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My question: is there a way to use javax.inject (or any other Java injection framework) for a consumer of a Provider to use multiple implementations at runtime if the number of implementations is unknown at build time?

Some background on my need for this: I work on reusable frameworks which, for the most part, combine the use of a factory and a service locator to load implementations. Several of these seem like they could be reworked to use proper dependency injection, at least insofar as removing the service locator, but there are some that require loading all implementations found on the class path. This is achieved through a simple "multi-implementation" implementation which then loads the other implementations, saving the instances off in a collection and looping over them when the API is called.

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  • the strategy used in spring is, use different bean name as ID to represent different bean Commented Jan 14, 2014 at 1:36
  • I don't think a client could "use" multiple implementations, at least not all at once, but you could specify a specific implementation from multiple candidates, is that what you meant? Commented Jan 14, 2014 at 1:36

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I assume you are running on the Java SE platform (as opposed to a Java EE platform) in which case I would highly recommend HK2 (see https://hk2.java.net/2.2.0/). It has a lot of support for efficiently instantiating services and it is certainly the case that multiple implementations of the same contract can be available at runtime. Then at runtime there are a whole manner of mechanisms that you can use to choose which particular implementation will satisfy the dependency (i.e., service ranking or assisted injection etc)

For build time with hk2 you can create "inhabitant" files that describe services to the point where they can be satisfied at runtime without classloading all of them (only the one that is picked will be classloaded if you do it properly). This can be a huge performance boost at boot time of your application (if that sort of thing matters to you).

If you are running on a Java EE platform you can also use HK2, but you should then also give a long look at CDI. Both CDI and HK2 are implementations of JSR-330, and so both work with javax.inject API

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The libraries for which I am investigating injection are bundled into applications in Java SE environments and Java EE environments (and not always in a web container in the case of Java EE portlets, etc.). Looking at the usage guide for HK2, the IterableProvider looks like it's exactly what I'm looking for.
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So you basically have one implementation that delegates a call to an API method to all other implementations. you will need to inject this bean in all the dependencies you can do this in spring by giving this bean (this implementation instance) an id and then inject it using @Autowired @Qualifier("bean_id"). now for listing all implementations that can be done easily in spring by injecting an applicationContext into your delegate implementation and then querying the applicationContext for all beans implementing the API interface.

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