Shiro No Securitymanager Accessible to the Calling Code

I am trying to integrate Spring Boot and Shiro. When I tried to call SecurityUtils.getSubject() in one of my controllers, an exception occurred: org.apache.shiro.UnavailableSecurityManagerException: No SecurityManager accessible to the calling code, either bound to the org.apache.shiro.util.ThreadContext or as a vm static singleton. This is an invalid application configuration.

I just followed some tutorials and docs to configure Shiro and here is my ShiroConfig class:

@Configuration
public class ShiroConfig {

    @Bean
    public Realm realm() {
        return new UserRealm();
    }

    @Bean
    public HashedCredentialsMatcher hashedCredentialsMatcher() {
        HashedCredentialsMatcher hashedCredentialsMatcher = new HashedCredentialsMatcher();
        hashedCredentialsMatcher.setHashAlgorithmName(PasswordEncoder.getALGORITHM()); 
        hashedCredentialsMatcher.setHashIterations(PasswordEncoder.getITERATION()); 
        return hashedCredentialsMatcher;
    }

    @Bean
    public UserRealm shiroRealm() {
        UserRealm userRealm = new UserRealm();
        userRealm.setCredentialsMatcher(hashedCredentialsMatcher()); 
        return userRealm;
    }

    @Bean
    public SessionsSecurityManager securityManager() {
        DefaultWebSecurityManager securityManager = new DefaultWebSecurityManager();
        securityManager.setRealm(shiroRealm());
        return securityManager;
    }

    @Bean
    public DefaultAdvisorAutoProxyCreator defaultAdvisorAutoProxyCreator() {
        DefaultAdvisorAutoProxyCreator defaultAdvisorAutoProxyCreator = new DefaultAdvisorAutoProxyCreator();
        defaultAdvisorAutoProxyCreator.setUsePrefix(true);
        return defaultAdvisorAutoProxyCreator;
    }

    @Bean
    public ShiroFilterChainDefinition shiroFilterChainDefinition() {
        DefaultShiroFilterChainDefinition definition = new DefaultShiroFilterChainDefinition();
        definition.addPathDefinition("/login", "anon");
        definition.addPathDefinition("/register", "anon");
        definition.addPathDefinition("/api/**", "user");
        return definition;
    }
}

And this is the code which caused exception:

    @PostMapping("/login")
    @ResponseBody
    public Object login(@RequestParam("username") String username,
                        @RequestParam("password") String password) {
        if (username.equals("") || password.equals("")) {
            return "please provide complete information";
        }
        UsernamePasswordToken token = new UsernamePasswordToken(username, password);
        Subject subject = SecurityUtils.getSubject(); // this line caused exception
        ...
    }

I am very confused about this exception. Could anyone help?

EDIT I am using Spring Boot 2.1.6.RELEASE and shiro-spring-boot-starter 1.4.0.