AuthenticationSuccessHandler in Spring Security

Rather than sublcassing AuthenticationSuccessHandler, It's worth knowing about the Spring security role-checking config:

@Configuration
@EnableWebSecurity
public class SecSecurityConfig extends WebSecurityConfigurerAdapter {
    @Override
    protected void configure(HttpSecurity http) throws Exception {
        http
          .authorizeRequests()
          .antMatchers("/admin/**").hasRole("ADMIN");
    }
    ...
} 

OR pre-checking a Role per endpoint:

@Autowired
@PreAuthorize("hasRole('ADMIN')")
@RequestMapping("/")
public ModelAndView home(HttpServletRequest request) throws Exception {

}

where the default Role prefix is ROLE_

https://docs.spring.io/spring-security/site/docs/3.0.x/reference/el-access.html https://www.baeldung.com/spring-security-expressions-basic

import java.io.IOException;
import java.util.Set;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.springframework.security.core.Authentication;
import org.springframework.security.core.authority.AuthorityUtils;
import org.springframework.security.web.authentication.AuthenticationSuccessHandler;
import org.springframework.stereotype.Component;

@Component
public class Securityhandler implements AuthenticationSuccessHandler {

     public void onAuthenticationSuccess(HttpServletRequest request,   HttpServletResponse response, Authentication authentication) throws IOException  {
        Set<String> roles = AuthorityUtils.authorityListToSet(authentication.getAuthorities());
        if (roles.contains("ROLE_ADMIN")) {
            response.sendRedirect("admin/home.html");
        }
    }
}

You've missed the @Component annotation in your success handler class.