@Test
    void testGet() throws Exception {

        Injector injector = createMock(Injector.class);
        PatternMatcher patternMatcher = createMock(PatternMatcher.class);

        underTest.injector = injector;
        underTest.setPatternMatcher(patternMatcher);

        FilterChainResolver resolver = underTest.get();

        Field chainsField = SimpleFilterChainResolver.class.getDeclaredField("chains");
        chainsField.setAccessible(true);
        Field injectorField = SimpleFilterChainResolver.class.getDeclaredField("injector");
        injectorField.setAccessible(true);
        Field patternMatcherField = SimpleFilterChainResolver.class.getDeclaredField("patternMatcher");
        patternMatcherField.setAccessible(true);

        assertSame(chains, chainsField.get(resolver));
        assertSame(injector, injectorField.get(resolver));
        assertSame(patternMatcher, patternMatcherField.get(resolver));
    }
This test relies on the internal structure of FilterChainResolver in order to check that it got created correctly.
support/guice/src/test/java/org/apache/shiro/guice/web/FilterChainResolverProviderTest.java