Java - Get all the implementations of an interface available in client package

I am developing one task scheduler which triggers the tasks in parallel using executor service. I want to make my task scheduler as generic and no code change/less code change in scheduler code base whenever any new type of task is added.

My tasks (mostly client package) can be of any type which basically just accepts particular request and execute the tasks.

To do this I am exposing interface (say ITask) which must be implemented by tasks (which will be on some other app/package) and that will be having one implementation method say example

doTask(IRequest request);

So the use case is if any clients who wants to trigger their job using my scheduler framework/API, just need to add my package in their dependency and rest (those are, getting the list of task classes which implements ITask > schedule it using executor service > retry failed tasks > finally provide the entire tasks status) should be taken care by my schedular API.

What is the optimal way to do this. I am thinking of solution how Junit gets its @Test methods (based on annotation) of client whoever adds Junit dependency in his package, similarly I want get classes based on interface.