Java OpenID Library - Configuration and Custom Messages

I previously described how message handling works in the Internet2 OpenID library, and how each OpenID message type requires a half dozen or so classes to handle everything. While this may seem like overkill to some, one of the nice things about this separation of logic is that it makes it quite simple to provide custom implementations of specific kinds of messages. While this was not specifically a core requirement of the library, it was an added bonus of the design, and just seemed like a good thing to support. I want to talk about it here, because it illustrates how this portion of the library is configured, which will be important to understand later.

Central Registry

As we mentioned, every OpenID message type has a number of supporting classes. Let’s take the authentication request message as an example. You have:

All classes except for the actual message implementation must be thread-safe, as only a single instance is maintained by the library (technically they don’t follow a singleton pattern, but only one instance is typically used). All of these are stored in central registries, so that they can be retrieved to marshall or unmarshall a message as needed. Each one has it’s own factory that allows registering and looking up of specific implementations:

MessageValidator implementations are registered based on the message class that it validates. For the other three factories, implementations are registered based on a QName which consists of the OpenID protocol namespace URI, and the value of the mode parameter. Yes, there are three OpenID message types that don’t actually have a ‘mode’ parameter, but I’ll save that discussion for another post. Also, the QName here doesn’t exactly represent a namespaced parameter name like it does in the ParameterMap, instead it is just a container for a namespace URI and a string value. Perhaps this is technically a misuse of the QName object, but it’s working fine for now. A static instance of each factory is available from the Configuration class.

Message Flow (redux)

So now let’s go through a message flow like we did last time, and look at how each of the factories are used. (At the time of this writing, I’m still working on hooking in the MessageValidators, so I won’t be talking much about that).

Remember that when a message comes in, it is in some kind of transport specific encoding. Depending on how the message was received and the format it is in, an appropriate MessageDecoder is used to convert it into a ParameterMap. The next step is to find an appropriate MessageUnmarshaller to convert this ParameterMap into an actual Message object. The MessageUnmarshallerFactory has a getUnmarshaller(ParameterMap) method that will lookup exactly what we need. Once we have an unmarshaller, we can call its unmarshall(ParameterMap). This method is responsible for building an appropriate Message object, and then populating it based on the data provided in the ParameterMap. Internally, the unmarshaller uses the MessageBuilderFactory to find an appropriate MessageBuilder using the getBuilder(ParameterMap) method. Once the correct builder is obtained, its buildObject() method is called to get an instance of the Message object. This instance is then populated using data from the ParameterMap and returned. (If anyone wants to volunteer a flow chart that illustrates this, I’d be greatly appreciative!)

When it comes time to send a message back out, the MessageMarshallerFactory’s getMarshaller(Message) method is called to get the correct MessageMarshaller for a given message. The marshaller’s marshall(Message) method is called and returns a ParameterMap, and that is passed through an appropriate MessageEncoder to send it out on the wire.

Custom Implementations

The library comes with default implementations for all of this, so a user can simply choose to ignore all of this plumbing and be just fine. But just in case you do want to customize part of this, how would you go about doing so? Simply by registering them with the appropriate factory. Let’s say you want to provide your own AssociationRequest implementation for whatever reason. But maybe you don’t necessarily care to customize the way the data is unmarshalled into and marshalled out of the object… the default implementations for those are fine. You would of course have your custom AssociationRequest:

public class MyAssociationRequest implements AssociationRequest {
    /* implementation here */

Then to make sure that your custom implementation is built instead of the default implementation provided by the library, you would also need to provide a MessageBuilder:

public class MyAssociationRequestBuilder implements
             MessageBuilder<AssociationRequest> {

    public AssociationRequest buildObject() {
        /* build and return an instance of MyAssociationRequest */

Then register your message builder:

MessageBuilder myBuilder = new MyAssociationRequestBuilder();
QName qname = new QName(OpenIDConstants.OPENID_20_NS, AssociationRequest.MODE);
Configuration.getMessageBuilders().registerBuilder(qname, myBuilder);

Once your builder is registered, it will be used to build AssociationRequest objects for all incoming messages of that type. However, the default marshaller and unmarshaller for that type will continue to be used… you don’t need to worry about that. And once I get the validators hooked in, that will just work as well with your custom class. Or, you could provide your own Validators if you like. You can customize as much or as little of the library as you want.

I don’t imagine that anyone will want to provide custom message implementations very often, but the option is most certainly there. What is far more likely is providing a custom message extension like Attribute Exchange or PAPE. That works in very much the same way, which I’ll explain next.

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