I am trying to write a Mockito Unit Test for my Spring security config class and it has this code

@Bean
public GrantedAuthoritiesMapper authoritiesMapper() {
        return (authorities) -> {
            Set<GrantedAuthority> mappedAuthorities = new HashSet<>();

            authorities.forEach(authority -> {
               if (OAuth2UserAuthority.class.isInstance(authority)) {
                    OAuth2UserAuthority oauth2UserAuthority = (OAuth2UserAuthority)authority;

                    Map<String, Object> userAttributes = oauth2UserAuthority.getAttributes();

        userAttributes.getOrDefault("field", List.of())).forEach(a-> mappedAuthorities.add( new SimpleGrantedAuthority(a)));
                }
            });
            return mappedAuthorities;
        };
    }

And this is my unit test method which does not cover any line of above code.

@Before(){
this.config = new Config(); //this is Config class which has authoritiesMapper
}

@Test
public void testMapper(){
    //I had some mockito code here but nothing worked.

    this.config.authoritiesMapper();

  
}

If I write any Mockito test case, nothing invokes whole method, maybe because of lambda expression. How to write a Mockito test when lambda expression like this exist??