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Let's say you've written a bunch of different services, each of which does something specific for a particular type of file (identified by the file's extension), and each implements the same interface, which we'll call FileServicer. And now let's say you need a central service that selects the one of your FileServicer implementations based on a given file extension. You start by providing a service builder method:

  public static FileServiceDispatcher buildFileServicerDispatcher(Map<String,FileServicer> contributions)
  {
    return new FileServiceDispatcherImpl(contributions);
  } 

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Or, instead of instantiating those services ourselves, we could inject them:

  public static void contributeFileServicerDispatcher(MappedConfiguration<String,FileServicer> configuration,
  
    @InjectService("TextFileServicer") FileServicer textFileServicer,
    @InjectService("PDFFileServicer") FileServicer pdfFileServicer)
  {
    configuration.add("txt", textFileServicer);
    configuration.add("pdf", pdfFileServicer);
  }  

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• It makes it much easier for an override of the service to get the configuration intended for the original service.

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Again, the order in which service contribution methods are invoked is unknown. Therefore, the order in which objects are added to the configuration is not known. Instead, we enforce an order on the items after all the contributions have been added. As with service decorators, we set the order by giving each contributed object a unique id, and identifying (by id) which items must preceded it in the list, and which must follow.

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For the "FileSystem" contribution, a constraint has been specified, indicating that FileSystem should be ordered after some other contribution named "CacheSetup". Any number of such ordering constraints may be specified (the add() method accepts a variable number of arguments).

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For mapped configurations where the key type is String, a CaseInsensitiveMap will be automatically used (and passed to the service builder method), to help ensure that case insensitivity is automatic and pervasive.

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