Does this unit test cover all edge cases?
As far as I can see the
@RequestParam
annotation can have a defaultValue parameter - once that's used the
method becomes a bit simpler:

@RequestMapping(method = RequestMethod.GET)
public List&lt;Tags&gt; getAllTags(@RequestParam(value = "query", required = false) String query,
        @RequestParam(value = "limit", required = false, defaultValue = "10") int limit,
        @RequestParam(value = "start", required = false, defaultValue = "0") int start,
        @RequestParam(value = "approved", required = false, defaultValue = "true") boolean approved,
        @ActiveLanguage Language language,
        @ActiveUser User user) {
    if (query != null) {
        return tagService.findTagsByQuery(user, query, limit, start, language, approved);
    } else {
        return tagService.findTags(user, limit, start, language);
    }
}


I'd also consider some edge cases for query, like the empty string or
a string with just whitespace for that matter.

Now for the unit tests, the naming could probably be improved, but I
also don't pay that much attention to it to be honest.

If the tagsList (or any other part like mocking methods etc.) is a
common thing in the tests consider moving it into the setUp method (or
equivalent) so it doesn't clutter up the test methods.

The assertSame is fine, you're basically just testing whether the
object is passed through directly, however the "not null" tests
afterwards are useless - tagsList shouldn't (can't) be null, so why
is it being verified here?

times(1) is default for Mockito, so leave it out.  Also the
verifyNoMoreInteractions is a bit discouraged in general since it
might make tests more brittle than they'd have to be, also since you're
not gaining much from it here I'd leave it out too.

IMO since this is at the outermost layer I'd rather have an integration
tests that actually checks the parameter parsing and validation too -
this particular test doesn't have that much value considering you're
just passing through values.