Eu queria adicionar um arquivo CSS ao meu arquivo HTML. O problema apareceu quando tentei adicionar CSS ao aplicativo Spring Security (trabalho no Conteúdo básico de introdução ao Spring). Eu culpo o Spring Security porque sem ele o arquivo CSS é carregado corretamente.

Application.java Arquivo:

package mainpack;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class Application {

    public static void main(String[] args) throws Throwable {
        SpringApplication.run(Application.class, args);
    }
}

MvcConfig.java Arquivo:

package mainpack;

import org.springframework.context.annotation.Configuration;
import org.springframework.web.servlet.config.annotation.ViewControllerRegistry;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurerAdapter;

@Configuration
public class MvcConfig extends WebMvcConfigurerAdapter {

    @Override
    public void addViewControllers(ViewControllerRegistry registry) {
        registry.addViewController("/home").setViewName("home");
        registry.addViewController("/").setViewName("home");
        registry.addViewController("/hello").setViewName("hello");
        registry.addViewController("/login").setViewName("login");
        registry.addViewController("/index").setViewName("index");
        registry.addViewController("/register").setViewName("register");
        registry.addViewController("/whatever").setViewName("whatever");
    }
}

WebSecurityConfig.java Arquivo:

package mainpack;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Configuration;
import org.springframework.security.config.annotation.authentication.builders.AuthenticationManagerBuilder;
import org.springframework.security.config.annotation.web.builders.HttpSecurity;
import org.springframework.security.config.annotation.web.configuration.WebSecurityConfigurerAdapter;
import org.springframework.security.config.annotation.web.configuration.EnableWebSecurity;

@Configuration
@EnableWebSecurity
public class WebSecurityConfig extends WebSecurityConfigurerAdapter {
    @Override
    protected void configure(HttpSecurity http) throws Exception {
        http
            .authorizeRequests()
                .antMatchers("/", "/home", "/index", "/register", "../static/css", "../static/images").permitAll()
                .anyRequest().authenticated()
                .and()
            .formLogin()
                .loginPage("/login")
                .permitAll()
                .and()
            .logout()
                .permitAll();
    }

    @Autowired
    public void configureGlobal(AuthenticationManagerBuilder auth) throws Exception {
        auth
            .inMemoryAuthentication()
                .withUser("user").password("password").roles("USER");
    }
}

Eu carrego CSS com a linha:

<link href="../static/css/index.css" th:href="@{/css/index.css}" rel="stylesheet" />

noindex.html Arquivo.