Reusable Components vs Extensions

[meetoblivion]

I'm just wondering, now that we're starting to get past the initial use cases, what is the difference between a reusable component and an extension?  To me, I see none.  The guide calls the Logger injection an extension, even though I can't see any extension related to logger itself in the code, it seems like it's just a reusable standalone producer for some 3PL.  There is no Extension implementation that does the logger config.  So is there a difference?  Does there need to be a difference?

[gavin.king]

I don't think it really matters. If it's generic code and it's portable, it's a portable extension, whether it uses the SPI or not. But note that the logging extension actually does have a very small dependency to the SPI, since InjectionPoint is actually part of the SPI package.

[meetoblivion]

In this case, would integrating a 3PL always end up being an extension, since it would have to use the SPI to work (Logger is integrating a 3PL, SLF4J)?

[gavin.king]

John Ament wrote on Dec 29, 2009 11:59:


In this case, would integrating a 3PL always end up being an extension, since it would have to use the SPI to work (Logger is integrating a 3PL, SLF4J)?

Not necessarily. But probably in most cases, yes.

[meetoblivion]

Which part is the not necessarily? Having to use the SPI?

[gavin.king]

Right.