Thursday, July 10, 2008

Grails Eclipse plugin

So I was messing around with Grails the other day, and heard about the wonderful Eclipse plugin. Like the documentation says, grails generates all the necessary files for Eclipse to recognize your project.

This is great and all, but a lot of people I spoke to encountered the same problem I did.
There's a file for your Spring DSL code, called resources.groovy located in

Many people will then receive the following error:

Invalid Package declaration in script: <path-to-your-project>/grails-app/conf/spring/resources.groovy is not in a source folder matching the package declaration: DonutStore/grails-app/conf/spring resources.groovy

I don't know enough Grails at the moment, but a quick fix is to add:

package spring;

to the top of the file.

Update:I've been coding Grails apps for several months now, and realize that this is just an error with the Eclipse plugin. Since I develop on the Mac, I prefer TextMate anyway. However, if you're an Eclipse fan, be glad that the SpringSource guys are working on a new and improved plugin and it should be available later this year.

Tuesday, July 08, 2008

Spring method injection

I recently inherited some legacy code and it's been extremely difficult to refactor. It's a catch-22: you can't refactor without unit tests in place, but it's difficult to unit test since it wasn't written to be testable. [TDD can cure this. :)]

So after much hair-pulling and thought, I was able to extract a few interfaces and configure some of the business logic classes as Spring managed beans. The problem was that some of the dependencies were still being acquired via the "new" keyword, and it had to remain that way.

In other words, I had a singleton bean which required a non-singleton, or prototype. Not just once, at bean creation time, but a new instance every time. How could this be done, without mucking up my code with the Spring API? We definitely do not want a reference to a BeanFactory or ApplicationContext in our bean.

Enter one of the rarer forms of dependency injection: Method Injection (see section 3.3.8).

The recipe is:
  1. Create a method for the dependency and either make it protected abstract, or public.
  2. In your applicationContext.xml, instead of specifying <property name="someProperty">, you use
    <lookup-method name="theMethodWhichReturnsYourPrototype" bean="idOfPrototypeBean"/>

Let's look at an example:

We have a class Homer, which wants a new instance of another bean, a Donut, every time, instead of reusing the same Donut over and over again.

Now, for the applicationContext.xml:

<!-- a stateful bean deployed as a prototype (non-singleton) -->
<bean id="donut" class="demo.DonutImpl" scope="prototype">
<!-- inject dependencies here as required -->

<!-- homer (singleton) uses donut (prototype) -->
<bean id="homer" class="demo.Homer">
<lookup-method name="getDonut" bean="donut"/>

So what's going on here? To quote the Spring docs:
The Spring Framework implements this method injection by dynamically generating a subclass overriding the method, using bytecode generation via the CGLIB library.

So when Homer asks for a donut via getDonut(), the subclassed Homer, generated via CGLIB, will return a new instance of Donut, which has its dependencies injected by Spring.

Pros of this approach: we are decoupled from BeanFactory and the Spring API
Cons: making the bean abstract makes testing a little difficult, since you have to subclass the class, however, making it concrete and implementing the getter method, you avoid this.