependency to your pom.xml:
Next we need to configure Spring's ContextLoaderListener in web.xml:
As you can see from above, we configured Spring to load all context configuration files under the directory WEB-INF/spring.
With these configurations in place, your project is already Spring enabled. Now we need a easy way to access the Spring's Application Context. One good way to do this is using a singleton that implements the ApplicationContextAware interface. To keep this post as small as possible, I borrowed an implementation from this blog post, where you can learn more about the details. Create the directory src/main/groovy and put the following SpringApplicationContext singleton there (in the correct package):
Configure the singleton in your spring context:
As you can see, I also declared some SimpleDateFormat instances as beans to be used in our examples.
Now everything is configured and ready to be used. Let's se how we can obtain a spring bean inside a Groovlet. Create the file WEB-INF/groovy/index.groovy with the following content
Now run your application with the command `mvn gae:run`, point your browser to http://localhost:8080/index.groovy and you should see something like this:
Well, that's it! Nothing so much different from what you are used to do in a normal Web application, right? But remember: Gaelyk is NOT your normal Web framework so let's spice things a little bit.
The solution for looking up beans depicted above is a bit cumbersome. Let's use Gaelyk's plugin system to make things a little more "groovy". Using the plugin descriptor bellow, we can provide shortcuts to our SpringApplicationContext's methods, getContext() and getBean(). Save it in the file WEB-INF/plugins/spring.groovy:
Before you can use these shortcuts, you need to tell Gaelyk about your descriptor by "installing" it in your project. Save the code bellow in the file WEB-INF/plugins.groovy:
Now you can use the shortcuts in your Groovlets this way:
Cool, isn't it? A note on the `autowire` binding: It creates bindings "automagically" for each bean you passed as a parameter, as if the beans were declared in your Groovlet.
You can download the sample project used in this post from GitHub: http://github.com/deluan/gaelyk-spring. You can follow each commit to see exactly what was changed in each step of this tutorial.
If you have any suggestion or question, please leave a comment.
