Thymeleaf if expression not working with theme param value

Question

I have a theme resolver in my application. There are two themes: dark and white.

<p th:text="${param.theme}">

</p>

This returns the current theme, dark or white. However, the

<p th:if="${param.theme == 'white'}">
    white theme
</p>

outputs nothing.

I want this to implement theme switching. How to make it work?

The Java configuration code.

public class WebConfig  implements WebMvcConfigurer {

    @Autowired
    private ApplicationContext applicationContext;

    @Bean
    public SpringResourceTemplateResolver templateResolver() {

        var templateResolver = new SpringResourceTemplateResolver();

        templateResolver.setApplicationContext(applicationContext);
        templateResolver.setPrefix("classpath:/templates/");
        templateResolver.setSuffix(".html");

        return templateResolver;
    }

    @Bean
    public SpringTemplateEngine templateEngine() {

        var templateEngine = new SpringTemplateEngine();
        templateEngine.setTemplateResolver(templateResolver());
        templateEngine.setEnableSpringELCompiler(true);

        return templateEngine;
    }

    @Bean
    public ViewResolver viewResolver() {

        var resolver = new ThymeleafViewResolver();
        var registry = new ViewResolverRegistry(null, applicationContext);

        resolver.setTemplateEngine(templateEngine());
        registry.viewResolver(resolver);

        return resolver;
    }

    @Bean
    public ThemeSource themeSource() {

        var themeSource = new ResourceBundleThemeSource();
        themeSource.setBasenamePrefix("themes/");
        return themeSource;
    }

    @Bean
    public ThemeResolver themeResolver(){

        var resolver = new CookieThemeResolver();
        resolver.setCookieMaxAge(604800);
        resolver.setCookieName("mytheme");
        resolver.setDefaultThemeName("dark");
        return resolver;
    }

    @Override
    public void addInterceptors(InterceptorRegistry registry) {

        var themeChangeInterceptor = new ThemeChangeInterceptor();
        themeChangeInterceptor.setParamName("theme");
        registry.addInterceptor(themeChangeInterceptor);
    }

    @Override
    public void addResourceHandlers(ResourceHandlerRegistry registry) {
        registry.addResourceHandler("/css/**").addResourceLocations("classpath:/static/css/");
    }

Answer 1

try to convert theme param to string ( using thymleaf expression #{} ) and then compare it to your theme name

as below :

<p th:if="${#strings.toString(param.theme) == 'white'}">
    white theme
</p>