How do I mock a static method that returns void with PowerMock?

You can stub a static void method like this:

PowerMockito.doNothing().when(StaticResource.class, "getResource", anyString());

Although I’m not sure why you would bother, because when you call mockStatic(StaticResource.class) all static methods in StaticResource are by default stubbed

More useful, you can capture the value passed to StaticResource.getResource() like this:

ArgumentCaptor<String> captor = ArgumentCaptor.forClass(String.class);
PowerMockito.doNothing().when(
               StaticResource.class, "getResource", captor.capture());

Then you can evaluate the String that was passed to StaticResource.getResource like this:

String resourceName = captor.getValue();