You can not use Sling annotations in OSGI service. currentPage, currentNode etc.. are global sling objects which will be created while Sling resolving particular resource. As Sling model has all the capabilities of Sling, those variables/objects will be directly injected in to Sling Model in context of current request.
In OSGI service this is not that straight forward, you should get the current Page and then get path.
Get the ResourceResolver
Get current Resource
Adapt resourceResolver to PageManager
Get the current Page by passing current resource
Get path from Page
Try the below code snippet, might help. Here I am getting Resource and ResourceResolver from request.