How to Mock WorkflowService

Below is my original class 

@3214626
private ResourceResolverFactory resourceResolverFactory;

@3214626
private WorkflowService workflowService;

How can I mock the above?

I tried something like below:

@Mock
WorkflowService workflowServiceMock;

then

workflowServiceMock = context.registerInjectActivateService(workflowServiceMock);

But it throws NullPointerException