[Hawkular-dev] WF9 requires us to change our agent subsystem extension classes

[John Mazzitelli]

The quick summary of this email: the Hawkular Wildfly Monitor Agent will not be able to run on WildFly 9 in its current state. It must be refactored due to changes that were made in the WildFly 9 subsystem extension API.

Details:

After staring at this for a while, I found that our currently WildFly8 code layout of the subsystem extensions in the agent needs to change for this to run in WF9 - I don't think I used this approach in the bus or nest subsystem extensions so nothing needs to change there.

The design pattern here is to use PersistentResourceDefinition subclasses for our the subsystem resource definitions (called the Definition classes). In here you define static members for all attributes and operations as well as an INSTANCE object of the definition itself, among other things.

The problem is, that same Definition class needs to grab (and thus create) an instance of the AddStepHandler within the Definition class' constructor. And THAT AddStepHandler needs to refer back to the Definition object for the attributes. Today, it does so in the populateModel() overloaded method, which isn't a problem. However, we needed to access a parent class' data field (this.attributes) to set our attributes in that populateModel() method. Unfortunately, that parent class in WF9 has changed that this.attributes data field to final. So we can't do that anymore. Instead, it appears the only way to set the attributes now is to pass them in via the AddStepHandler class' constructor (calling the superclass's "super(Collection<?> attributes")).

But this introduces an ugly circular dependency in the Definition and AddStepHandler constructors - it bombs with an NPE.

So this means in order for us to move the agent to WF9, I would need to refactor the subsystem definition classes - I would probably have to extract out all of the attribute/operation definitions into some independent class that can be shared between the *Definition and *Add classes to avoid this constructor-circularity dependency. It probably won't be hard to do this, just a hour or two hopefully. But as it is now, it can't run on WF9.