discovering services/modules via SPI

[steve christensen]

I stumbled on Google's auto-service project the other day, which made me dislike the SPI pattern less. You'd annotated your Guice modules with auto-service's @AutoService(Module.class) annotation. That generates a META-INF/services/com.google.inject.Module at compile time for the annotated class.

So... rather than compose an application via Maven dependencies, and manually adding each dependency's modules into the injector like: 

final Injector injector = Guice.createInjector(

new ExampleModule(),

new ConfigModule(new File(args[0]), ExampleConfig.class),

new LoggingModule(),

new RestModule(),

new HibernateModule(),

new MetricsModule());

injector.getInstance(Run.class).start();

You'd compose the application via Maven, then to get all the default Modules loaded into your injector, you'd do something like:

final Injector injector = Guice.createInjector(

new ExampleModule(),

new ConfigModule(new File(args[0]), ExampleConfig.class),

GwizardModuleLoader.loadModules() 

);

// start services

injector.getInstance(Run.class).start(); // if you depend  on gwizard-services

I took a stab at integrating it here: https://github.com/stevesea/gwizard/tree/spi/gwizard-spi 

It still has lots of rough edges. And... I'm not sure I like it. But, thought I'd throw it out to see if there's interest

[Jeff Schnitzer]

Hmmmmmm... not sure how I feel about this either. It does seem kind of nice, but it adds complication when building a fat jar (need to enable the ServicesResourceTransformer in the shade plugin). And I'm just generally allergic to the META-INF dir.

It seems a no-brainer to include the autoservice flag on the modules, though. Then just not document it until we're sure we like it :)

Maybe the ModuleLoader will be what pushes us to finally create a gwizard-common jar.

Jeff

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[steve christensen]

I like that google auto-service eliminated my major problem with the SPI pattern -- having to add that file to META-INF, and not fat-finger the spelling of the filename, and not fat-finger the spelling of the contents within the file, and keep both of those synchronized with whatever refactoring was going on in the codebase. 

thinking about it more today... I think I favor the explicitness of: 

     final Injector injector = Guice.createInjector(

new ExampleModule(),

new ConfigModule(new File(args[0]), ExampleConfig.class),

new LoggingModule(),

new RestModule(),

new HibernateModule(),

new MetricsModule());

over a SPI module loader approach.

I also like that the individual gwizard components can depend on each other with just an install() statement in their Modules. So, if someone was tired of stringing all those "new *Module()"s together, they could create a 'StandardGwizardWebAppModule' that just install()s all the standard components that make up a web app, and people could use it or not.

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