Spring Security Crud应用程序中的nullPointerExeption

Question

在我的基于Spring的应用程序中，访问被拒绝页面存在问题。 这是一个错误：

java.lang.NullPointerException
        com.finalProject.controller.MainController.accessDenied(MainController.java:88)
        sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)

这是我的控制器文件的一部分：

@RequestMapping(value = "/403", method = RequestMethod.POST)
    public ModelAndView accessDenied() {

        ModelAndView model = new ModelAndView();

        //check if user is login
        Authentication auth = SecurityContextHolder.getContext().getAuthentication();
        if (!(auth instanceof AnonymousAuthenticationToken)) {
            UserDetails userDetail = (UserDetails) auth.getPrincipal();
            System.out.println(userDetail);

            model.addObject("username", userDetail.getUsername());

        }
        model.setViewName("403");
        return model;
    }

这是发生错误的行：

UserDetails userDetail = (UserDetails) auth.getPrincipal();

这是我的spring-security.xml

<http auto-config="true" use-expressions="true">
        <intercept-url pattern="/admin/**" access="hasRole('ROLE_ADMIN')" />
        <intercept-url pattern="/user/**" access="hasAnyRole('ROLE_ADMIN' , 'ROLE_USER')" />
        <!-- access denied page -->
        <access-denied-handler error-page="/403" />
        <form-login
                login-page="/login"
                default-target-url="/welcome"
                authentication-failure-url="/login?error"
                username-parameter="username"
                password-parameter="password" />
        <logout logout-success-url="/login?logout" delete-cookies="JSESSIONID" />
        <!-- enable csrf protection -->
        <csrf/>
    </http>

请任何帮助