Adding Functionality to Apache NiFi
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Developing a ControllerService

Just like with the Processor interface, the ControllerService interface exposes methods for configuration, validation, and initialization. These methods are all identical to those of the Processor interface except that the initialize method is passed a ControllerServiceInitializationContext, rather than a ProcessorInitializationContext.

Controller Services come with an additional constraint that Processors do not have. A Controller Service must be comprised of an interface that extends ControllerService. Implementations can then be interacted with only through their interface. A Processor, for instance, will never be given a concrete implementation of a ControllerService and therefore must reference the service only via interfaces that extends ControllerService.

This constraint is in place mainly because a Processor can exist in one NiFi Archive (NAR) while the implementation of the Controller Service that the Processor lives in can exist in a different NAR. This is accomplished by the framework by dynamically implementing the exposed interfaces in such a way that the framework can switch to the appropriate ClassLoader and invoke the desired method on the concrete implementation. However, in order to make this work, the Processor and the Controller Service implementation must share the same definition of the Controller Service interface. Therefore, both of these NARs must depend on the NAR that houses the Controller Service's interface. See NiFi Archives (NARs) for more information.