On 8/24/05, Ken Weiner <[EMAIL PROTECTED]> wrote: > Martin, yes, I will file this as an issue in Jira and reference this > email thread. > > Craig, thank you so much for your response. I now understand the > relationship between a java spec and the physical API source code much > better. You brought up an interesting point about JSF being different > in that there is more executable code than in other frameworks. What > was the reason that classes like javax.faces.render.Renderer were > defined as abstract classes rather than interfaces?
Trust me ... you don't want to read all the EG threads on that topic :-). But the short answer is, consider what happens in a future version of JSF, when (for good reason) a new method signature is added to the Renderer API. If it were an interface, you've just broken every current implementation in the world. As an abstract base class, you can add a default implementation that mirrors the previous behavior, so the only potential breakage is if an implementation had their own version of that very same method signature. Breakage like this is not totally prohibited -- consider what has happened in servlet (when new methods get added to the HttpServletRequest interface) or JDBC (where there has been lots of changes over the versions). But that kind of breakage affects relatively small audiences (app server and database vendors) versus the potential for breaking every JSF component written by every component developer in the world. > In Section 8.2, > the spec refers to what an "instance" of Renderer should do. Does the > word "instance" imply a class rather than interface or is it up to the > implementation? No, "instance" is used in the O-O sense of the word, referring to how many objects are instantiated into the JVM's memory. In JSF, a Renderer is an application-level singleton that is shared by all the instances of the components (just like a Servlet is an application level singleton that is shared by all requests mapped to the same URI pattern). With UIComponents, for example, there is a separate object instance for every component on your page -- but if you have four text fields, the four component instances will all share the same renderer instance. An actual instance, of course, must be of a concrete class that can be instantiated. But whether that concrete class extends an abstract base class, or implements an interface, is irrelevant to how many copies there are. In this particular case, it simply has to be that "instanceof Renderer" returns true, since the JSF implementation is going to cast it to this before calling its methods. > > -Ken > Craig
