Practical Singleton & Dependency Injection question

Question

Say I have a class called PermissionManager which should only exist once for my system and basically fulfills the function of managing various permissions for various actions in

Answer 1

So should I actually require API users to pass in a PermissionManager instance in the constructor of the class? Even though I only want a single PermissionManager instance to exist for my application?

Yes, this is all you need to do. Whether a dependency is a singleton / per request / per thread or a factory method is the responsibility of your container and configuration. In the .net world we would ideally have the dependency on an IPermissionsManager interface to further reduce coupling, I assume this is best practice in Java too.