A passive API

A passive entity must employ a passive API. A passive API is one that is acted upon, versus one that acts itself. See the Inversion of Control pattern for an explanation.

A specific Role

The concept of roles comes from the theater. A play, musical, or movie will have a certain number of roles that actors play. Although there never seems to be a shortage of actors, there are a finite number of roles. I am not going to make reference to different types of roles at this point, but simply bring the concept to light. The function or action of a role is defined by its script.

We are introducing this concept now because you need to have it in mind when you are designing your system architecture. Think of the different roles in your system, and you will have your "cast" of components so to speak.

For each role, you need to specify its script, or interface to the rest of the system. To be honest the interface is not enough. There are specific contracts that you must define and keep in mind when you specify your interfaces. In other words, what users of the component must provide, and what the component produces. When the interfaces and contracts are defined, you can work on your implementation.

The Service Manager

For the majority of all cases, you will need to use the ServiceManager to get the instance of the component you need. If you recall the discussion on component roles in the component documentation, you already have a head start. In Avalon, roles are defined by the work interface a component has. A work interface is different from any other interface because it is the interface that defines the component's role. Serviceable and Component are concern interfaces because they address specific concerns about the component.

The ServiceManager has one method to retrieve all of your components. The lookup method will look up the component based on the fully qualified name (FQN) of the work interface (Role). See the following example:

final MyComponent component = (MyComponent)manager.
     lookup( "com.mycompany.myproject.MyComponent" );
        

It is important to note that role is not the same thing as functional equivalence. In other words, if you have a MailSpooler that is functionally equivalent to a FileStore (they do the same thing), it does not mean that they perform the same role. The FileStore is used to store objects to files, and the MailSpooler is used to temporarily store messages until they are sent. Thus they are separate roles. Some containers require that the interface name match the key used to lookup component. In this situation you may need to create a new interface that does nothing more than extend another interface and add a new role.

The Service Selector

Sometimes you will have several components that function in the same role. For those cases, you will use the ServiceSelector to choose the exact one you need. The best way to describe its proper use is the scenario described here. You have several formatters that have the same role: to take an input document and format it according to the rules in the individual component implementations. One formatter may take a text file and remove all tabs and replace them with four spaces. Another formatter may reverse the formerly mentioned one. Yet another takes the text file and formats it for a canvas object. For the Serviceable, it makes no difference what the implementation does--just that it formats the text.

Using the processing chain example in the previous paragraph, we realize the unsuitability of the ServiceManager for getting the right component. The component addresses the concern of one component per role. Fortunately, the ServiceSelector is a component. That means we use the ServiceManager to lookup the ServiceSelector. The ServiceSelector is designed to choose the specific component out of many that perform the same role. The following code will help:

final ServiceSelector selector = (ServiceSelector)manager.
    lookup( "org.mycompany.myproject.FormatterSelector" );
final Formatter formatter = (Formatter)selector.select( myURL );
        

The selector does not discriminate against lookup keys. In that respect it acts much like a hashtable lookup. Keep in mind that the implementation of the selector does not limit you to a hashtable lookup--you can dynamically instantiate objects as well. It takes an object (a hint), and returns the specific component based on that hint.

Discussions are currently taking place about the ServiceSelector interface and concept. It might be deprecated at some point in the future. There are indications that usage of the ServiceSelector is only required when an application is badly designed.

When you are done with the component

Both the ServiceManager and the ServiceSelector require you to release your component when you are done with it. The method used to do this is "release". One way of handling this is to use the try/catch/finally construct. For your convenience, the following code can help:

MyComponent component = null;

try
{
    component = (MyComponent) manager.lookup("org.mycom.MyComponent");
    component.myMethod();
}
catch (Exception e)
{
    getLogger().debug("Error using MyComponent", e);
}
finally
{
    if (component != null) manager.release(component);
}
	

The reason for this is so that smart component managers that select components from a pool can properly manage the resources.

DefaultComponentManager

The DefaultComponentManager is nothing more than a Hashtable lookup of roles and Components. It even gives you the method put to populate the ServiceManager. One feature of the DefaultComponentManager is that it can cascade. In other words, if the role is not found in this ServiceManager, the default implementation will look in the parent ServiceManager.

For the paranoid developer, the cascading feature of the ServiceManager can be seen as a security hole as opposed to a usability enhancement. You are free to create your own implementation that does not use the cascading feature--but you have to manually populate it with anything that would have been in the parent ServiceManager that your child Serviceable needs. Truth be told, there is very little risk due to the set-once contract for ComponentManagers. The method is never exposed to hostile agents before the ServiceManager is set.

DefaultComponentSelector

The DefaultComponentSelector again is simply a Hashtable selection of components based on hints. It gives the method put to populate the ServiceSelector. The ServiceSelector does not have the cascading feature of the ServiceManager, nor should it. A ServiceSelector simply holds a number of components that implement the same role--there really is no need to cascade.

After the ServiceSelector is populated, you must put it in the ServiceManager. Please use the role of the component you are selecting, not the role of the selector itself. An acceptable convention is to add the "Selector" name to the end of the role you are looking up. Just be consistent.